DC Power Electronics Student Manual 86356 00

Electric Power / Controls
1-800-Lab-Volt
www.labvolt.com
86356-00
|3086356000000R~
DC Power Electronics
Student Manual
Electric Power / Controls
DC Power Electronics
Student Manual
86356-00
A
First Edition
Published September 2014
© 2010 by Lab-Volt Ltd.
Printed in Canada
All rights reserved
ISBN 978-2-89640-396-7 (Printed version)
ISBN 978-2-89747-236-8 (CD-ROM)
Legal Deposit – Bibliothèque et Archives nationales du Québec, 2010
Legal Deposit – Library and Archives Canada, 2010
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form by any means,
electronic, mechanical, photocopied, recorded, or otherwise, without prior written permission from Lab-Volt Ltd.
Information in this document is subject to change without notice and does not represent a commitment on the part of
Lab-Volt. The Lab-Volt® materials described in this document are furnished under a license agreement or a
nondisclosure agreement.
The Lab-Volt® logo is a registered trademark of Lab-Volt Systems.
Lab-Volt recognizes product names as trademarks or registered trademarks of their respective holders.
All other trademarks are the property of their respective owners. Other trademarks and trade names may be used in
this document to refer to either the entity claiming the marks and names or their products. Lab-Volt disclaims any
proprietary interest in trademarks and trade names other than its own.
Safety and Common Symbols
The following safety and common symbols may be used in this manual and on
the Lab-Volt equipment:
Symbol
Description
DANGER indicates a hazard with a high level of risk which, if not
avoided, will result in death or serious injury.
WARNING indicates a hazard with a medium level of risk which,
if not avoided, could result in death or serious injury.
CAUTION indicates a hazard with a low level of risk which, if not
avoided, could result in minor or moderate injury.
CAUTION used without the Caution, risk of danger sign ,
indicates a hazard with a potentially hazardous situation which,
if not avoided, may result in property damage.
Caution, risk of electric shock
Caution, hot surface
Caution, risk of danger
Caution, lifting hazard
Caution, hand entanglement hazard
Notice, non-ionizing radiation
Direct current
Alternating current
Both direct and alternating current
Three-phase alternating current
Earth (ground) terminal
A DC Power Electronics
v
Safety and Common Symbols
Symbol
Description
Protective conductor terminal
Frame or chassis terminal
Equipotentiality
On (supply)
Off (supply)
Equipment protected throughout by double insulation or
reinforced insulation
In position of a bi-stable push control
Out position of a bi-stable push control
vi
DC Power Electronics A
Table of Contents
Preface ...................................................................................................................xi
About This Manual ............................................................................................... xiii
Introduction DC Power Electronics .................................................................. 1
DISCUSSION OF FUNDAMENTALS ....................................................... 1
Exercise 1
The Diode and Switching Transistor .......................................... 3
DISCUSSION ..................................................................................... 3
Description of a diode .............................................................. 3
Operating principles of a diode ................................................ 4
Characteristic voltage-current curve of a diode ....................... 6
Description of a transistor ........................................................ 7
Using a transistor as a switch .................................................. 7
Cycle, period, and frequency ................................................... 9
Duty cycle and pulse width modulation .................................. 10
Average value ........................................................................ 10
Safety rules ............................................................................ 11
PROCEDURE ................................................................................... 12
Setup and connections .......................................................... 12
Forward-biased diode in a simple circuit ............................... 14
Reverse-biased diode in a simple circuit ............................... 15
Switching Transistor ............................................................... 16
Setup and connection ............................................................... 16
Static control of a switching transistor ...................................... 17
Dynamic control of a switching transistor ................................. 20
Frequency and duty cycle......................................................... 21
Average value .......................................................................... 22
Exercise 2
The Buck Chopper...................................................................... 25
DISCUSSION ................................................................................... 25
The buck chopper .................................................................. 25
Operation of a buck chopper with a resistive load ................. 26
High-side versus low-side switching ...................................... 27
PROCEDURE ................................................................................... 28
Setup and connections .......................................................... 28
Output voltage versus duty cycle ........................................... 30
Output voltage versus switching frequency ........................... 33
Low-side and high-side switching .......................................... 33
A DC Power Electronics
vii
Table of Contents
Exercise 3
Introduction to High-Speed Power Switching ......................... 37
DISCUSSION ................................................................................... 37
High-speed power switching circuits ...................................... 37
Voltage-type circuit................................................................. 37
Current-type circuit ................................................................. 38
Free-wheeling diodes ............................................................. 39
Power efficiency ..................................................................... 40
Interconnecting voltage-type and current-type circuits .......... 40
PROCEDURE ................................................................................... 41
Setup and connections .......................................................... 41
Voltage-type circuit................................................................. 43
Connecting a voltage-type circuit to a current-type circuit
via an electronic switch without a free-wheeling diode .......... 45
Connecting a voltage-type circuit to a current-type circuit
via an electronic switch with a free-wheeling diode ............... 46
Power efficiency ..................................................................... 48
Interconnecting two voltage-type circuits via an electronic
switch ..................................................................................... 50
Exercise 4
Ripple in Choppers..................................................................... 53
DISCUSSION ................................................................................... 53
Ripple ..................................................................................... 53
Ripple versus inductance / capacitance ................................ 54
Ripple versus switching frequency......................................... 56
PROCEDURE ................................................................................... 56
Setup and connections .......................................................... 56
Current ripple versus switching frequency and inductance ... 58
Current ripple versus frequency ................................................ 58
Current ripple versus inductance .............................................. 61
Voltage ripple versus switching frequency and
capacitance ............................................................................ 62
Exercise 5
The Lead-Acid Battery Charger ................................................ 69
DISCUSSION ................................................................................... 69
Battery fundamentals ............................................................. 69
Lead-acid battery charger fundamentals ............................... 70
Implementing a lead-acid battery charger using a buck
chopper with feedback loops ................................................. 71
viii
DC Power Electronics A
Table of Contents
PROCEDURE ................................................................................... 74
Setup and connections .......................................................... 74
Voltage regulation .................................................................. 76
Current regulation .................................................................. 78
Partially discharging the batteries in the Lead-Acid
Battery Pack ........................................................................... 81
The lead-acid battery charger ................................................ 82
Initial stage of a lead-acid battery fast-charging – Constant
current charge .......................................................................... 82
Middle stage of a lead-acid battery fast-charging –
Constant voltage charge at gassing voltage ............................. 85
Finishing stage of a lead-acid battery fast-charging –
Constant voltage charge at float level....................................... 86
Exercise 6
The Boost Chopper .................................................................... 89
DISCUSSION ................................................................................... 89
The boost chopper ................................................................. 89
Power efficiency ..................................................................... 91
PROCEDURE ................................................................................... 92
Setup and connections .......................................................... 92
Output voltage versus duty cycle ........................................... 94
Output voltage versus switching frequency ........................... 96
Power efficiency ..................................................................... 97
Exercise 7
The Buck/Boost Chopper .......................................................... 99
DISCUSSION ................................................................................... 99
The Buck/Boost Chopper ....................................................... 99
PROCEDURE ................................................................................. 102
Setup and connections ........................................................ 102
Switching control signals of a buck/boost chopper .............. 104
Operation of a buck/boost chopper ...................................... 106
Exercise 8
The Four-Quadrant Chopper ................................................... 111
DISCUSSION ................................................................................. 111
The Four-Quadrant Chopper ............................................... 111
PROCEDURE ................................................................................. 114
Setup and connections ........................................................ 114
Switching control signals of a four-quadrant chopper .......... 116
Operation of a four-quadrant chopper ................................. 118
Partially discharging the batteries in the Lead-Acid
Battery Pack ......................................................................... 120
Demonstrating four-quadrant operation ............................... 120
A DC Power Electronics
ix
Table of Contents
Appendix A Equipment Utilization Chart .................................................... 125
Appendix B Resistance Table for the Resistive Load Module .................. 127
Appendix C Preparation of the Lead-Acid Battery Pack ........................... 129
Charging procedure ............................................................. 129
Sulfation test ........................................................................ 130
Battery maintenance ............................................................ 131
Appendix D Glossary of New Terms............................................................ 133
Appendix E Circuit Diagram Symbols ......................................................... 135
Index of New Terms ........................................................................................... 141
Acronyms ........................................................................................................... 143
Bibliography ....................................................................................................... 145
We Value Your Opinion!..................................................................................... 147
x
DC Power Electronics A
Preface
The production of energy using renewable natural resources such as wind,
sunlight, rain, tides, geothermal heat, etc., has gained much importance in recent
years as it is an effective means of reducing greenhouse gas (GHG) emissions.
The need for innovative technologies to make the grid smarter has recently
emerged as a major trend, as the increase in electrical power demand observed
worldwide makes it harder for the actual grid in many countries to keep up with
demand. Furthermore, electric vehicles (from bicycles to cars) are developed and
marketed with more and more success in many countries all over the world.
To answer the increasingly diversified needs for training in the wide field of
electrical energy, Lab-Volt developed the Electric Power Technology Training
Program, a modular study program for technical institutes, colleges, and
universities. The program is shown below as a flow chart, with each box in the
flow chart representing a course.
The Lab-Volt Electric Power Technology Training Program.
A DC Power Electronics
xi
Preface
The program starts with a variety of courses providing in-depth coverage of basic
topics related to the field of electrical energy such as ac and dc power circuits,
power transformers, rotating machines, ac power transmission lines, and power
electronics. The program then builds on the knowledge gained by the student
through these basic courses to provide training in more advanced subjects such
as home energy production from renewable resources (wind and sunlight), largescale electricity production from hydropower, large-scale electricity production
from wind power (doubly-fed induction generator [DFIG], synchronous generator,
and asynchronous generator technologies), smart-grid technologies (SVC,
STATCOM, HVDC transmission, etc.), storage of electrical energy in batteries,
and drive systems for small electric vehicles and cars.
xii
DC Power Electronics A
About This Manual
This course covers power electronic components and circuits (choppers)
required to manage dc power, such as dc power stored in batteries or produced
from wind power or solar power. The course presents the diode and the
switching transistor, two semiconductor components that are widely used in
power electronics. The course also provides in-depth coverage of various types
of chopper, a power electronics device used in many DC power circuits
(e.g., dc motor drives, battery chargers, dc-to-dc converters, etc.).
Manual objectives
When you have completed this manual, you will be familiar with the main types of
choppers. You will also be familiar with high-speed power switching (voltage-type
and current-type circuits, free-wheeling diodes, etc.). Finally, you will know how
to control ripple in choppers, and to build a battery charger using a buck chopper.
Prerequisite
As a prerequisite to this course, you should have read the manual titled
DC Power Circuits, part number 86350.
Safety considerations
Safety symbols that may be used in this manual and on the Lab-Volt equipment
are listed in the Safety Symbols table at the beginning of the manual.
Safety procedures related to the tasks that you will be asked to perform are
indicated in each exercise.
Make sure that you are wearing appropriate protective equipment when
performing the tasks. You should never perform a task if you have any reason to
think that a manipulation could be dangerous for you or your teammates.
Systems of units
Units are expressed using the International System of Units (SI) followed by the
units expressed in the U.S. customary system of units (between parentheses).
A DC Power Electronics
xiii
Introduction
DC Power Electronics
DISCUSSION OF
FUNDAMENTALS
Power electronics circuits can be found wherever there is a need to modify a
form of electrical energy (i.e., change in voltage, current, or frequency). In
modern systems the conversion is performed with semiconductor switching
devices such as diodes, thyristors and transistors. In contrast to electronic
systems concerned with transmission and processing of signals and data,
substantial amounts of electrical energy are processed.
As efficiency is at a premium in power electronics, the losses that a power
electronic device generates should be as low as possible. The instantaneous
dissipated power of a device is equal to the product of the voltage across the
device and the current through it. From this, one can see that the losses of a
power device are at a minimum when the voltage across it is zero or when no
current flows through it. Therefore, a power electronic converter is built around
one (or more) device operating in switching mode (either on or off). With such a
structure, the energy is transferred from the input of the converter to its output by
bursts.
Power electronics circuits are found in virtually every electronic device. For
example, they are found in most mobile devices to maintain the voltage at a fixed
value whatever the voltage level of the battery is.
A DC Power Electronics
1
Exercise
1
The Diode and Switching Transistor
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the operation of
the diode and the switching transistor.
a
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:









DISCUSSION
The direction of the diode
symbol’s “arrowhead” points
in the direction of current flow.
th
Scientists of the 17 century
arbitrarily decided that current
flows from the positive terminal to the negative terminal.
This so-called conventional
current direction is still used
today, and is the accepted
direction of current flow, but it
is worth noting that the actual
direction of electron flow is
opposite to the conventional
current direction.
A DC Power Electronics
The hands-on exercises in this manual require you to be familiar with the Lab-Volt
computer-based instruments. Refer to user guides Data Acquisition and Control
System (86716-E) and Computer-Based Instruments for EMS (86718-E) to
become familiar with the operation and use of the Lab-Volt computer-based
instruments.
Description of a diode
Operating principles of a diode
Characteristic voltage-current curve of a diode
Description of a transistor
Using a transistor as a switch
Cycle, period, and frequency
Duty cycle and pulse width modulation
Average value
Safety rules
Description of a diode
A diode is a semiconductor device that allows electrical current to flow in one
direction only. Figure 1 shows a typical low-power diode. The diode has two
terminals, called the anode (A) and the cathode (K). A ring mark on the diode
case identifies the terminal corresponding to the cathode. The other terminal
corresponds to the anode.
Anode (A)
Cathode (K)
Schematic symbol
Ring mark
Physical representation
Figure 1. Schematic symbol and physical representation of a diode.
3
Exercise 1 – The Diode and Switching Transistor  Discussion
Figure 2 shows the construction and schematic symbol of a diode.
P layer
A
The arrowhead points toward
the cathode, i.e., in the direction
of conventional current.
N layer
Anode
Cathode
Construction
K
A
K
Symbol
Figure 2. Construction and schematic symbol of a diode.
As Figure 2 shows, the diode consists of two layers of semiconducting material
(semiconductors):

A P-type semiconductor layer containing positive charge carriers (holes).
The P-type layer corresponds to the anode (A) terminal of the diode.

An N-type semiconductor layer containing negative charge carriers
(electrons). The N-type layer corresponds to the cathode (K) terminal of the
diode.
Operating principles of a diode
The diode is an essential component of many circuits, as you will see in this
manual. The diode operates as a high-speed switch with no movable parts:

When no voltage is present across the diode terminals, the diode is in the
“off” (blocked) state. No current flows through the diode, and the diode acts
like an open switch as shown in Figure 3.
No voltage
A
Switch open
K
Figure 3. When no voltage is present across the diode terminals, the diode acts as an open
switch. Therefore, no current flows through the diode.
4
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Discussion

The + and – signs next to
voltage
in the figure
indicate the convention of
measurement of this voltage.
These signs indicate that the
voltage at point A ( ) in
the figure is higher than
the voltage at point K ( )
when voltage
is positive
(e.g., when
10 V).
Conversely, the value of
is
negative when the voltage at
point A ( ) is lower than the
voltage at point K ( )
(e.g., when
10 V).
When a voltage is present across the diode terminals and the voltage at
the anode is lower than the voltage at the cathode, the diode acts as an
open switch. Therefore, no current flows through the diode. In this
condition, the diode is said to be reverse biased, as shown in Figure 4.
Reverse voltage
Switch open
A
0V
10 V
K
Figure 4. When the voltage at the anode is lower than the voltage at the cathode (i.e., when
voltage
is negative), the diode acts as an open switch: no current flows through the diode.

When a voltage is applied across the diode terminals, and the voltage at
the anode is higher than the voltage at the cathode, the diode passes from
the “off” (blocked) state to the “on” (conducting) state. In this case, the
diode is said to be forward biased: it acts as a closed switch, allowing the
current to flow from the anode to the cathode, as shown in Figure 5.
Forward voltage
Switch closed
A
9.2 V
10 V
K
Figure 5. When the voltage at the anode is higher than the voltage at the cathode (i.e., when
is positive), the diode acts as a closed switch and the current flows through the
voltage
diode in the direction indicated.

As long as current flows through the diode, the diode remains forward
biased and acts like a closed switch. When the current stops flowing
through the diode (even for a very brief period), the diode becomes like an
open switch and the voltage across the diode terminals drops to 0 V, as
shown in Figure 6.
⇩ 0V
Switch opens
A
K
⇩ 0A
Figure 6. When the current stops flowing through the diode, the diode becomes like an open
switch and the voltage across the diode terminals drops to 0 V.
A DC Power Electronics
5
Exercise 1 – The Diode and Switching Transistor  Discussion
Characteristic voltage‐current curve of a diode
The characteristic curve of a diode represents the current flowing through the
diode as a function of the voltage across its terminals. Figure 7 shows the
characteristic curve of an ideal diode and that of a real diode.
 Ideal diode: when the diode is reverse biased, it acts like a perfect insulator.
Therefore, no current flows through the diode. When the diode is forward
biased, it acts like a perfect conductor. Therefore, current flows through the
diode without a voltage drop across the diode.
 Real diode: when the diode is reverse biased, a small leakage current flows
through it. When the diode is forward biased, the current flowing through it
increases very rapidly as the voltage increases, until the diode becomes fully
conducting. Note that the diode conducts little when the forward voltage is
below a minimum value, called the knee voltage. The knee voltage is the
voltage drop across the diode (typically 0.7 V in the case of a silicon diode)
when the current starts to increase very rapidly.
Ideal diode
Real diode
Knee voltage (typical
voltage drop) across
a silicon diode
0.7 V
Figure 7. Characteristic voltage-current curves of an ideal diode and a real diode.
6
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Discussion
Description of a transistor
A transistor is a semiconductor device commonly used to amplify or switch
electronic signals. It is made of three layers of semiconductor materials rather
than two layers as in a diode. The transistor has three terminals for connection to
an external circuit: base (B), collector (C), and emitter (E). There are two types of
transistors, NPN and PNP, with different circuit symbols as shown in Figure 8.
The letters N and P refer to the type (N or P) of semiconductor in each layer of
material used to make the transistor. Note that the amplifier mode of transistors
is not covered in this manual.
C
C
B
B
E
E
NPN
PNP
Figure 8. Transistor circuit symbols.
Using a transistor as a switch
The current flowing through
the transistor (between the
collector and the emitter) is
referred to as the collector
current .
When a transistor is used as an electronic switch, it must be either off (open
circuit) or on (fully conductive). Without current applied to the base of the
0), no current can flow between the collector and the emitter, and
transistor (
the transistor is said to be off. Conversely, when a current is applied to the base
of the transistor, current flows through the transistor (between the collector and
the emitter). When sufficient current flows through the base, the transistor
becomes fully conductive and is said to be on. In the On state, the voltage
across the transistor is almost zero and the transistor is said to be saturated
because it cannot pass any more collector current . In conclusion, a relatively
small flow of current through the base of the transistor has the ability to exert
control over a large flow of current through the transistor (between the collector
and the emitter). See Figure 9.
Load
(large)
C
B
(small)
E
Figure 9. A relatively small flow of current through the base of a transistor has the ability to
exert control over a large flow of current through the transistor.
A DC Power Electronics
7
Exercise 1 – The Diode and Switching Transistor  Discussion
The power dissipated by a switching transistor ( ×
) is very small. In the Off
state, the collector current equals 0, so no power is dissipated in the transistor.
is almost 0, so that very little power is
In the On state, the collector voltage
dissipated in the transistor.
The two devices most often used as switching transistors in power electronics
circuits are the MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
and the IGBT (Insulated Gate Bipolar Transistor). The MOSFET has three
terminals: gate, drain, and source that correspond roughly to the base, collector,
and emitter of the generic (bipolar) transistor shown previously. The IGBT also
has three terminals: gate, collector, and emitter. In the case of MOSFETs and
IGBTs, the control signal is a voltage, applied between the gate and drain of
MOSFETs or the gate and emitter of IGBTs instead of a current as for bipolar
transistors. The symbols used to represent a MOSFET and an IGBT are shown
in Figure 10.
S
C
G
G
D
MOSFET
E
IGBT
Figure 10. Symbols used to represent a MOSFET and an IGBT.
In order to simplify circuit representation, the symbol of an electronic switch is
often used in schematic circuits to represent a MOSFET or an IGBT acting as
a switching transistor (see Figure 11). In this symbol, the arrowhead indicates
the direction of current flow when the switch is closed (transistor in the on state).
Symbol of an electronic switch
Figure 11. The symbol of an electronic switch is often used to represent a MOSFET or an IGBT
acting as an electronic switch in schematic circuits.
During the lab experiment, you will observe the operation of a switching transistor
by applying a voltage to the gate of an IGBT. You will observe that the transistor
switches on when the voltage applied to its gate is 5 V dc, and switches off when
the voltage is null. The application of the voltage to the gate of the transistor will
be controlled manually then electronically. You will observe that the manual
control of the switching control signal (voltage applied to the gate) does not allow
rapid switching of the transistor. Conversely, the electronic control of the
switching control signal allows rapid changes in state (0 V and 5 V) of the
switching control signal, and thus, rapid switching of the transistor. Successive
changes in the state of the switching control signal produce a series of
rectangular pulses as shown in Figure 12.
8
DC Power Electronics A
Voltage (V)
Exercise 1 – The Diode and Switching Transistor  Discussion
5
0
Time
Figure 12. Successive changes in the state of the switching control signal produce a series of
rectangular pulses (typical).
Cycle, period, and frequency
The duration a signal is at high level (5 V) plus the duration the signal is at low
level (0 V) form a complete cycle of the signal, as shown in Figure 13.
Duration
at high
level
Duration
at low
level
Signal level
High
Low
One complete cycle
Time
Figure 13. The duration a signal is at high level plus the duration the signal is at low level form
a complete cycle.
The time required for one complete cycle to occur is called the period ( ).
The number of cycles that occur in one second is called the frequency ( ).
Frequency is measured in hertz (Hz), 1 Hz being equal to 1 cycle per
second (1/s).
The equation used for calculating the frequency is:
1
where
(1)
is the frequency of the signal, in hertz (Hz) or cycles per
second (1/s).
is the period of the signal, in seconds (s).
The period is simply the reciprocal of the frequency:
1
(2)
For instance, a control signal having a 10 Hz frequency, repeats itself 10 times
per second. Therefore, the period of one cycle is 1/10 Hz = 0.1 s.
A DC Power Electronics
9
Exercise 1 – The Diode and Switching Transistor  Discussion
Duty cycle and pulse width modulation
Electronic control of switching control signals generally allows the proportion of
time the signal is at high level (or low level) during one cycle to be adjusted. The
proportion of on-time (high level) to the duration of one cycle (period) of the
control signal is called the duty cycle ( ). The duty cycle is expressed by a
percentage or a decimal. In Figure 14a for instance, the on-time of the switching
control signal corresponds to 25% of the period, which corresponds to a duty
cycle of 25% or 0.25. Figure 14b and Figure 14c show switching control signals
having a duty cycle of 50% and 75%, respectively. The modulation of the pulse
width, i.e., the variation of the width of the high-level pulse in the switching
control signal, obtained by varying the duty cycle is called pulse-width
modulation (PWM).
Amplitude of
the switching
control signal
High
Low
Time
On-time
Cycle
(a) 25% duty cycle
Amplitude of
the switching
control signal
High
Low
Time
On-time
(b) 50% duty cycle
Amplitude of
the switching
control signal
High
Low
Time
On-time
(c) 75% duty cycle
Figure 14. Switching control signals having various duty cycles.
Average value
The average (AVG) or mean value of a voltage or current waveform corresponds
to the height of a rectangle having an area equal to that of the waveform during
one period . This is also referred to as the dc value of the waveform. In
Figure 15a, the height of the rectangle having the same area as that of the
voltage waveform during 25% of a complete cycle is 12.5 V. Consequently, the
average value of the voltage is 12.5 V. Figure 15b and Figure 15c show the
average value of other voltage waveforms (rectangular pulse signals)
having 50% and 75% duty cycles.
10
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Discussion
As Figure 15 shows, the average voltage value
of a rectangular pulse signal
corresponds to the maximum voltage value
times the duty cycle .
(3)
where
is the average voltage value.
is the maximum voltage value.
is the duty cycle.
Voltage (V)
Maximum value
50
Average value
12.5
Time
Cycle
(a) 25 % duty cycle
Voltage (V)
Maximum value
50
Average value
25
Time
(b) 50 % duty cycle
Voltage (V)
Maximum value
50
37.5
Average value
0
Time
(c) 75 % duty cycle
Figure 15. Average values of various voltage waveforms (pulses) with their corresponding
duty cycle.
In the Metering application of LVDAC-EMS and multimeters, the average value is
obtained by selecting the DC mode. In the Oscilloscope window of LVDAC-EMS
the value is indicated in the column "AVG" located under the Oscilloscope
display.
Safety rules
Observe the following safety rules when using electrical equipment:
1.
A DC Power Electronics
Always make sure that the power source is disabled when connecting or
disconnecting leads or components.
11
Exercise 1 – The Diode and Switching Transistor  Procedure Outline
PROCEDURE OUTLINE
2.
Never leave any electrical lead unconnected. Touching the unconnected
end of a lead while the power source is enabled could give you an electric
shock. A short circuit could also occur if the unconnected end of a lead
touches a conducting surface.
3.
When connecting an electrical circuit, make sure that the contact terminals
are free of dirt, oil, and water. Dirt and oil are insulators and impair the
connection between two components. Water is a conductor and might
make a connection where it is not wanted.
The Procedure is divided into the following sections:




PROCEDURE
Setup and connections
Forward-biased diode in a simple circuit
Reverse-biased diode in a simple circuit
Switching Transistor
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Install the required equipment in the Workstation.
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply. Turn the 24 V ac power supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to a
USB port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply. This connects the internal power supply of the
module to the Power Supply terminals on the front panel.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
12
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Procedure
5. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
6. Set up the circuit shown in Figure 16. In this circuit, is a positive dc voltage
source implemented using the Four-Quadrant Dynamometer/Power Supply,
E1 and I1 are voltage and current inputs of the Data Acquisition and Control
is one of the power diodes in the IGBT Chopper/Inverter
Interface, diode
is a combination of the resistors of the Resistive Load.
module. Resistor
is used to observe the behavior of diodes, and resistor
is used
Diode
as a load that limits the current in the circuit.
a
The Chopper/Inverter Control is not used in this part of the exercise. For this
reason, the IGBT Chopper/Inverter module does not need to be powered.
57 Ω
25 V
Figure 16. Circuit used to observe the operation of a forward-biased diode.
7. Make the necessary connections and switch settings on the Resistive Load
module in order to obtain the resistance value required.
a
A DC Power Electronics
Appendix B lists the switch settings required on the Resistive Load in order to
obtain various resistance values.
13
Exercise 1 – The Diode and Switching Transistor  Procedure
Forward‐biased diode in a simple circuit
In this part of the exercise, you will apply a forward dc voltage to a diode, and
observe if the diode acts as a closed switch or as an open switch in this
operating condition.
8. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:

Set the Function parameter to Voltage Source (+).This setting makes
the internal power source of the Four-Quadrant Dynamometer/Power
Supply operate as a positive dc voltage source.

Set the Voltage parameter to 25 V. This sets the source voltage
to 25 V.

Start the voltage source.
9. In LVDAC-EMS, open the Metering window. Make sure that meters E1 and
I1 are enabled. Set the AC/DC button at the bottom of each meter to DC.
This sets the meters to display dc (direct current or average) values.
Select the Continuous Refresh mode by clicking on the Continuous Refresh
button.
10. Record the voltage across diode
Voltage across diode
Diode current
and the diode current
.
: _____
: _____
11. Do the measured voltage and current indicate that the diode acts as a closed
switch or as an open switch?
12. Is the diode forward biased or reverse biased in this operating condition?
13. Explain why the voltage measured across the diode is very small.
14
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Procedure
14. In the Four-Quadrant Dynamometer/Power Supply window, set the Voltage
parameter to 50 V.
Does the voltage across the diode double? Explain why.
15. Stop the voltage source by clicking on the Start/Stop button or by setting the
Status parameter to Stopped.
Reverse‐biased diode in a simple circuit
In this part of the exercise, you will apply a reverse voltage to a diode, and
observe if the diode acts as a closed switch or as an open switch in this
operating condition.
16. Modify the circuit as shown in Figure 17. This reverses the diode orientation
in your circuit, and thus, applies a reverse voltage to the diode. Note that the
connections to voltage input E1 are also reversed so that its positive (+) and
negative (-) terminals remain connected to the anode and cathode of
diode , respectively.
57 Ω
25 V
Figure 17. Circuit used to observe the operation of a reverse-biased diode.
17. In the Four-Quadrant Dynamometer/Power Supply window, set the Voltage
parameter to 25 V, and then start the voltage source.
18. Record the voltage across diode
Voltage across diode
Diode current
A DC Power Electronics
and the diode current
.
: _____
: _____
15
Exercise 1 – The Diode and Switching Transistor  Procedure
19. Do the measured voltage and current indicate that the diode acts as a closed
switch or as an open switch?
20. Is the diode forward biased or reverse biased in this operating condition?
21. Close the Metering window and stop the voltage source.
Switching Transistor
In this part of the exercise, you will observe the operation of a switching
transistor. You will begin by switching one transistor manually, and then by using
switching control signals produced by the Data Acquisition and Control Interface.
You will also familiarize yourself with the frequency and duty cycle.
Setup and connection
22. Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface.
23. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable. This connection
provides the signals that control the switching transistors in the
IGBT Chopper/Inverter. The switching control signals are generated in the
DACI under the supervision of the Chopper/Inverter Control function of
LVDAC-EMS.
Connect Switching Control Input 1 of the IGBT Chopper/Inverter to Analog
Input 1 of the Data Acquisition and Control Interface using a miniature
(2 mm) banana plug lead. This connection allows observation of the control
signal applied to switching transistor . Note that the numbering (1 to 6) of
the Switching Control Inputs on the IGBT Chopper/Inverter matches the
to
) of the transistors in this module. Consequently, the
numbering (
signal applied to Switching Control Input 1 controls transistor , the signal
applied to Switching Control Input 2 controls transistor , and so on.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
16
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Procedure
24. Set up the circuit shown in Figure 18. Use transistor
in the
(shown as
IGBT Chopper/Inverter to implement switching transistor
in Figure 18). In this circuit, the component
is a
electronic switch
capacitor that is used to prevent high voltage spikes from being induced
when the transistor switches on. The use of capacitors is explained in more
detail later in this manual. Use the 210 µF capacitor of the Filtering
Inductors/Capacitors to implement
and make sure that the (+) terminal of
the capacitor is connected as shown in Figure 18. Notice that many
components included in the IGBT Chopper/Inverter are not shown in
Figure 17 because they are not used in the circuit.
Chopper/Inverter
25 V
210 µf
57 Ω
Switching control signals
from digital outputs
on DACI
Figure 18. Circuit used to observe the operation of a switching transistor.
25. Make sure that the Resistive Load is still set to obtain the resistance value
required.
Static control of a switching transistor
In this part of the exercise, you will control the state (on or off) of transistor
manually by changing the level (voltage) of the corresponding switching control
signal.
26. In the Four-Quadrant Dynamometer/Power Supply window, make the
following settings:
A DC Power Electronics

Make sure that the Function parameter is set to Voltage Source (+).

Make sure that the Voltage parameter is set to 25 V.

Start the voltage source.
17
Exercise 1 – The Diode and Switching Transistor  Procedure
27. In LVDAC-EMS, open the Oscilloscope window and make the following
settings:

Display the output voltage (input E1), the output current (input I1),
and the switching control signal (AI-1) on channels 1, 2, and 3
respectively.

Select the Continuous Refresh mode by clicking on the Continuous
Refresh button. The Continuous Refresh mode enables updated
data to be seen on screen at all times.

Set the sensitivity of Channel 1 to 10 V/div.

Set the sensitivity of Channel 2 to 0.2 A/div.

Set the sensitivity of Channel 3 to 2 V/div.

Position the waveforms to facilitate observation.
28. In LVDAC-EMS, open the Chopper/Inverter Control window, and make the
following settings:
18

Set the Function parameter to Buck Chopper (high-side switching).
in the
This setting is required to allow control of transistor
IGBT Chopper/Inverter module. Notice that you do not have to be
familiar with the operation of choppers to perform this exercise.

Leave the Switching Frequency, Duty Cycle Control, Duty Cycle,
Acceleration Time, and Deceleration Time parameters to their default
values. These parameters are not functional when the Q parameter
is set to On or Off.

Start the buck chopper, by clicking on the Start/Stop button or by
setting the Status parameter to Started.

Set the Q parameter to On while observing the Oscilloscope. This
on and off by changing the
parameter allows you to turn transistor
to switch
level (voltage) of its control signal. This causes transistor
on when the buck chopper is started.

is an example of what the Oscilloscope should display.
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Procedure
Oscilloscope Settings
Channel-1 Input .............................. E1
Channel-1 Scale ..................... 10 V/div
Channel-1 Coupling ........................DC
Channel-2 Input ................................ I1
Channel-2 Scale .................... 0.2 A/div
Channel-2 Coupling ........................DC
Channel-3 Input ............................ AI-1
Channel-3 Scale ....................... 2 V/div
Channel-3 Coupling ........................DC
Time Base .............................. 5 ms/div
Figure 19. Example of what the Oscilloscope should display when switching transistor
is on.
29. Record the level (voltage) of switching control signal
Level (voltage) of switching control signal
.
: _____
30. Is there current flowing in the circuit and a voltage across resistor
 Yes
?
 No
31. What does this indicate about the state of switching transistor
?
32. In the Chopper/Inverter Control window, set the Q parameter to Off while
observing the Oscilloscope. Record the level (voltage) of switching control
signal .
Level (voltage) of switching control signal
: _____
33. Is there current flowing in the circuit and a voltage across resistor
 Yes
A DC Power Electronics
?
 No
19
Exercise 1 – The Diode and Switching Transistor  Procedure
34. What does this indicate about the state of switching transistor
?
35. Toggle the level (voltage) of the switching control signal of transistor
between high and low a few times by toggling the Q parameter in the
Chopper/Inverter Control window between On and Off.
How do the resistor voltage and current vary when the level (voltage) of
is toggled between low and high?
switching control signal
36. Which parameter represents the number of times switching control signal
is set to on and off during one second?
37. Stop the buck chopper by clicking on the Start/Stop button or by setting the
Status parameter to Stopped.
Dynamic control of a switching transistor
In this part of the exercise, you will control the state of switching transistor
using PWM switching control signals.
38. In the Chopper/Inverter Control window of LVDAC-EMS, make the following
settings:
20

Make sure that the Function parameter is set to Buck Chopper (highside switching).

Set the Switching Frequency parameter to 1000 Hz by entering 1000
in the field next to the parameter name or by using the Switching
Frequency (Hz) control knob at the bottom of the Chopper/Inverter
Control window. This sets the frequency of switching control
to 1000 Hz.
signal

Set the Duty Cycle Control parameter to Knob. This allows the value
of the command used in the buck chopper to be set using the Duty
Cycle Control parameter or the Duty Cycle control knob at the
bottom of the Chopper/Inverter Control window.

Set the Duty Cycle parameter to 50%. This sets the duty cycle of
to 50%.
switching control signal
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Procedure

Make sure that the Acceleration Time parameter is set to 0.0 s. This
parameter usually applies to motor control. Thus, the Acceleration
Time sets the time a motor at rest takes to attain full speed, when the
load of the chopper is a motor. This parameter prevents abrupt
changes in system operation.

Make sure that the Deceleration Time parameter is set to 0.0 s. This
parameter usually applies to motor control. The Deceleration Time
sets the time a motor running at full speed takes to stop rotating.
This parameter prevents abrupt changes in system operation.

Set the Q parameter to PWM. This enables control of electronic
using a pulse width modulated PWM switching control
switch
signal.

Start the buck chopper.
39. In the Oscilloscope, make the following settings:

Set the time base to 0.2 ms in order to display two complete cycles
of the switching control signal (AI-1).

Set the trigger controls so that the Oscilloscope triggers when the
rising edge of the switching control signal (AI-1) observed using
Channel 3 reaches 2 V.

Do not modify the other parameters set.
40. Describe the on/off sequence of the switching control signal displayed on the
Oscilloscope.
41. How do the voltage (input E1) and current (input I1) vary with respect to the
successive changes of state in switching control signal ?
Frequency and duty cycle
In this part of the exercise, you will calculate the period and the frequency of a
signal from the time base setting and the waveform displayed on the
Oscilloscope.
A DC Power Electronics
21
Exercise 1 – The Diode and Switching Transistor  Procedure
42. In the Chopper/Inverter Control window, set the Switching Frequency
parameter to 400 Hz. Do not modify the setting of the other parameters in
this window.
43. In the Oscilloscope window, change the time base to display exactly two
complete cycles of the switching control signal. Record the time base setting
and the number of horizontal divisions occupied by the waveform of the
switching control signal during one complete cycle on the Oscilloscope.
Time base setting: _____
Number of divisions occupied by the waveform during one cycle: _____
44. Determine the period of one complete cycle using the time base setting,
the number of divisions occupied by the waveform during one complete cycle
on the Oscilloscope, and the following equation.
Period
= Time base × number of divisions _____
45. Determine the frequency of the switching control signal using the period
calculated in step 44 and the following equation:
= 1/ = _____
46. Does the frequency calculated in the previous step correspond to the value
set in the Chopper/Inverter Control window and shown in the column "f (Hz)"
of the Oscilloscope?
 Yes
 No
Average value
In this part of the exercise, you will measure the average value of the voltage
using the Oscilloscope and the Metering applications of
across resistor
LVDAC-EMS.
shown in the
47. Record the average value of the voltage across resistor
column AVG located under the Oscilloscope display, in the appropriate cell of
Table 1.
22
DC Power Electronics A
Exercise 1 – The Diode and Switching Transistor  Procedure
Table 1. Average voltage across resistor
Method
obtained using different methods.
Average voltage across resistor
(V)
Measured using the Oscilloscope application
Calculated
Measured using the Metering application
Note: Input voltage = 25 V, duty cycle = 50%
48. Calculate the average output voltage across resistor
by determining the
maximum (peak) value of the voltage waveform and multiplying this value by
the duty cycle (proportion of the on-time to the period of the voltage
waveform measured using the Oscilloscope). Record the average voltage
you calculated in the appropriate cell of Table 1.
Maximum (peak) voltage: _____
On-time: _____
Period : _____
Duty cycle : _____
Average voltage: _____
49. Do your results confirm that the average value of a rectangular voltage
times the duty cycle ?
waveform corresponds to the maximum voltage
 Yes
 No
50. Open the Metering application of LVDAC-EMS. Make sure that meter E1 is
enabled, then select the DC mode by clicking on the AC/DC button at the
bottom of the meter. Click on the Refresh button and record the average (dc)
displayed by meter E1 in the appropriate cell of
voltage across resistor
Table 1.
51. Do your results confirm that the dc value and the average value of a
rectangular voltage waveform are the same?
 Yes
 No
52. Stop the buck chopper and the voltage source.
Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
A DC Power Electronics
23
Exercise 1 – The Diode and Switching Transistor  Conclusion
CONCLUSION
In this exercise, you learned how a diode operates. You saw that when the
voltage at the anode is higher than the voltage at the cathode, the diode acts as
a closed switch. You also saw that when the voltage at the anode is lower than
the voltage at the cathode, the diode acts as an open switch.
You were introduced to the switching transistor. You saw that a transistor used
as a switch is either off (open circuit) or on (fully conductive). You saw that
without a voltage applied to the gate of a MOSFET or IGBT, the switching
transistor is off. Conversely, when a voltage is applied to the gate, the switching
transistor is on. You learned that the power dissipated by a switching transistor is
very low.
You were also introduced to the following new terms: cycle, period, frequency,
duty cycle, and pulse width modulation. You learned that the time required for
one complete cycle to occur is called the period, and that the number of cycles
that occur in one second is called the frequency. You learned that the duty cycle
corresponds to the proportion of time a signal is at high level during one cycle.
You saw that the average or mean value (AVG) of a current or voltage waveform
corresponds to the height of a rectangle of equal area as the waveform of a
signal during one period, and that this also corresponds to the dc value of the
signal waveform.
REVIEW QUESTIONS
1. Is the voltage at the anode lower or higher than the voltage at the cathode
when a forward voltage is applied across the diode?
2. Explain why the power dissipated by a switching transistor is always small.
3. Determine the frequency of a waveform having a period 0.2 ms.
4. Determine the duty cycle of a switching control signal which is at high level
during 0.9 ms of a cycle having a period of 3.0 ms.
5. Determine the average value of a rectangular pulse wave having a maximum
(peak) voltage of 100 V and a duty cycle of 80%.
24
DC Power Electronics A
Exercise
2
The Buck Chopper
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the operation of
the buck chopper.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:



DISCUSSION
The buck chopper
Operation of a buck chopper with a resistive load
High-side versus low-side switching
The buck chopper
Many electric devices that are powered by batteries contain sub-circuits that
require a voltage level different than that supplied by the battery or the external
supply (sometimes higher or lower than the supply voltage, and possibly even of
opposite polarity).
The chopper, also called dc-to-dc converter, is a power electronic circuit which
converts the voltage of a dc source from one value to another. When the initial
voltage is converted to a higher voltage, the chopper is called a boost chopper;
conversely, when the initial voltage is converted to a lower voltage, it is called a
buck chopper.
Choppers use electronic switches to convert dc voltages and currents from high
to low levels and vice versa. The electronic switches can be designed with
IGBTs (insulated gate bipolar transistors), MOSFETs (metal-oxide semiconductor
field-effect transistors), etc. Figure 20 shows a buck chopper built with electronic
switch , and some waveforms related to this circuit.
A DC Power Electronics
25
Exercise 2 – The Buck Chopper  Discussion
Resistive
load
Switching control
signal generator
Switching
control
signal
on
off
Amplitude
on
Time
Output
voltage
( )
Amplitude
on
Output
current
( )
Amplitude
Time
Time
Figure 20. Operation of a buck chopper with a resistive load.
Operation of a buck chopper with a resistive load
When the electronic switch
shown in Figure 20 switches on, the dc power
is applied to the resistive load and current flows through the
supply voltage
load. When the electronic switch switches off, the dc power supply voltage is
no longer applied to the load, and the current stops flowing through the load.
is
The dc voltage (i.e., the average voltage) at the buck chopper output
and the time that
proportional to the voltage at the buck chopper input
electronic switch is on during each cycle. This time, which is referred to as the
, is in turn proportional to the duty cycle of the switching control
on-time
signal applied to the control signal input of electronic switch .
26
DC Power Electronics A
Exercise 2 – The Buck Chopper  Discussion
The equation relating voltages
and
is given by the expression:
(4)
where
is the dc (average) voltage at the buck chopper output.
is the dc (average) voltage applied at the buck chopper input.
is the duty cycle expressed as a decimal (50% = 0.5).
can be varied by varying the duty cycle . Since the duty
Thus, voltage
cycle can vary between 0% and 100%, voltage
can vary between 0 and .
A low duty cycle corresponds to a low output voltage , because the voltage at
the buck chopper output is zero most of the time. Pulse-width modulation (PWM)
of the switching control signals is a very efficient way of providing intermediate
amounts of electrical power between 0 and maximum.
Note that varying the frequency of the switching control signal while maintaining
the duty cycle constant does not vary the average value of the output voltage
and current at the buck chopper output.
High‐side versus low‐side switching
Figure 21 shows two buck choppers implemented with electronic switch placed
and the load. When the
at two different locations relative to the power supply
electronic switch is placed on the high-voltage side of the power supply as shown
in Figure 21a, the configuration is called high-side switching. Conversely, when
the electronic switch is placed on the low-voltage side of the power supply as
shown in Figure 21b, the configuration is called low-side switching.
Load
(a) High-side switching
Load
(b) Low-side switching
Figure 21. High-side and low-side switching configurations.
Although both configurations produce the same voltage and same current in the
load, the low-side switching configuration is often preferred to the high-side
switching configuration because it is easier to implement and requires fewer
electronic components. In fact, for current to flow through the switching transistor,
a low voltage (typically 5 V) must be applied between the gate and emitter of the
switching transistor. The voltage at the gate must be higher than the voltage at
the emitter of the switching transistor for current to flow through the transistor.
Figure 22b shows a typical case of a switching transistor in the low-side
switching configuration using an NPN type transistor.
A DC Power Electronics
27
Exercise 2 – The Buck Chopper  Procedure Outline
In order to maintain a positive voltage of 5 V across the gate and emitter of a
switching transistor in a high-side switching configuration, the gate voltage must
be equal to the voltage (47 V) across the load when the transistor is on plus
about 5 V. This corresponds to a voltage of approximately 52 V in the example
shown in Figure 22a. In comparison, a gate voltage of only 5 V is required to turn
on the transistor in a low-side switching configuration as shown in Figure 22b.
5 V + 47 V
G
C
≈ 47 V
E
48 V
48 V
G
≈ 47 V
C
5V
E
(a) High-side switching
(b) Low-side switching
Figure 22. Voltage required at the gate in high-side and low-side switching configurations.
The high-side switching configuration is generally used in applications where one
terminal of the load must be at the same voltage level as the common of the
power supply (negative terminal of the power supply). Since the load is not
connected to the common of the power supply in the low-side switching
configuration, the load is said to be floating.
PROCEDURE OUTLINE
The Procedure is divided into the following sections:




Setup and connections
Output voltage versus duty cycle
Output voltage versus switching frequency
Low-side and high-side switching
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Install the required equipment in the Workstation.
28
DC Power Electronics A
Exercise 2 – The Buck Chopper  Procedure
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to a USB
port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply. This connects the internal power supply of the
module to the Power Supply terminals on the front panel.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
Connect Switching Control Input 1 of the IGBT Chopper/Inverter to Analog
Input 1 of the Data Acquisition and Control Interface using a miniature
banana plug lead.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
A DC Power Electronics
29
Exercise 2 – The Buck Chopper  Procedure
7. Set up the circuit shown in Figure 23. Notice the presence of capacitor
in the Chopper/Inverter. Capacitor
is used to filter the
and diode
dc voltage at the buck chopper input. The use of capacitors and diodes in
chopper circuits is explained in more detail in the next exercise. Notice that
many components included in the Chopper/Inverter are not shown in
Figure 23 because they are not used in the circuit.
Chopper/Inverter
+
57 Ω
50 V
Switching control signals
from digital outputs
on DACI
Figure 23. Buck chopper circuit with resistive load (high-side switching).
8. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance value required.
Output voltage versus duty cycle
In this part of the exercise, you will vary the duty cycle of the switching control
signal to vary the time the electronic switch is on while observing the voltage at
the buck chopper output. This will allow you to verify the relationship between the
duty cycle and the voltage conversion performed by the buck chopper.
9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:

Select the Voltage Source (+) function.

Set the voltage to 50 V.

Start the voltage source.
10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:

30
Make sure that the Function parameter is set to Buck Chopper (highside switching). This setting allows the Data Acquisition and Control
DC Power Electronics A
Exercise 2 – The Buck Chopper  Procedure
Interface to generate the switching control signal required by a buck
chopper implemented with an electronic switch located on the highvoltage side of the power supply.

Set the Switching Frequency parameter to 2000 Hz. This sets the
frequency of the switching control signal to 2000 Hz.

Set the Duty Cycle Control parameter to Knob.

Set the Duty Cycle parameter to 50%. This sets the duty cycle of the
switching control signal to 50 %.

Set the Acceleration Time parameter to 0 s.

Set the Deceleration Time parameter to 0 s.

Make sure that the Q parameter is set to PWM.

Notice that the Status parameter is set to Stopped. This indicates
that the buck chopper is disabled.
11. For each duty cycle shown in Table 2, calculate the average output
of the buck chopper using an input voltage
equal to 50 V and
voltage
. Record your results in the column "Calculated" of
the equation
Table 2.
Table 2. Average output voltage versus duty cycle (high-side switching).
DC input voltage
(V)
Duty cycle
(%)
Average output voltage
(V)
Calculated
Measured
0
20
50
40
60
80
100
Note: switching frequency = 2000 Hz
12. In the Chopper/Inverter Control window, start the buck chopper by clicking on
the Start/Stop button (or by setting the Status parameter to Started).
13. In LVDAC-EMS, open the Oscilloscope window and display the following
parameters: voltage (input E1) at the buck chopper input, switching control
(input E2),
signal (AI-1), average output voltage measured across resistor
(input I2).
and the output current flowing through resistor
A DC Power Electronics
31
Exercise 2 – The Buck Chopper  Procedure
Select the Continuous Refresh mode, then set the time base to display at
least two complete cycles, and set the trigger controls so that the
Oscilloscope triggers when the rising edge of the switching control
signal (AI-1) reaches 2 V.
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation. Figure 24 is an example of what the Oscilloscope
should display.
Oscilloscope Settings
Channel-1 Input .............................. E1
Channel-1 Scale ..................... 50 V/div
Channel-1 Coupling ........................DC
Channel-2 Input ............................ AI-1
Channel-2 Scale ....................... 5 V/div
Channel-2 Coupling ........................DC
Channel-3 Input ............................. E-2
Channel-3 Scale ..................... 20 V/div
Channel-3 Coupling ........................DC
Channel-4 Input ...............................I-2
Channel-4 Scale .................... 0.5 A/div
Channel-4 Coupling ........................DC
Time Base ........................... 0.1 ms/div
Trigger Source .............................. Ch2
Trigger Level .................................. 2 V
Trigger Slope ............................. Rising
Figure 24. Waveforms at the input and output of the buck chopper (high-side configuration).
14. For each duty cycle value shown in Table 2, measure the average output
across resistor
using input E2 of the Data Acquisition and
voltage
Control Interface. The average values of the observed waveforms are
indicated in the column "AVG" located under the Oscilloscope display.
Record your results in the column "Measured" of Table 2.
15. Compare the calculated voltages with the measured voltages. Are they
approximately the same? Notice that a slight difference is normal due to the
voltage drop across the switching transistor.
 Yes
 No
16. Do your measurement results confirm that the average output voltage
is
)?
proportional to the duty cycle of the switching control signal (
 Yes
32
 No
DC Power Electronics A
Exercise 2 – The Buck Chopper  Procedure
Output voltage versus switching frequency
In this part of the exercise, you will vary the switching frequency while observing
the voltage at the buck chopper output. This will allow you to verify the
relationship between the switching frequency and the voltage conversion
performed by the buck chopper.
17. In the Chopper/Inverter Control window, set the duty cycle to 50%. This sets
of the buck chopper to about 25 V.
the average output voltage
For each switching frequency shown in Table 3, measure the average output
across resistor
using input E2. The average output voltage
voltage
value is indicated in the column "AVG" located under the
Oscilloscope display. Record your results in Table 3.
Table 3. Average output voltage versus switching frequency (high-side switching).
Switching frequency
(Hz)
DC input voltage
(V)
Average output voltage
(V)
500(1)
1000(1)
50
5000(1)
10 000(1)
(1)
Change the time base setting in the Oscilloscope as required to display at least two
complete cycles
(2)
Duty cycle = 50%
18. Does the buck chopper output voltage vary with the switching frequency?
 Yes
 No
19. Do your results confirm that the switching frequency has no effect on the
output voltage of the buck chopper?
 Yes
 No
20. Stop the buck chopper and the voltage source.
Low‐side and high‐side switching
In this part of the exercise, you will change the position of the electronic switch in
the buck chopper to obtain a low-side switching configuration. Then you will
verify if the buck chopper still converts the voltage in the same way as with the
high-side configuration.
A DC Power Electronics
33
Exercise 2 – The Buck Chopper  Procedure
21. Modify your circuit as shown in Figure 25 to set up a low-side switching
configuration.
Disconnect Switching Control Input 1 of the Chopper/Inverter from Analog
Input 1 of the Data Acquisition and Control Interface. Connect Switching
Control Input 4 of the Chopper/Inverter to Analog Input 1 of the Data
Acquisition and Control Interface.
Chopper/Inverter
+
50 V
57 Ω
Switching control signals
from digital outputs
on DACI
Figure 25. Buck chopper circuit with resistive load (low-side switching).
22. In the Chopper/Inverter Control window, make the following settings:

Set the function to Buck Chopper (low-side switching). This setting
allows the Data Acquisition and Control Interface to generate the
switching control signal required by a buck chopper implemented
with an electronic switch located on the low-voltage side of the power
supply.

Set the switching frequency to 2000 Hz.

Set the duty cycle to 0%.

Make sure that the acceleration time is set to 0 s.

Make sure that the deceleration time is set to 0 s.

Set the Q parameter to PWM.

Start the buck chopper.
23. In the Four-Quadrant Dynamometer/Power Supply window, start the voltage
source.
34
DC Power Electronics A
Exercise 2 – The Buck Chopper  Conclusion
24. For each duty cycle value shown in Table 4, measure the average output
across resistor
using input E2 of the of the Data Acquisition
voltage
and Control Interface. Record your results in Table 4.
Table 4. Average output voltage versus duty cycle (low-side switching).
DC input voltage
(V)
Duty cycle
(%)
Average output voltage
(V)
0
20
40
50
60
80
100
25. Compare the average output voltages measured in the low-side
configuration (Table 4)
with
those
measured
in
the
high-side
configuration (Table 2). Are they approximately the same?
 Yes
 No
26. Do your results confirm that the switching transistor can be located on either
side of the voltage source without affecting the voltage conversion performed
by the buck chopper?
 Yes
 No
27. Stop the buck chopper and the voltage source.
Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
CONCLUSION
In this exercise, you learned that the average voltage at the output of a buck
chopper is proportional to the duty cycle. You learned that the maximum average
voltage that can be obtained at the buck chopper output is slightly lower than the
voltage at its input. You observed that the switching frequency has no effect on
the voltage conversion performed by the buck chopper.
You also observed that the electronic switch can be located on the low-voltage
side or the high-voltage side of the power supply without affecting the voltage
conversion taking place in the buck chopper.
A DC Power Electronics
35
Exercise 2 – The Buck Chopper  Review Questions
REVIEW QUESTIONS
1. A buck chopper is powered by a 300 V dc power supply. What is the output
voltage range of this chopper if the duty cycle can vary between 5%
and 95%?
2. Give a brief description of a chopper.
3. What differentiates a buck chopper from a boost chopper?
4. How does the frequency of the switching control signal affect the average
output voltage provided by a buck chopper?
5. Explain why the low-side switching configuration is often preferred to the
high-side switching configuration.
36
DC Power Electronics A
Exercise
3
Introduction to High‐Speed Power Switching
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the concept of
voltage-type and current-type circuits. You will also be familiar with the use of
free-wheeling diodes.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:






DISCUSSION
High-speed power switching circuits
Voltage-type circuit
Current-type circuit
Free-wheeling diodes
Power efficiency
Interconnecting voltage-type and current-type circuits
High‐speed power switching circuits
In high-speed power switching circuits, electronic switches are opened and
closed rapidly to transfer electric power from one circuit to another with minimal
power dissipation. In many cases, some characteristics of the electric power,
such as the voltage, current, and frequency are modified.
A solid-state converter is a
converter whose operation
depends on the control of
electrical or magnetic phenomena in solids, such as a
transistor, crystal diode, or
ferrite device.
Basically, electronic switches open, close, short-circuit, or interconnect circuits to
modify the voltages and currents related to these circuits. In power electronics,
however, the characteristics of a circuit must be taken into account before
opening or short-circuiting a circuit, or before interconnecting a circuit to another,
to avoid damaging the circuits. Two types of circuits are distinguished in solidstate converters: voltage-type and current-type circuits.
Voltage‐type circuit
A voltage-type circuit opposes voltage variations but not current variations. The
voltage across a voltage-type circuit tends to remain constant. Batteries,
capacitors, and dc voltage sources are examples of voltage-type circuits. A very
high current flows through a voltage-type circuit when it is short-circuited.
Therefore, a voltage-type circuit should never be short-circuited because the high
current which results could damage the circuit or trigger an overcurrent protection
circuit.
Because capacitors oppose voltage variations, they are widely used in electronic
circuits to smooth the output voltage of power supplies, this is why they are often
called smoothing or filtering capacitors. The ability of a capacitor to oppose
voltage variations is called capacitance, and is measured in units of farad. The
larger the capacitor, the larger the capacitance and the opposition to voltage
variations.
A DC Power Electronics
37
Exercise 3 – Introduction to High-Speed Power Switching  Discussion
Electrical symbol of a capacitor
In electrical circuit schematics, capacitors are identified with the letter and
represented by the symbol shown in the left margin. Figure 26 shows various
types of capacitors. The plus (+) sign that is sometimes included in the symbol
indicates that the capacitor is polarized. Polarized capacitors must be used in
such a way that their positive terminal is connected to the positive side (high
side) of a circuit.
Figure 26. Various types of capacitors.
Current‐type circuit
A current-type circuit opposes current variations but not voltage variations. The
current flowing in a current-type circuit tends to remain constant. Inductors,
motors, and current sources are examples of current-type circuits. A very high
voltage develops across a current-type circuit when it is opened while a current is
flowing through the circuit. Therefore, a current-type circuit should never be
opened when current is flowing through the circuit, because the high voltage
which results could damage the circuit or trigger an overvoltage protection circuit.
Because inductors oppose current variations, they are widely used in electronic
circuits to smooth the current, this is why they are often called smoothing or
filtering inductors. The ability of an inductor to oppose current variations is called
inductance, and is measured in units of Henry. The larger the inductor, the
larger the inductance and the opposition to current variations.
Electrical symbol of an inductor.
38
In electrical circuit schematics, inductors are identified with the letter and
represented by the symbol shown in the left margin. Figure 27 shows various
types of inductors.
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Discussion
Figure 27. Various types of inductors.
Free‐wheeling diodes
In order to prevent fast current variations in a current-type circuit, and thereby
prevent high voltages from being induced across it, a diode can be connected in
parallel with the current-type circuit as shown in Figure 28. This diode is usually
referred to as a free-wheeling diode. In the circuit of Figure 28, an electronic
switch
is used to connect a voltage source (a voltage-type circuit) to a
current-type circuit consisting of inductor . If diode
were removed, a high
voltage would develop across inductor when the switch opens, that is, when
electronic switch turns off.
Note that the circuit shown
in Figure 28 is in fact a buck
chopper. The free-wheeling
diode is considered as part
of the buck chopper.
Voltage-type circuit
Current-type circuit
Free-wheeling diode
Figure 28. Free-wheeling diode connected in parallel with a current-type circuit.
Such a high voltage could destroy electronic switch . However, no high voltage
is allowed to develop across inductor because of the presence of diode in
parallel with inductor , as is explained below.
A DC Power Electronics
39
Exercise 3 – Introduction to High-Speed Power Switching  Discussion
When electronic switch turns on, voltage is applied to inductor and the
current in it gradually builds up. Since diode is reverse-biased, it is blocked and
no current flows through it. When electronic switch turns off, voltage source
is disconnected from inductor and the current begins to decrease. The inductor
across the inductor
reacts to the current variation and this causes the voltage
to decrease extremely rapidly until its polarity reverses and its value reaches
approximately -0.7 V (consequently, the voltage across electronic switch
increases very rapidly until it reaches approximately voltage + 0.7 V). As a
result, a forward-bias voltage of approximately 0.7 V appears across diode .
Consequently, diode
turns on and the current continues to flow through
inductor , returning now through diode , while gradually decreasing. The rate
of decrease depends upon the
constant of inductor , where
is the
internal resistance of inductor . When electronic switch turns on again, the
current in inductor builds up to its previous value, and the cycle repeats.
Power efficiency
The power which a chopper (e.g., a buck chopper) delivers at its output
is
minus the power dissipated in the
equal to the power it receives at its input
electronic switch. The power dissipated in the electronic switch is small
compared to the output power . The power efficiency of choppers, thus, often
exceeds 90% and can even approach 100%. Notice that the power efficiency is
the ratio of the output power on the input power times 100%, as stated in
Equation (5).
Power efficiency =
where
100%
(5)
is the power the chopper delivers.
is the power the chopper receives.
Interconnecting voltage‐type and current‐type circuits
When two batteries having the same polarity but different voltages are connected
in parallel, the battery having the higher voltage discharges into the other until
the voltages of the two batteries are equal. The current flowing in the batteries
reaches a very high value since it is only limited by the internal resistance of the
batteries. Obviously, this could damage the batteries.
A similar phenomenon occurs when two charged capacitors are connected in
parallel. One capacitor discharges into the other until the voltage across the
capacitors is the same. Since the internal resistance of capacitors is very low, a
high current flows momentarily in the capacitors. If an electronic switch is used to
connect these two capacitors, it could be damaged by the high current peak
which flows every time the electronic switch closes.
These two examples show that it is not recommended to directly connect two
voltage-type circuits in parallel, unless their voltages and polarities are identical.
40
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Procedure Outline
A similar reasoning applies to current-type circuits. They should not be directly
connected together unless the current in both circuits is equal and flows in the
same direction. In effect, when two current-type circuits are connected together,
a high voltage develops across each circuit when the current in these circuits is
not the same. If the circuit contains electronic switches, these could be damaged
by the high voltages that are produced.
However, a voltage-type circuit can be connected to a current-type circuit without
creating any problems. For example, a voltage-type source can be connected to
an inductor by means of an electronic switch which opens and closes at a high
rate as long as a free-wheeling diode is connected in parallel with the inductor.
This corresponds to the circuit shown in Figure 28.
When electronic switch turns on, the current drawn from voltage-type source
passes rapidly from zero to the value of the current which was flowing in
inductor just before electronic switch turned off. Simultaneously, voltage
across inductor passes rapidly from nearly zero (i.e., approximately -0.7 V) to
voltage . This causes no problems because voltage-type source does not
oppose current variations and inductor does not oppose voltage variations.
When electronic switch
turns off, the current drawn from the voltage-type
source
decreases rapidly to zero. At the same time, the voltage across
inductor passes rapidly from voltage to nearly zero (more precisely -0.7 V),
thereby applying a forward-bias voltage across diode
which enters into
conduction. Again, this causes no problem because the voltage-type source
does not oppose current variations and inductor does not oppose voltage
variations.
PROCEDURE OUTLINE
The Procedure is divided into the following sections:






Setup and connections
Voltage-type circuit
Connecting a voltage-type circuit to a current-type circuit via an
electronic switch without a free-wheeling diode
Connecting a voltage-type circuit to a current-type circuit via an
electronic switch with a free-wheeling diode
Power efficiency
Interconnecting two voltage-type circuits via an electronic switch
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Install the required equipment in the Workstation.
A DC Power Electronics
41
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to a USB
port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
Connect Switching Control Input 1 of the IGBT Chopper/Inverter to Analog
Input 1 of the Data Acquisition and Control Interface using a miniature
banana plug lead.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
42
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
7. Set up the circuit shown in Figure 29. Use the 210 μF capacitor in the
Filtering Inductors/Capacitors module to implement . Notice that for this
replaces capacitor
of the
part of the exercise, capacitor
form a
Chopper/Inverter. In this circuit, the voltage source and capacitor
voltage-type circuit.
Chopper/Inverter
Voltage-type circuit
25 V
210 µF
86 Ω
Switching control signals
from digital outputs
on DACI
Figure 29. Voltage-type circuit connected to a resistive load via an electronic switch.
8. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance value required.
Voltage‐type circuit
In this part of the exercise, you will observe the behavior of a voltage-type circuit
supplying a resistive load via an electronic switch.
9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:

Select the Voltage Source (+) function.

Set the voltage to 25 V.

Start the voltage source.
10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:
A DC Power Electronics

Make sure that the Buck Chopper (high-side switching) function is
selected.

Set the switching frequency to 500 Hz.

Set the Duty Cycle Control parameter to Knob.

Set the duty cycle to 50%.
43
Exercise 3 – Introduction to High-Speed Power Switching  Procedure

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.

Make sure that the

Start the buck chopper.
parameter is set to PWM.
11. In LVDAC-EMS, open the Oscilloscope window and display the voltage and
current (inputs E1 and I1) at the buck chopper input, the switching control
(input E2), and the output
signal (AI-1), the output voltage across resistor
(input I2).
current flowing through resistor
Select the Continuous Refresh mode, set the time base to display at least
two complete cycles, and set the trigger controls so that the Oscilloscope
triggers when the rising edge of the switching control signal (AI-1)
reaches 2 V.
is an example of what the Oscilloscope should display.
Oscilloscope settings
Channel-1 Input .............................. E1
Channel-1 Scale ..................... 20 V/div
Channel-1 Coupling ........................DC
Channel-2 Input ...............................I-1
Channel-2 Scale .................... 0.5 A/div
Channel-2 Coupling ........................DC
Channel-3 Input ............................ AI-1
Channel-3 Scale ....................... 5 V/div
Channel-3 Coupling ........................DC
Channel-4 Input ............................. E-2
Channel-4 Scale ..................... 20 V/div
Channel-4 Coupling ........................DC
Channel-5 Input ...............................I-2
Channel-5 Scale .................... 0.5 A/div
Channel-5 Coupling ........................DC
Time Base ........................... 0.5 ms/div
Trigger Source .............................. Ch3
Trigger Level .................................. 2 V
Trigger Slope ............................. Rising
Figure 30. Waveforms at the input and output of a buck chopper connected to a resistive load.
12. Observe the waveforms displayed on the Oscilloscope. Are the voltage and
current (inputs E1 and I1) waveforms at the buck chopper input in
accordance with the characteristics of a voltage-type circuit? Explain.
44
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
13. Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
14. Stop the buck chopper and the voltage source.
Connecting a voltage‐type circuit to a current‐type circuit via an
electronic switch without a free‐wheeling diode
In this part of the exercise, you will modify your circuit by adding inductor
in
as shown in Figure 31 to observe what happens when a
series with resistor
voltage-type circuit is connected to a current-type circuit via an electronic switch
without a free-wheeling diode.
in series with resistor
as shown
15. Modify your circuit to connect inductor
in Figure 31. Use the 50 mH inductor in the Filtering Inductors/Capacitors
limits the current in
module to implement . In this circuit, resistor
and prevents excessively high voltages from developing in the
inductor
and inductor
form a current-type circuit.
circuit. Resistor
Do not modify the values of resistor
(86 Ω) and capacitor
Chopper/Inverter
Voltage-type circuit
(210 μF).
Current-type circuit
50 mH
25 V
210 µF
86 Ω
Switching control signals
from digital outputs
on DACI
Figure 31. Voltage-type circuit connected to a current-type circuit via an electronic switch
without a free-wheeling diode.
A DC Power Electronics
45
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
16. Start the buck chopper and the voltage source.
Wait two minutes for the voltage at the buck chopper output to stabilize, then
select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation.
Describe what happens to the voltage across the current-type
turns off (when the
circuit (input E2) at the instant when electronic switch
current stops flowing).
17. Determine the value of the voltage which appears across the current-type
turns off.
circuit at the instant when electronic switch
18. What can be done to prevent a large voltage from being induced across the
turns off?
current-type circuit when electronic switch
19. Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
20. Stop the buck chopper and the voltage source. Do not modify the other
settings in LVDAC-EMS.
Connecting a voltage‐type circuit to a current‐type circuit via an
electronic switch with a free‐wheeling diode
In this part of the exercise, you will add a free-wheeling diode in parallel with the
current-type circuit and observe its effect on the voltage induced across the
turns off.
current-type circuit when electronic switch
46
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
21. Modify your circuit to add diode
in parallel with the current-type circuit as
has been removed from the
shown in Figure 32. Notice that capacitor
, which is part of
previous circuit (Figure 31). It is replaced by capacitor
is automatically inserted
the IGBT Chopper/Inverter module. Capacitor
is added to the circuit, and is used to smooth the
in the circuit when diode
dc bus voltage at the input of the buck chopper.
Voltage-type circuit
Chopper/Inverter
Current-type
circuit
50 mH
+
25 V
86 Ω
Switching control signals
from digital outputs
on DACI
Figure 32. Voltage-type circuit connected to a current-type circuit via an electronic switch
with a free-wheeling diode (i.e., a buck chopper).
22. Start the buck chopper and the voltage source.
23. On the Oscilloscope, display the voltage (input E1) at the buck chopper input,
the switching control signal (AI-1), the voltage across the current-type
circuit (input E2), and the current flowing through the voltage-type
circuit (input I2).
Make sure that the time base is set to display at least two complete cycles.
Also make sure that the trigger controls are set so that the Oscilloscope
triggers when the rising edge of the switching control signal (AI-1)
reaches 2 V.
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation.
24. Observe the waveforms displayed on the Oscilloscope. Describe the effect
which adding a free-wheeling diode has on the voltage waveform at the buck
chopper output.
A DC Power Electronics
47
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
25. Describe the effect which adding a free-wheeling diode has on the current
waveform at the buck chopper output.
a
Notice that the opposition to current variation produced by inductor
causes
the current to flow without interruptions in the circuit. The time the electronic
switch is turned off is too short to allow the current to decrease down to zero.
26. Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
Power efficiency
In this part of the exercise, you will measure the power at the input and output of
the buck chopper to determine the power efficiency.
27. In the Chopper/Inverter Control window, set the duty cycle to 50%, and the
switching frequency to 1000 Hz.
28. In the Four-Quadrant Dynamometer/Power Supply window, set the source
voltage to 50 V.
29. In the Oscilloscope window, momentarily disable the Continuous Refresh
mode by clicking on the Continuous Refresh button.
30. Open the Metering window and enable the power meters PQS1 (E1,I1)
and PQS2 (E2,I2) to display the input power
and output power
respectively.
Make sure that power meters PQS1 (E1,I1) and PQS2 (E2,I2) show active
power [the letter W (watts) should appear at the bottom of each meter].
Make sure that meters E1, E2, I1, and I2 are enabled, then select the
DC mode for each of these meters by clicking on the AC/DC button at the
bottom of each meter.
Measure the dc voltage, dc current, and power at the input and output of the
buck chopper by clicking on the Single Refresh button in the Metering
window, and record the values in Table 5.
48
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
Table 5. Power at the input and output of the buck chopper.
Buck chopper input
DC voltage
(V)
DC current
(A)
Buck chopper output
Power
(W)
DC voltage
(V)
DC current
(A)
Power
(W)
Duty cycle = 50%, switching frequency = 1000 Hz
31. Determine the power efficiency of the buck chopper using the values in
Table 5 and the following equation.
Power efficiency =
100% = _____
32. Determine the power dissipated in the electronic switch by subtracting the
from the input power .
output power
Power dissipated in the electronic switch: _____
33. Do your results confirm that the power dissipated in the electronic switch is
small compared to the output power?
 Yes
 No
34. Using the values in Table 5, determine the current ratio
ratio ⁄ for a 50% duty cycle.
Current ratio
⁄ ∶ _____
Voltage ratio
⁄ : _____
⁄
and voltage
You should observe that the voltage ratio ⁄ is virtually equal to the duty
cycle whereas the current ratio ⁄ is virtually equal to the reciprocal of
the duty cycle (1/ ).
35. Stop the buck chopper and the voltage source.
Close the Metering window.
A DC Power Electronics
49
Exercise 3 – Introduction to High-Speed Power Switching  Procedure
Interconnecting two voltage‐type circuits via an electronic switch
In this part of the exercise, you will interconnect two voltage-type circuits via a
buck chopper, and observe the current flowing between the two circuits.
36. Set up the circuit shown in Figure 33. Use the 5 μF capacitor in the Filtering
is
Inductors/Capacitors module to implement . In this circuit, resistor
and capacitor form a voltage-type
used as a current limiter. Resistor
circuit.
Voltage-type circuit
Chopper/Inverter
86 Ω
25 V
5 µF
171 Ω
Voltage-type
circuit
Switching control signals
from digital outputs
on DACI
Figure 33. Two voltage-type circuits connected via a buck chopper.
37. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance values required.
38. In the Four-Quadrant Dynamometer/Power Supply window, set the source
voltage to 25 V, then start the voltage source.
39. In the Chopper/Inverter Control window, make sure that the duty cycle is set
to 50%, then set the switching frequency to 500 Hz. Start the buck chopper.
40. In the Oscilloscope, enable the Continuous Refresh mode by clicking on the
Continuous Refresh button. Make sure that the voltage (input E1) at the buck
chopper input, the switching control signal (AI-1), the voltage across
resistor
(input E2), and the current (input I2) flowing through the two
voltage-type circuits are displayed.
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation.
50
DC Power Electronics A
Exercise 3 – Introduction to High-Speed Power Switching  Conclusion
Describe what happens to the voltage waveform across resistor
(i.e., the voltage across the voltage-type circuit at the buck chopper output)
when the switching control signal turns on and off. Explain why.
41. Observe the waveform of the current (input I2) flowing through the two
voltage-type circuit. You should observe a sharp increase in current followed
by a gradual decrease to a lower value at the instant when the electronic
switch closes. Explain why.
42. Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
43. Stop the buck chopper and the voltage source.
Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
CONCLUSION
In this exercise, you learned that a voltage-type circuit opposes voltage
variations, and that a current-type circuit opposes current variations. You learned
that a free-wheeling diode connected in parallel with a current-type circuit
supplied by a chopper prevents high voltages from being induced across the
current-type circuit.
You learned that a voltage-type circuit can be connected to a current-type circuit
without creating any problems, but that two voltage-type circuits or two currenttype circuits, should not be connected together using an electronic switch unless
precautions are taken.
You also learned that the power efficiency of a buck chopper is high and that the
power dissipated in the electronic switch is small compared to the output power.
REVIEW QUESTIONS
A DC Power Electronics
1. What is the main characteristic of a voltage-type circuit?
51
Exercise 3 – Introduction to High-Speed Power Switching  Review Questions
2. Explain why a voltage-type circuit should never be short circuited.
3. Give examples of voltage-type circuits.
4. Explain the role of the free-wheeling diode in a buck chopper.
5. Explain why two circuits of the same type should not be interconnected
directly using an electronic switch.
52
DC Power Electronics A
Exercise
4
Ripple in Choppers
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with ripple in
choppers.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:



Ripple
Ripple versus inductance / capacitance
Ripple versus switching frequency
Ripple
DISCUSSION
Output current
The residual current or voltage variation at the output of a chopper is called
“ripple”. Figure 34 shows a typical current waveform with ripple at a buck chopper
output. Because of ripple, the maximum current value is higher than the average
current value. Without ripple, the maximum current and the average current are
the same.
Max (peak)
Average
Ripple
Time
Figure 34. Current ripple at the output of a buck chopper.
Ripple voltage is commonly expressed as a peak-to-peak value because it is
easy to measure on an oscilloscope and simple to calculate theoretically. Filter
circuits intended for the reduction of ripple are usually called smoothing or
filtering circuits.
The presence of ripple is unwanted because it has many undesirable effects.
Some of the effects are:

A DC Power Electronics
High current ripple increases the maximum current in a circuit. High
currents may reduce the life of some electric and electronic
components. For instance, high currents reduce the life of the
brushes in dc motors, and require high capacity components.
53
Exercise 4 – Ripple in Choppers  Discussion

High current ripple produces electromagnetic interference
(disturbance) that can affect the operation of other electrical and
electronic circuits. The disturbance can cause interruptions, or limit
the performance of some circuits for instance.

When the ripple frequency and its harmonics are within the audio
band, they may be audible.

High voltage ripple increases the maximum voltage in a circuit. High
voltages may reduce the life of some electric and electronic
components. For instance, high voltages also require components
having high breakdown voltages.
Ripple versus inductance / capacitance
You have seen in the previous exercise that inductors oppose current variations
and that capacitors oppose voltage variations. When the electronic switch
shown in Figure 35 closes, current starts to flow through inductor and a
magnetic field builds up around the coil of the inductor (energy is stored in the
magnetic field). While the magnetic field is building up, the current in
inductance increases gradually at a rate inversely proportional to the opposition
to the current flow produced by inductor (this opposition is proportional to the
inductance of the inductor). When electronic switch opens, the magnetic field
around the coil collapses gradually. This provides electrical energy that keeps
current flowing in inductor until the magnetic field collapsed completely. This
explains why current continues to flow even after switch closes, as shown in
Figure 35. Larger inductors have a larger inductance and build larger magnetic
fields (store more energy), and thus the time taken to build the magnetic field
(and to make the magnetic field collapse) is also longer. For this reason, large
inductors oppose current variations more than small inductors as shown in
Figure 35.
The opposition to current flow causes a reduction in ripple. Without inductor in
the circuit of Figure 35, the output current waveform and the switching control
signal waveform would be similar. In a complete cycle, there would be periods
when the current is null and other periods when current is high, two operating
conditions resulting in a maximal amount of ripple (this is not desirable).
54
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Discussion
Buck chopper
Switching control signal
on
Amplitude (V)
Switching
control
signal
off
5
0
Time
Small inductor
Large inductor
Output current
No inductor
Time
Figure 35. Opposition to current variations caused by inductors.
Similarly, capacitors oppose voltage variations by storing electrical energy. Inside
a capacitor, the terminals connect to two metal plates separated by a
dielectric (non-conducting material). When a voltage is applied across the plates
of a capacitor, an electric field builds up in the dielectric (electrical energy is
stored). When the voltage applied to the capacitor is removed, the energy stored
in the capacitor supplies the circuit for a period of time. The amount of electrical
energy a capacitor can store is proportional to the capacitance. Large capacitors
store more energy (i.e., have a higher capacity) than small capacitors and
oppose more voltage variations. Increasing the size of capacitors is a means of
reducing voltage ripple.
A DC Power Electronics
55
Exercise 4 – Ripple in Choppers  Procedure Outline
Ripple versus switching frequency
It is not always possible and economical to increase the size of inductors and
capacitors to reduce the ripple. As you will observe in the procedure of this
exercise, increasing the size of an inductor requires more space and adds weight
to a device. Instead of increasing the size of inductors and capacitors, the ripple
can also be reduced by increasing the switching frequency.
Output current
Figure 36 shows a waveform having the same average value as the one shown
in Figure 34 but with less ripple. The difference in ripple is caused by an increase
in frequency of the switching control signal: doubling the switching frequency
reduced the ripple by 50% and the maximum voltage reached by
approximately 20%. Increasing the switching frequency is a good way of
reducing both the current and voltage ripples. In modern equipment, high
switching frequency (10 000 Hz and more) is common practice to minimize
ripple.
Max (peak)
Average
Ripple
Time
Figure 36. Increasing the switching frequency reduces ripple.
PROCEDURE OUTLINE
The Procedure is divided into the following sections:



Setup and connections
Current ripple versus switching frequency and inductance
Voltage ripple versus switching frequency and capacitance
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Before installing the Filtering Inductors/Capacitors module in the Workstation,
take a look inside the module and observe the difference in size between the
two inductors. It is quite evident that the biggest inductor (50 mH) is heavier
56
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Procedure
and requires more space. The difference in size and weight between the two
capacitors is also evident.
Install the required equipment in the Workstation.
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to a USB
port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
Connect Switching Control Input 1 of the IGBT Chopper/Inverter to Analog
Input 1 of the Data Acquisition and Control Interface using a miniature
banana plug lead.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
A DC Power Electronics
57
Exercise 4 – Ripple in Choppers  Procedure
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
7. Set up the circuit shown in Figure 37. Use the 50 mH inductor in the Filtering
is a
Inductors/Capacitors module to implement . In this circuit, resistor
resistive load and is a smoothing inductor.
Chopper/Inverter
50 mH
100 V
57 Ω
Switching control signals
from digital outputs
on DACI
Figure 37. Buck chopper circuit with a resistor load and a smoothing inductor.
8. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance value required.
Current ripple versus switching frequency and inductance
In this part of the exercise, you will observe the ripple on the output current of a
buck chopper. You will first observe how the ripple varies when the switching
frequency is changed and then when the size of the inductor is changed.
Current ripple versus frequency
9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:
58

Select the Voltage Source (+) function.

Set the voltage to 100 V.

Start the voltage source.
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Procedure
10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:

Select the Buck Chopper (high-side switching) function.

Set the switching frequency to 400 Hz.

Set the Duty Cycle Control parameter to Knob.

Set the duty cycle to 50%.

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.

Set the Q parameter to PWM.

Start the buck chopper.
11. In LVDAC-EMS, open the Oscilloscope window and display the following
parameters: the voltage at the buck chopper input (input E1), the switching
control signal (AI-1), and the load voltage and current (inputs E2 and I2).
Select the Continuous Refresh mode, then set the time base to display two
complete cycles, and set the trigger controls so that the Oscilloscope triggers
when the rising edge of the switching control signal (AI-1) reaches 2 V.
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation of the waveforms. Figure 38 shows an example of what
the Oscilloscope should display.
Oscilloscope Setting
Channel-1 Input .............................. E1
Channel-1 Scale ................... 100 V/div
Channel-1 Coupling ........................DC
Channel-2 Input ............................ AI-1
Channel-2 Scale ....................... 5 V/div
Channel-2 Coupling ........................DC
Channel-3 Input ............................. E-2
Channel-3 Scale ..................... 50 V/div
Channel-3 Coupling ........................DC
Channel-4 Input ...............................I-2
Channel-4 Scale ....................... 1 A/div
Channel-4 Coupling ........................DC
Time Base ........................... 0.5 ms/div
Trigger Source .............................. Ch2
Trigger Level .................................. 2 V
Trigger Slope ............................. Rising
Figure 38. Voltage and current waveforms of a buck chopper operating at 400 Hz with a 50 mH
inductor.
A DC Power Electronics
59
Exercise 4 – Ripple in Choppers  Procedure
12. Determine the maximum (peak) value and the ripple magnitude (peak-topeak value) of the current at the buck chopper output (input I2) using the
Oscilloscope. Record your results in the corresponding cells of Table 6.
a
Use the horizontal cursors of the Oscilloscope to obtain good measuring
accuracy.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
Table 6. Maximum current and current ripple magnitude at the buck chopper output.
Inductance
(mH)
Switching frequency
(Hz)
Maximum current
(A)
Current ripple magnitude
[peak-to-peak value]
(A)
400
50
2000
2000
2
10 000
13. Slowly vary the switching frequency from 400 Hz to 5000 Hz using the
Switching Frequency knob in the Chopper/Inverter Control window. If the
ambient noise in your laboratory is low, you should be able to hear a tone
associated with the switching frequency.
14. In the Chopper/Inverter Control window, set the switching frequency
to 2000 Hz.
Determine the maximum value and the ripple magnitude of the current at the
buck chopper output (input I2) using the Oscilloscope. Record your results in
the corresponding cells of Table 6. Change the time base setting as
necessary.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
15. Compare the ripple magnitude measured at 400 Hz and 2000 Hz (shown in
Table 6). Do your results confirm that the ripple decreases when the
switching frequency is increased?
 Yes
60
 No
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Procedure
16. What is the effect of a ripple reduction on the maximum value of current at
the output of the buck chopper?
17. Stop the buck chopper and the voltage source.
Current ripple versus inductance
18. Replace the 50 mH inductor in your circuit with the 2 mH inductor of the
Filtering Inductors/Capacitors module to implement .
Do not modify the parameters set in LVDAC-EMS.
Start the buck chopper and the voltage source.
19. In the Oscilloscope, make sure that convenient vertical scales and positions
are set to facilitate observation of the waveforms.
Determine the maximum value and the ripple magnitude of the current at the
buck chopper output (input I2) using the Oscilloscope. Record your results in
the corresponding cells of Table 6.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
20. Referring to the values in Table 6, compare the magnitude of the ripple
measured at 2000 Hz with both inductors. Is the magnitude of the ripple
measured using the 2 mH inductor lower or higher than that measured using
the 50 mH inductor?
21. Do your results confirm that a large inductor is more effective to smooth the
current ripple than a small inductor?
 Yes
A DC Power Electronics
 No
61
Exercise 4 – Ripple in Choppers  Procedure
22. Increase the switching frequency from 2000 Hz to 10 000 Hz.
Determine the maximum value and the ripple magnitude of the current at the
buck chopper output (input I2) using the Oscilloscope. Record your results in
the corresponding cells of Table 6.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
23. Compare the magnitude of the ripple measured at 2000 Hz with the 50 mH
inductor to that measured at 10 000 Hz with the 2 mH inductor. They should
be rather similar.
Vary the switching frequency until the magnitude of the current ripple
measured using the 2 mH inductor equals that measured using the 50 mH
inductor at 2000 Hz.
Record the switching frequency at which the current ripple magnitude equals
that measured using the 50 mH inductor at 2000 Hz.
Switching frequency: _____
24. Do your measurements confirm that a small inductor can replace a large
inductor to reduce the current ripple when the switching frequency is
increased?
 Yes
 No
25. Stop the buck chopper and the voltage source.
Voltage ripple versus switching frequency and capacitance
In this part of the exercise, you will observe the ripple on the output voltage of a
buck chopper. You will observe how the voltage ripple varies when the switching
frequency is changed and when the size of the capacitor is changed.
26. Modify your circuit as shown in Figure 39. Use the 210 µF capacitor in the
Filtering Inductors/Capacitors module to implement . In this circuit,
is used as a current limiter, resistor
is a resistive load, and
is
resistor
a filtering capacitor. Make sure that you connect the positive (+) terminal of
and .
the 210 µF capacitor to the junction of resistors
62
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Procedure
Chopper/Inverter
86 Ω
100 V
210 µF
171Ω
Switching control signals
from digital outputs
on DACI
Figure 39. Buck chopper circuit with a resistive load and a filtering capacitor.
27. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance values required.
28. In the Four-Quadrant Dynamometer/Power Supply window, make the
following settings:

Make sure that the Voltage Source (+) function is selected.

Make sure that the voltage is set to 100 V.

Start the voltage source.
29. In the Chopper/Inverter Control window, make the following settings:
A DC Power Electronics

Make sure the Buck Chopper (high-side switching) function is
selected.

Set the switching frequency to 400 Hz.

Make sure that the duty cycle is set to 50%.

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.

Make sure that the Q parameter is set to PWM.

Start the buck chopper.
63
Exercise 4 – Ripple in Choppers  Procedure
30. Make sure that the Oscilloscope is set to display the following parameters:
the voltage at the buck chopper input (input E1), the switching control
(input E2), and the load
signal (AI-1), the voltage across the resistive load
current (input I2).
Select the Continuous Refresh mode, then set the time base to display at
least two complete cycles. Set the trigger controls so that the Oscilloscope
triggers when the rising edge of the switching control signal (AI-1)
reaches 2 V.
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation of all waveforms. Notice that the ripple magnitude is
very low in this part of the exercise.
31. Determine the maximum (peak) value and the ripple magnitude (peak-topeak value) of the voltage across resistive load
(input E2) using the
Oscilloscope. Record your results in the corresponding cells of Table 7.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
Table 7. Maximum voltage and voltage ripple magnitude at the buck chopper output.
Capacitance
(µF)
210
5
Switching frequency
(Hz)
Maximum voltage
(V)
Voltage ripple magnitude
[peak-to-peak value]
(V)
400
2000
2000
10 000
32. In the Chopper/Inverter Control window, set the switching frequency
to 2000 Hz.
Determine the maximum value and the ripple magnitude of the voltage
(input E2) using the Oscilloscope. Record your
across resistive load
results in the corresponding cells of Table 7.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
33. Stop the buck chopper and the voltage source.
64
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Procedure
34. Replace the 210 µF capacitor in your circuit by the 5 µF capacitor of the
Filtering Inductors/Capacitors module to implement .
In the Chopper/Inverter Control window, Make sure that the switching
frequency is set to 2000 Hz. Do not modify the other parameters set in
LVDAC-EMS.
Start the buck chopper and the voltage source.
35. In the Oscilloscope, make sure that convenient vertical scales and positions
are set to facilitate observation of the waveforms.
Determine the maximum value and the ripple magnitude of the voltage
across resistive load
(input E2) using the Oscilloscope. Record your
results in the corresponding cells of Table 7.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
36. Referring to the values in Table 7, compare the magnitude of the ripple
measured using both capacitors at 2000 Hz. Is the magnitude of the ripple
measured using the 210 µF capacitor lower or higher than that measured
using the 5 µF capacitor?
37. Do your results confirm that a large capacitor is more effective for smoothing
the voltage ripple than a small capacitor?
 Yes
 No
38. Increase the switching frequency from 2000 Hz to 10 000 Hz.
Determine the maximum value and the ripple magnitude of the voltage
(input E2) using the Oscilloscope. Record your
across resistive load
results in the corresponding cells of Table 7.
Print or save the waveforms displayed on the Oscilloscope for future
reference. It is suggested that you include these waveforms in your lab
report.
A DC Power Electronics
65
Exercise 4 – Ripple in Choppers  Conclusion
39. Compare the ripple magnitude measured at 2000 Hz and 10 000 Hz (shown
in Table 7) with the 5 µF capacitor. Do your results confirm that the
magnitude of the ripple decreases when the switching frequency is
increased?
 Yes
 No
40. Compare the maximum value of voltage measured at 2000 Hz
and 10 000 Hz with the 5 µF capacitor to the magnitude of the ripple
measured at each frequency (shown in Table 7). Does your comparison
confirm that the maximum value of voltage decreases when the magnitude of
the ripple decreases?
 Yes
 No
41. Compare the magnitude of the voltage ripple measured at 400 Hz with
the 210 µF capacitor to that measured at 10 000 Hz with the 5 µF capacitor.
They should be similar.
Do your measurements confirm that a small capacitor can replace a large
capacitor to smooth the voltage ripple when the switching frequency is
increased?
 Yes
 No
42. Stop the buck chopper and the voltage source.
Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
CONCLUSION
In this exercise, you learned that voltage ripple and current ripple are undesired.
For instance, high levels of current ripple are associated with high circuit currents
that require high capacity components and can also cause premature wear of
components. Furthermore, ripple produces disturbances that can affect the
operation of electrical circuits.
You saw that because inductors oppose current variations, they are widely used
to smooth current waveforms. You observed that increasing the size of inductors
and increasing the switching frequency are two means of reducing the current
ripple.
You also saw that because capacitors oppose voltage variations, they are widely
used to smooth (filter) voltage waveforms. You observed that increasing the size
of capacitors and increasing the switching frequency are two means of reducing
the voltage ripple.
66
DC Power Electronics A
Exercise 4 – Ripple in Choppers  Review Questions
REVIEW QUESTIONS
1. Explain why it is important to reduce the ripple in chopper as much as
possible?
2. What is a smoothing inductor used for?
3. What can be done to limit the voltage ripple?
4. Should a smoothing capacitor be connected in series or in parallel with the
load in a circuit?
5. What can be done to improve the smoothing capability of an inductor?
A DC Power Electronics
67
Exercise
5
The Lead‐Acid Battery Charger
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the operation of
a lead-acid battery charger implemented using a buck chopper with feedback
loops.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:



DISCUSSION
Battery fundamentals
Lead-acid battery charger fundamentals
Implementing a lead-acid battery charger using a buck chopper with
feedback loops
Battery fundamentals
A battery is a device that converts chemical energy into electric energy by means
of an electrochemical reaction. When a load such as a resistor or a small light
bulb, is connected to the terminals of a battery, a chemical reaction starts and
electricity is produced and consumed by the load. When no load is connected to
the battery terminals, no chemical reaction occurs and no electrical energy is
produced.
Schematic symbol
of a battery
There are three major types of batteries: primary (single use), secondary
(rechargeable), and reserve (for long storage periods). The primary batteries are
discarded when discharged. They are commonly used as a power source for
portable electronic and electrical devices. The secondary batteries can be
electrically recharged. They can be used as an energy-storage device, which is
usually electrically connected to and charged by an energy source that delivers
energy to a load on demand, as in automotive applications, emergency systems,
and stationary energy storage systems. The lead-acid batteries are secondary
batteries.
The basic electrochemical unit that produces electric energy is referred to as a
cell. A battery consists of a series/parallel arrangement of several cells. For
example, the 12 V lead-acid battery shown in Figure 40 and used in any internalcombustion engine (ICE) car for starting, lighting, and ignition (SLI battery)
consists of six cells connected in series, each cell having a nominal output
voltage (voltage under load) of about 2 V. The Lead-Acid Battery Pack of your
training system consists of four 12 V batteries connected in series, each battery
containing 6 cells.
A DC Power Electronics
69
Exercise 5 – The Lead-Acid Battery Charger  Discussion
Figure 40. A 12 V lead-acid battery contains six cells.
The capacity ( ) of a battery is a measure of the amount of energy that can be
delivered by a battery when it is fully charged, and it is usually expressed in
ampere-hour (Ah). The capacity of a lead-acid battery is usually determined by
measuring the average load current during a discharge test that lasts a specific
time and brings the battery voltage down to a preset end value at which the
battery is considered to be completely discharged. When the capacity is
.
determined using a test that lasts 20 hours, it is designated as the capacity
For instance, a lead-acid battery rated for 200 Ah (10 A × 20 h) will deliver 10 A
of current for 20 hours under standard temperature conditions.
Lead‐acid battery charger fundamentals
A lead-acid battery can be recharged by connecting a source of electric power to
the battery. The battery charging process uses the electric power supplied by the
source to convert the active chemicals in the battery to their original high-energy
state.
For the conversion of the active chemicals to their original high-energy state to
be safe and cause no harm to the battery, the voltage and current of the
dc power source must be carefully controlled during battery charge. In particular,
the lead-acid battery should not be allowed to overheat or to produce too much
oxygen and hydrogen gas (this is commonly referred to as gassing) due to
electrolysis of the water contained in the electrolyte of the battery. When too
much gassing occurs, water is lost from the battery electrolyte, thereby reducing
the amount of electrolyte and modifying its chemical composition – two harmful
consequences leading to a deterioration in performance of any lead-acid battery
and a shortened life. Gassing occurs in a lead-acid battery when the charging
voltage measured across the battery exceeds a certain value, referred to as the
gassing voltage. The gassing voltage is about 57.6 V for a 48 V lead-acid battery
pack.
Many methods of charging can be used to charge lead-acid batteries. In this
exercise, you will use the “modified constant-voltage charging method” (fast
charging method).
70
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Discussion
In the initial stage of the fast charging method, the battery charge starts with a
constant current set to the maximum charging current recommended by the
battery manufacturer. The charging current is maintained at this value until a
certain voltage is reached, usually the gassing voltage (about 2.4 V/cell). See
Figure 41.
When the gassing voltage is reached, battery charging continues with a constantvoltage just equal to or slightly below the gassing voltage until the current flowing
through the battery decreases to a rate of one tenth of the battery
capacity (0.1 20). This stage corresponds to the middle stage of the battery
charge as shown in Figure 41. Note that the charging current rate shown in
Figure 41 applies to a 48 V lead-acid battery.
In the finishing stage of charge, the voltage is reduced to the float voltage to
complete and maintain the battery charge. Float voltage refers to the constant
voltage that is applied continuously to a battery to maintain it in a fully charged
condition. Typical float voltage is about 55.2 V for a 48 V battery pack.
Initial stage
(constant-current
charge)
Middle stage
(constant-voltage
charge at gassing
voltage)
Finishing stage
(constant-voltage
charge at float
voltage)
Float voltage
Charging current rate (
Cell voltage (V)
Gassing
voltage
)
Charging current
Cell voltage
Time (h)
Figure 41. Modified constant-voltage charging method (fast charging method).
Implementing a lead‐acid battery charger using a buck chopper with
feedback loops
As learned in the previous exercises, the voltage conversion occurring in a buck
chopper is controlled by adjusting the duty cycle of the signal controlling the
electronic switch in the chopper. The average voltage at the chopper output is a
portion of the voltage applied to the chopper input. Thus, when the voltage
applied to the chopper input is varied, the chopper output voltage varies. Also,
the higher the duty cycle, the closer the average output voltage to the input
voltage. Similarly, when the load at the buck chopper output varies, the load
current also varies if the buck chopper input voltage and the duty cycle remain
constant.
A DC Power Electronics
71
Exercise 5 – The Lead-Acid Battery Charger  Discussion
To charge a battery using the fast charging method, the current during the initial
stage of battery charge and the voltage during the middle and finishing stages of
battery charge must be consistently maintained across the battery, even if the
battery characteristics change due to variations in the battery state of charge. To
do this, the battery current or voltage must be continuously monitored and the
duty cycle of the switching control signal of the switch in the buck chopper
readjusted to take into account the load variations due to variations in the battery
state of charge. Figure 42 shows a simplified diagram of a battery charger
implemented using a buck chopper with the voltage and current feedback loops
required. In this battery charger circuit, the battery voltage and current are
monitored by voltage sensor E and current sensor I. In brief, each of these
sensors transmits a feedback signal to a controller, which sets the duty cycle of
the switching control signal in the buck chopper to the value required to maintain
the current or the voltage across the battery at the desired value.
Battery charger
Buck chopper
Current sensor
Current
feedback
loop
Switching
control
signals
Command
inputs
Measured
current
Controller
Measured
voltage
Battery
under
charge
Voltage sensor
Voltage
feedback
loop
Figure 42. Simplified block diagram of a lead-acid battery charger implemented using a buck
chopper with voltage and current feedback loops.
A command signal is a
target value that an automatic control system aims
to reach.
72
The voltage and current feedback loops in the battery charger shown above can
be implemented using the Buck Chopper with Feedback function of LVDACEMS. Inputs E4 and I4 of a Data Acquisition and Control Interface (DACI) are
used as voltage and current sensors to monitor the battery voltage and current.
The voltage and current signals from these sensors are filtered to remove the
ripple and noise, and each signal is compared with a command signal in an error
detector. The difference (error) between the measured signal value and the
command signal is then amplified in a proportional and integral (PI) amplifier and
sent to a PWM signal generator to readjust the duty cycle of the buck-chopper
switching control signal in order to increase or decrease the buck-chopper output
voltage so that the measured voltage or current equals the voltage or current
command. The block diagram of this battery charger is shown in Figure 43.
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Discussion
Chopper/Inverter
Current sensor (DACI)
Voltage sensor (DACI)
Battery
under
charge
Error detector
Current
command
Filter
Filter
PI
amplifier
Error detector
Voltage
command
PWM
signal
generator
PI
amplifier
Measured current
Measured voltage
Figure 43. Block diagram of the lead-acid battery charger that can be implemented using the
Buck Chopper with Feedback function of LVDAC-EMS, a Data Acquisition and Control
Interface (DACI), a Chopper/Inverter module, and a dc power source.
For instance, when the voltage measured across the battery is less than the
voltage set by the voltage command, the error detector signal is positive thereby
increasing the duty cycle of the buck chopper switching control signal. This
causes the voltage across the battery to increase until the error is corrected. At
this point, the error detector signal is zero and the PI amplifier produces a fixed
signal that sets the duty cycle of the buck chopper switching control signal to the
exact value required to maintain equilibrium.
Similarly, when the current flowing through the battery is less than the current set
by the current command, the error detector signal is positive thereby increasing
the duty cycle of the buck chopper switching control signal. This causes the
current flowing through the battery to increase until the error is corrected. At this
point, the error detector signal is zero and the PI amplifier produces a fixed signal
that sets the duty cycle of the buck chopper switching control signal to the exact
value required to maintain equilibrium.
The current loop is used during the first phase of the battery charge and the
voltage loop is used during the second and third phases of the battery charge.
A DC Power Electronics
73
Exercise 5 – The Lead-Acid Battery Charger  Procedure Outline
PROCEDURE OUTLINE
The Procedure is divided into the following sections:





Setup and connections
Voltage regulation
Current regulation
Partially discharging the batteries in the Lead-Acid Battery Pack
The lead-acid battery charger
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
a
Before beginning this exercise, measure the open-circuit voltage across the
Lead-Acid Battery Pack (8802), using a multimeter. If the open-circuit voltage
is lower than 51 V, ask your instructor for assistance as the Lead-Acid Battery
Pack is probably not fully charged. Appendix C of this manual indicates how to
prepare (charge) the Lead-Acid Battery Pack before each laboratory period.
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Install the required equipment in the Workstation.
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to a
USB port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
74
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Procedure
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
7.
Set up the circuit shown in Figure 44. In this circuit, inputs E4 and I4 are
used by LVDAC-EMS as feedback inputs for the buck chopper. Note
that when inputs E4 and I4 are used by LVDAC-EMS as feedback inputs,
they are no longer available for the LVDAC-EMS instrumentation,
i.e., the Metering, Oscilloscope, Phasor Analyzer, and Harmonic Analyzer.
Other voltage and current inputs of the DACI must be used to measure and
observe the voltage and current sensed by inputs E4 and I4.
Chopper/Inverter
2 mH
5 µF
171 Ω
Switching control signals
from digital outputs
on DACI
Figure 44. Circuit used to observe voltage and current regulation.
A DC Power Electronics
75
Exercise 5 – The Lead-Acid Battery Charger  Procedure
8. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance value required.
Voltage regulation
In this part of the exercise, you will apply a voltage across a resistor connected to
the output of a buck chopper with feedback, and observe that the voltage is
maintained by the voltage regulation circuit although the input voltage of the
chopper is changed.
9. In the Data Acquisition and Control Settings of LVDAC-EMS, set the range of
E4 to Low. This corresponds to a 0 V to 80 V range.
10. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:

Select the Voltage Source (+) function.

Set the voltage to 60 V.

Do not start the voltage source now.
11. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:
76

Select the Buck Chopper with Feedback function.

Set the switching frequency 20 000 Hz.

Set the Command Input parameter to Knob.

Set the Command parameter to 100%. This parameter is used to set
the command (i.e., the value at which the voltage will be regulated).
The Command parameter is expressed as a percentage of the
Feedback Range parameter.

Set the Controller Proportional Gain Kp parameter to 1.0.

Set the Controller Integral Gain Ki parameter to 20.

Set the Feedback Input parameter to E4. This sets input E4 as the
feedback input (sensor). This selection allows regulation of the
voltage measured using input E4 (i.e., the buck chopper output
voltage).

Set the Feedback Range parameter to 50 V. This parameter
determines the value at the selected feedback input that corresponds
to a command value of 100%. In the present case, this makes a
voltage of 50 V measured at input E4 correspond to a command
value of 100%, so that the regulated voltage will be 50 V
(100% × 50 V).
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Procedure

Set the Feedback Filter Cutoff Frequency to 100 Hz. This sets the
cutoff frequency of the feedback filter to 100 Hz.

Make sure that the Acceleration Time is set to 0.0 s.

Make sure that the Deceleration Time is set to 0.0 s.

Make sure that the Q parameter is set to PWM.

Start the buck chopper with feedback.
12. From the parameters set in the Chopper/Inverter Control window, predict the
voltage across resistor .
Voltage across resistor
: _____
13. What effect will a voltage variation at the buck chopper input have on the
voltage across resistor ? Explain.
14. What will the duty cycle set by the regulation circuit be if the chopper input
voltage is successively set to 80 V, 100 V, and 120 V?
Duty cycle when the input voltage is 80 V: _____
Duty cycle when the input voltage is 100 V: _____
Duty cycle when the input voltage is 120 V: _____
15. In LVDAC-EMS, open the Metering window. Make sure that meters E1, E2,
I1, and I2 are enabled. Select the DC mode by setting the AC/DC button at
the bottom of each meter to DC. Disable meter E4.
Select the Continuous Refresh mode by clicking on the Continuous Refresh
button.
16. Start the voltage source.
A DC Power Electronics
77
Exercise 5 – The Lead-Acid Battery Charger  Procedure
17. Successively set the chopper input voltage (meter E1 in the Metering
window) to each of the values shown in Table 8. For each value, record the
duty cycle (indicated by meter D1 in the Chopper/Inverter Control window) as
(meter E2 in the Metering window).
well as the voltage across resistor
Table 8. Duty cycle and voltage across resistor
Input voltage
(V)
Duty cycle
(%)
.
Voltage across
resistor
(V)
80
100
120
18. Do your measurements confirm the predictions you made in step 14?
 Yes
 No
19. What will the duty cycle and the voltage across resistor
be if the input
voltage were set to a value lower than the expected regulated voltage such
as 40 V?
Duty cycle: _____
Voltage across resistor
: _____
20. In the Four-Quadrant Dynamometer/Power Supply window, set the source
voltage to 40 V and validate your predictions made in the previous step. Are
your predictions confirmed?
 Yes
 No
21. Stop the buck chopper and the voltage source.
Current regulation
In this part of the exercise, you will make a current flow through a resistor
connected to the output of a buck chopper with feedback, and observe that the
current is maintained by the current regulation circuit although the resistor value
is changed. You will also observe that the current remains constant even when
the voltage at the buck chopper input is changed.
22. In the Data Acquisition and Control Settings window of LVDAC-EMS, make
sure that the range of input E4 is set to Low.
Make sure that the range of input I4 is set to Low. This corresponds to a
range of 0 A to 4 A.
78
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Procedure
23. Make sure that the connections and the switches on the Resistive Load are
set to obtain 171 Ω as the resistance value for .
24. In the Four-Quadrant Dynamometer/Power Supply window, make the
following setting:

Set the source voltage to 100 V.
25. In the Chopper/Inverter Control window, make the following settings:

Make sure that the Buck Chopper with Feedback function is
selected.

Make sure that the Switching Frequency is set to 20 000 Hz.

Make sure that the Command Input parameter is set to Knob.

Make sure that the Command parameter is set to 100%. This sets
the regulated output current to 100% of the Feedback Range
parameter value.

Make sure that the Controller Proportional Gain Kp parameter is set
to 1.0

Make sure that the Controller Integral Gain Ki parameter is set to 20.

Set the Feedback Input parameter to I4. This sets input I4 as the
feedback input (sensor). This selection allows regulation of the
current measured using input I4 (i.e., the buck chopper output
current).

Set the Feedback Range parameter to 0.5 A. This makes a current
of 0.5 A measured using input I4 correspond to a command value
of 100%.

Make sure that the Feedback Filter Cutoff Frequency is set
to 100 Hz.

Make sure that the Acceleration Time is set to 0.0 s.

Make sure that the Deceleration Time is set to 0.0 s.

Make sure that the Q parameter is set to PWM.

Start the buck chopper with feedback.
26. From the parameters set in the Chopper/Inverter Control window, what will
be?
the current flowing through resistor
Current flowing through resistor
A DC Power Electronics
: _____
79
Exercise 5 – The Lead-Acid Battery Charger  Procedure
27. What effect will reducing the value of resistor
through resistor ? Explain.
have on the current flowing
28. What effect will increasing the voltage at the buck chopper input have on the
current flowing through resistor ? Explain.
29. In the Metering window of LVDAC-EMS, make sure that meters E1, E2, I1,
and I2 are enabled, and that the DC mode is selected on each meter. Make
sure that meter I4 is disabled.
Make sure that the Continuous Refresh mode is selected.
30. Start the voltage source.
to each of the values shown in
Successively set the resistance of resistor
Table 9. For each value, record the duty cycle (meter D1 in the
Chopper/Inverter Control window) as well as the current flowing through
(meter I2 in the Metering window).
resistor
Table 9. Duty cycle and current flowing through resistor
Resistance
(Ω)
Duty cycle
(%)
.
Current flowing
through resistor
(A)
171
86
57
31. Do your measurements confirm your predictions made in step 26 and
step 27?
 Yes
80
 No
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Procedure
32. Increase the source voltage to 140 V. Record the duty cycle (meter D1 in the
Chopper/Inverter Control window) as well as the current flowing through
(meter I2 in the Metering window).
resistor
Duty cycle: _____
Current flowing through resistor
: _____
33. Do your measurements confirm your predictions made in step 28?
 Yes
 No
34. What will the current flowing through resistor
parameter is set to 80%?
Current flowing through resistor
be if the Command
: _____
35. In the Chopper/Inverter Control window, set the Command parameter to 80%
and validate your prediction made in the previous step. Is your prediction
confirmed?
 Yes
 No
36. Stop the buck chopper and the voltage source.
Partially discharging the batteries in the Lead‐Acid Battery Pack
In this part of the exercise, you will partially discharge the batteries in the LeadAcid Battery Pack to obtain the operating conditions required in the next part of
the exercise.
37. Disconnect the Four-Quadrant Dynamometer/Power Supply from the current
circuit by disconnecting the leads at the yellow and white terminals. The rest
of the circuit will be used later in the exercise.
Connect the positive (red) terminal of the Lead-Acid Battery Pack to the
Power Supply yellow terminal of the Four-Quadrant Dynamometer/Power
Supply. Connect the negative (black) terminal of the Lead-Acid Battery Pack
to the Power Supply white terminal of the Four-Quadrant Dynamometer/
Power Supply.
A DC Power Electronics
81
Exercise 5 – The Lead-Acid Battery Charger  Procedure
38. In the Four-Quadrant Dynamometer/Power Supply window of LVDAC-EMS,
make the following settings:

Set the Function parameter to Battery Discharger (Constant-Current
Timed Discharge with Voltage Cutoff). When this function is selected, the
Four-Quadrant Dynamometer/Power Supply operates as a negative
current source whose operation is controlled by parameters associated
with battery discharge.

Set the Discharge Current parameter to 4 A. This sets the discharge
current of the battery discharger to 4 A.

Set the Discharge Duration parameter to 20 min. This sets the discharge
duration to 30 minutes.

Set the Cutoff Voltage parameter to 40 V. This sets the minimum
value (40 V) to which the battery voltage is allowed to decrease during
discharge. This limits battery discharging to prevent damage to the
batteries.

Start the Battery Discharger by setting the Status parameter to Started or
by clicking on the Start/Stop button.
a
During battery discharging, the time left is indicated at the bottom of the FourQuadrant Dynamometer/Power Supply window. Once battery discharge is
completed, the Battery Discharger automatically stops and the Status
parameter is set to Stopped.
Once battery discharge is completed, disconnect the Power Supply terminals
of the Four-Quadrant Dynamometer/Power Supply from the Lead-Acid
Battery Pack and proceed with the next part of the exercise.
a
Make sure that the Battery Discharger is stopped (i.e., make sure that the
Status parameter is set to Stopped) before proceeding with the next part of the
exercise.
The lead‐acid battery charger
In this part of the exercise, you will perform the initial stage and the middle stage
of the modified constant-voltage charging method (fast charging method) for
lead-acid batteries, which are illustrated in Figure 41.
Initial stage of a lead‐acid battery fast‐charging – Constant current charge
In the initial stage of the battery charge, you will make the maximum
recommended charge current flow through the Lead-Acid Battery Pack, and
maintain this current using automatic regulation until the voltage measured
across the terminals of the Lead-Acid Battery Pack reaches the gassing
voltage (57.6 V).
39. Set up the circuit shown in Figure 45. In this circuit, the voltage and current
supplied to the Lead-Acid Battery Pack are measured using inputs E2 and I2.
Inputs E4 and I4 are used by LVDAC-EMS as feedback inputs for regulation
purposes.
82
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Procedure
Chopper/Inverter
2 mH
Switching control signals
from digital outputs
on DACI
Battery to be charged
(48 V battery-pack)
Figure 45. Circuit used to charge the Lead-Acid Battery Pack.
40. In the Data Acquisition and Control Settings window of LVDAC-EMS, set the
range of the inputs as follows: input E1 to High (0 V to 800 V), inputs E2
and E4 to Low (0 V to 80 V), inputs I1, I2 and I4 to Low (0 A to 40 A).
41. In the Four-Quadrant Dynamometer/Power Supply window, make the
following settings:

Select the Voltage Source (+) function.

Set the source voltage to 100 V.

Do not start the voltage source now.
42. In the Chopper/Inverter Control window, make the following settings:
A DC Power Electronics

Make sure that the Buck Chopper with Feedback function is
selected.

Make sure that the Switching Frequency parameter is set
to 20 000 Hz.

Make sure that the Command Input parameter is set to Knob.

Make sure that the Command parameter is set to 100%. This sets
the regulated output current to 100% of the Feedback Range
parameter value.

Set the Controller Proportional Gain Kp parameter to 1.0.
83
Exercise 5 – The Lead-Acid Battery Charger  Procedure

Set the Controller Integral Gain Ki parameter to 20.0.

Set the Feedback Input parameter is set to I4. This sets input I4 as
the feedback input. This selection allows regulation of the current
measured using input I4 (i.e., the battery charging current).

Set the Feedback Range parameter to the maximum charge current
value of the Lead-Acid Battery Pack. When this current is measured
at input I4, it corresponds to a command value of 100%.

Make sure that the Feedback Filter Cutoff Frequency is set
to 100 Hz.

Make sure that the Acceleration Time is set to 0.0 s.

Make sure that the Deceleration Time is set to 0.0 s.

Make sure that the Q parameter is set to PWM.

Start the buck chopper.
43. In the Metering window, make the following settings:

Make sure that meters E1, E2, I1, and I2 are enabled, and the
DC mode is selected on each meter.

Make sure that meters E4 and I4 are disabled.

Make sure that the Continuous Refresh mode is selected.
44. Start the voltage source and immediately record the duty cycle displayed by
the Duty Cycle meter in the Chopper/Inverter Control window.
Duty cycle: _____
45. Record the voltage across the Lead-Acid Battery Pack (meter E2 in the
Metering window).
Voltage across the Lead-Acid Battery Pack: _____
46. Record the current flowing through the Lead-Acid Battery Pack (meter I2 in
the Metering window).
Current flowing through the Lead-Acid Battery Pack: _____
47. Does the current flowing through the Lead-Acid Battery Pack correspond to
the parameters set in the Chopper/Inverter Control window?
 Yes
84
 No
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Procedure
48. Observe the voltage across the Lead-Acid Battery Pack displayed by
meter E2. The voltage should slowly increase. Once the voltage has reached
the gassing voltage (57.6 V), record the duty cycle displayed by the Duty
Cycle meter in the Chopper/Inverter Control window, and immediately stop
the buck chopper with feedback. Permanent damage to the batteries in the
Lead-Acid Battery Pack may occur if the voltage exceeds the gassing voltage
significantly.
a
The duty cycle value displayed is slightly higher than the theoretical value, due
of the Chopper/Inverter and inductor .
to voltage drops across transistor
Duty cycle: _____
49. Compare the duty cycle measured at the beginning of the initial stage of
battery charge (step 44) with the duty cycle measured at the end of the initial
stage (step 48).
You should observe that the duty cycle has increased slightly to maintain the
charge current constant despite the increase in the battery-pack voltage.
Middle stage of a lead‐acid battery fast‐charging – Constant voltage charge at
gassing voltage
In the middle stage of the battery charge, you will apply a voltage of 57 V (a little
below the gassing voltage) across the Lead-Acid Battery Pack and maintain this
voltage constant using automatic regulation until the current flowing through the
battery pack has decreased to a value corresponding to 0.1 20.
50. Do not modify the settings in the Data Acquisition and Control Settings, FourQuadrant Dynamometer/Power Supply window, and Metering window.
Make the following settings in the Chopper/Inverter Control window:

Set the Feedback Input parameter to E4. This sets input E4 as the
feedback input, and allows regulation of the voltage measured using
input E4 (i.e., the battery charging voltage).

Set the Feedback Range parameter to 57.0 V. This makes a voltage
of 57.0 V measured at input E4 correspond to a command value
of 100%.

Do not modify the settings of the other parameters in the
Chopper/Inverter Control window.

Do not start the buck chopper with feedback for now.
51. From the settings made in LVDAC-EMS, determine the duty cycle which the
regulation circuit will use at the beginning of the middle stage of the battery
charge.
Duty cycle at the beginning of the middle stage of battery charge: _____
A DC Power Electronics
85
Exercise 5 – The Lead-Acid Battery Charger  Procedure
52. Start the buck chopper with feedback. Wait for the duty cycle displayed by
the Duty Cycle meter in the Chopper/Inverter Control window to stabilize,
then record the value.
Duty cycle: _____
53. Does the measured duty cycle confirm your prediction in step 51?
 Yes
 No
54. Record the voltage across the Lead-Acid Battery Pack (meter E2 in the
Metering window).
Voltage across the Lead-Acid Battery Pack: _____
55. Wait about 10 minutes for the battery charge to progress.
In the Four-Quadrant Dynamometer/Power Supply window, decrease the
voltage to 80 V.
Does the voltage measured across the Lead-Acid Battery Pack decrease
when the voltage is decreased? Explain why.
56. Observe the current flowing through the Lead-Acid Battery Pack displayed by
meter I2. The current should decrease slowly. Once the current flowing
through the battery pack has decreased to a value corresponding to a value
of 0.1 , stop the voltage source and the buck chopper with feedback. This
completes the middle stage of the battery charge.
Finishing stage of a lead‐acid battery fast‐charging – Constant voltage charge at
float level
At this point, most of the energy supplied by the Lead-Acid Battery Pack during
the discharge has been returned to the battery pack. To complete and maintain
the battery charge, the voltage applied to the battery pack should be reduced to
the float voltage value (52.2 V) and maintained at this value indefinitely. During
the finishing stage, the current flowing through the battery pack decreases to a
very low value. You do not have to perform this stage of battery charging.
57. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
86
DC Power Electronics A
Exercise 5 – The Lead-Acid Battery Charger  Conclusion
CONCLUSION
In this exercise, you were introduced to lead-acid batteries and learned how to
charge a lead-acid battery using the modified constant-voltage charging method
(also called fast charging method). You were also introduced to the buck chopper
with feedback, an automatic regulation system which uses a voltage or current
sensor to return a feedback signal to a controller in order to maintain the voltage
or current equal to the command value. You implemented a lead-acid battery
charger using a buck chopper with feedback and charged a 48 V lead-acid
battery pack using the fast charging method.
REVIEW QUESTIONS
1. What is the basic electrochemical unit that produces electric energy in a
battery (fast charging method)?
2. Briefly describe the three stages of the modified constant-voltage charging
method (fast charging method).
3. Explain how the voltage at the output of a buck chopper can be maintained to
a fixed value despite changes in the load current or the input voltage.
4. How does the duty cycle of the switching control signal vary when the voltage
at the input of a buck chopper with feedback is doubled?
5. Briefly describe the gassing voltage and the float voltage.
A DC Power Electronics
87
Exercise
6
The Boost Chopper
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the operation of
the boost chopper.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:


DISCUSSION
The boost chopper
Power efficiency
The boost chopper
You learned in the previous exercises that buck choppers are used to convert a
dc voltage into a dc voltage having a lower value. You will learn in this exercise,
that a dc voltage can also be converted to a dc voltage having a higher value
using a boost chopper.
Figure 46 shows a boost chopper built with inductor , electronic switch ,
diode , and capacitor , and some waveforms related to this circuit. When
electronic switch switches on, the voltage across its terminals becomes almost
is applied to inductor , and current
null, the dc power supply voltage
flowing in inductor starts to increase. Simultaneously, diode
switches off
since it becomes reverse-biased. At this moment, capacitor starts to discharge
and
into the resistive load
and both the boost chopper output current
start to decrease. Notice that without the opposition to current
voltage
variations caused by inductor , the dc power source would be short-circuited
when electronic switch is switched on.
A DC Power Electronics
89
Switching
control
signal
Amplitude (V)
Exercise 6 – The Boost Chopper  Discussion
5
0
Inductor
current
( )
Amplitude (A)
Time
0
Output
Current
( )
Amplitude (A)
Time
0
Output
voltage
( )
Amplitude (V)
Time
2
0
Time
Figure 46. Operation of a boost chopper.
When electronic switch switches off, the voltage across its terminals increases
+ 0.7 V. This applies a forward-bias
very rapidly until it reaches approximately
voltage of approximately 0.7 V to diode , which therefore switches on. At this
starts to charge up capacitor , and
moment, a current equal to
and start to increase.
both
90
DC Power Electronics A
Exercise 6 – The Boost Chopper  Discussion
The dc voltage at the boost chopper output
is proportional to the dc voltage at
and the time the electronic switch is on during each
the boost chopper input
, is in turn proportional to
cycle. This time, which is referred to as the on-time
the duty cycle
of the switching control signal of electronic switch . The
and is given by Equation (6).
equation relating voltages
(6)
1
where
is the dc voltage at the boost chopper output.
is the dc voltage at the boost chopper input.
is the duty cycle expressed as a decimal (e.g., 50% = 0.5).
can be varied by varying the duty cycle . This equation
Thus, voltage
can range between voltage
and an infinite voltage
indicates that voltage
when the duty cycle varies between 0% and 100%. In practice, however, the
can vary
duty cycle only approaches 0% and 100%. Therefore, voltage
and many times voltage . In
between a voltage slightly higher than voltage
most circuits, the maximum value of duty cycle must be limited to limit the
maximum voltage the boost chopper can produce.
Varying the switching frequency while maintaining the duty cycle constant does
and current
at the boost chopper output.
not vary the output voltage
However, the ripple magnitude decreases as the switching frequency is
increased.
Power efficiency
The power
which the boost chopper delivers at its output is equal to the
it receives at its input minus the power dissipated in the electronic
power
switch and the inductor. The power dissipated in the electronic switch and the
inductor is usually small compared to the output power . The power efficiency
of boost choppers, thus, often exceeds 80%. Notice that the power efficiency is
the ratio of the output power to the input power times 100%, as stated in
Equation (7).
Power efficiency =
where
100%
(7)
is the power the boost chopper delivers.
is the power the boost chopper receives.
A DC Power Electronics
91
Exercise 6 – The Boost Chopper  Procedure Outline
PROCEDURE OUTLINE
The Procedure is divided into the following sections:




Setup and connections
Output voltage versus duty cycle
Output voltage versus switching frequency
Power efficiency
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Install the required equipment in the Workstation.
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to
a USB port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
92
DC Power Electronics A
Exercise 6 – The Boost Chopper  Procedure
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
Connect Switching Control Input 4 of the IGBT Chopper/Inverter to Analog
Input 1 of the Data Acquisition and Control Interface using a miniature
banana plug lead.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
7. Set up the circuit shown in Figure 47. Use the 50 mH inductor in the Filtering
Inductors/Capacitors module to implement .
Chopper/Inverter
50 mH
+
20 V
300 Ω
Switching control signals
from digital outputs
on DACI
Figure 47. Boost chopper circuit with resistive load.
8. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance value required.
A DC Power Electronics
93
Exercise 6 – The Boost Chopper  Procedure
Output voltage versus duty cycle
In this part of the exercise, you will vary the duty cycle of the switching control
signal while observing the voltage conversion performed by the boost chopper.
9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:

Select the Voltage Source (+) function.

Set the voltage to 20 V.

Start the voltage source.
10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:

Select the Boost Chopper function.

Set the switching frequency to 400 Hz.

Set the Duty Cycle Control to Knob.

Set the duty cycle to 20%.

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.

Make sure that the Q parameter is set to PWM.

Start the boost chopper.
11. For each duty cycle shown in Table 10, calculate the theoretical dc output
when the dc voltage
at the boost chopper input is 20 V. Use
voltage
/ 1
. Record your results in the column "Calculated" of
equation
Table 10.
94
DC Power Electronics A
Exercise 6 – The Boost Chopper  Procedure
Table 10. Output voltage versus duty cycle.
DC voltage at the boost
chopper input
(V)
DC voltage across
electronic switch
(V)
Duty cycle
(%)
DC output voltage
(V)
Measured Calculated
20
20
20
40
20
60
20
80
Switching frequency = 400 Hz
12. In LVDAC-EMS, open the Oscilloscope window and display the voltage and
current (inputs E1 and I1) at the boost chopper input, the switching control
signal (AI-1), the dc voltage across electronic switch Q (input E3), the
dc output voltage (input E2), and the dc output current (input I2).
Select the Continuous Refresh mode, set the time base to display at least
two complete cycles, and set the trigger controls so that the Oscilloscope
triggers when the rising edge of the switching control signal (AI-1)
reaches 2 V.
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation of the waveforms. Figure 48 shows an example of what
the Oscilloscope should display.
a
To facilitate the readings on the Oscilloscope, momentarily select the Single
Refresh mode by pressing the Single Refresh button. This will automatically
disable the Continuous Refresh mode. Do not forget to refresh the display
when a new setting is made.
Oscilloscope Setting
Channel-1 Input .............................. E1
Channel-1 Scale ..................... 50 V/div
Channel-1 Coupling ........................DC
Channel-2 Input ...............................I-1
Channel-2 Scale .................... 0.2 A/div
Channel-2 Coupling ........................DC
Channel-3 Input ............................ AI-1
Channel-3 Scale ....................... 5 V/div
Channel-3 Coupling ........................DC
Channel-4 Input ............................. E-3
Channel-4 Scale ..................... 50 V/div
Channel-4 Coupling ........................DC
Channel-5 Input ............................. E-2
Channel-5 Scale ..................... 20 V/div
Channel-5 Coupling ........................DC
Channel-6 Input ...............................I-2
Channel-6 Scale .................... 0.5 A/div
Channel-6 Coupling ........................DC
Time Base ........................... 0.1 ms/div
Trigger Source .............................. Ch3
Trigger Level .................................. 2 V
Trigger Slope ............................. Rising
Figure 48. Voltage and current waveforms in a boost chopper operating at 400 Hz.
A DC Power Electronics
95
Exercise 6 – The Boost Chopper  Procedure
13. Successively set the duty cycle to each of the values in Table 10. For each
value, measure the dc voltage (input E1) at the boost chopper input, the
dc voltage across electronic switch (input E3), and the dc output
(input E2) across the resistive load . The average (dc) voltage
voltage
values are indicated in the column "AVG" located under the Oscilloscope
display. Record your results in the column "Measured" of Table 10.
14. Do your measurement results confirm that the voltage at the output of the
boost chopper increases when the duty cycle is increased?
 Yes
a
 No
Note that the measured and calculated voltages differ slightly. The difference,
as
which increases with the duty cycle, is caused by the losses in inductor
and diode
that are not
well as the voltage drops across electronic switch
taken into account in the equation (
/ 1
). These are significant
and must be considered for precise estimations.
Output voltage versus switching frequency
In this part of the exercise, you will vary the frequency of the switching control
signal while observing the voltage conversion performed by the boost chopper.
15. In the Chopper/Inverter Control window, set the duty cycle to 50%.
Make sure that the source voltage is still set to 20 V in the Four-Quadrant
Dynamometer/Power Supply.
In the Chopper/Inverter Control window, successively set the switching
frequency to each of the values in Table 11. For each value, measure the
dc voltage
(input E2) at the output of the boost chopper using the
Oscilloscope. Record your results in the corresponding cells of Table 11.
a
To facilitate the reading of the voltage, you may select the Single Refresh
mode by pressing the Single Refresh button. This will automatically disable the
Continuous Refresh mode. Do not forget to return to the Continuous Refresh
mode after each reading.
Table 11. Output voltage versus switching frequency.
DC input voltage
(V)
Switching frequency
(Hz)
20
500(1)
20
1000(1)
20
1500(1)
20
2000(1)
20
5000(1)
DC output voltage
(V)
(1) Change the time base setting in the Oscilloscope as required to display two complete cycles
(2) Duty cycle = 50%
96
DC Power Electronics A
Exercise 6 – The Boost Chopper  Procedure
16. Do your results confirm that the switching frequency has no effect on the
output voltage of a boost chopper?
 Yes
 No
Power efficiency
In this part of the exercise, you will determine the power at the input and output
of the boost chopper to determine the power efficiency of the boost chopper. You
will also observe the direction of power flow.
17. In the Chopper/Inverter Control window, set the switching frequency
to 500 Hz and the duty cycle to 80%.
18. In Four-Quadrant Dynamometer/Power Supply window, make sure that the
source voltage is still equal to 20 V.
19. In the Oscilloscope window, display the value of power P1 and power P2.
Power P1 is the power in watts determined from the voltage and current
measured using inputs E1 and I1, and power P2 is the power in watts
determined from the voltage and current measured using inputs E2 and I2.
Since inputs E1 and I1 measure the input voltage and current of the boost
chopper, power P1 corresponds to the input power. Similarly, since inputs E2
and I2 measure the output voltage and current of the boost chopper,
power P2 corresponds to the output power of the boost chopper.
Record the input power
and the output power
.
: _____
Input power
Output power
: _____
20. Determine the power efficiency of the boost chopper using your measured
powers and the following equation.
Power efficiency =
100% = _____
21. Do your results confirm that the power efficiency is high (exceeding 80%)?
 Yes
 No
22. Slowly vary the duty cycle between 20% and 80% while observing the load
current (input I2). Notice that the load current polarity is positive and remains
positive as the duty cycle of the switching control signal is varied. This
indicates that the power flow is always from the power source to the load.
A DC Power Electronics
97
Exercise 6 – The Boost Chopper  Conclusion
23. Stop the boost chopper and the voltage source.
Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
CONCLUSION
In this exercise, you verified that the voltage at the output of a boost chopper
increases as the duty cycle of the switching control signal is increased. You
observed that the frequency of the switching control signal has no effect on the
output voltage of a boost chopper. Nevertheless, you observed that as the
switching frequency is increased, the current ripple decreases. You verified that
the power efficiency of a boost chopper exceeds 80%. You saw that power
always flows in the same direction in a boost chopper.
REVIEW QUESTIONS
1. Describe the effect the switching frequency has on the output voltage of a
boost chopper.
2. A boost chopper is powered by a 12-V battery. What is the output voltage
range of this chopper if the duty cycle can vary between 20% and 95%?
3. Explain why the dc power source
shown in Figure 47, does not become
is switched on.
shorted when electronic switch
4. Explain why the maximum value of the duty cycle must be limited in certain
boost choppers.
5. Is the diode
in the Figure 47 forward-biased or reverse-biased when
is switched off?
electronic switch
98
DC Power Electronics A
Exercise
7
The Buck/Boost Chopper
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the operation of
the buck/boost chopper.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:

DISCUSSION
The Buck/Boost Chopper
The Buck/Boost Chopper
As discussed in the previous exercises of this manual, the buck chopper converts
a dc voltage into a dc voltage having a lower value and the boost chopper
converts a dc voltage into a dc voltage having a higher value. In these choppers,
the current always flows in the same direction, that is, from the input to the output
of the chopper as shown in Figure 49a and Figure 49b.
In a buck/boost chopper, the current can flow in either direction. When the
current flows from the low voltage side to the high voltage side, power also flows
from the low voltage side to the high voltage side, and the buck/boost chopper
operates as a boost chopper. On the other hand, when the current flows from the
high voltage side to the low voltage side, power also flows from the high voltage
side to the low voltage side, and the buck/boost chopper operates as a buck
chopper. This is summarized in Figure 49c.
A DC Power Electronics
99
Exercise 7 – The Buck/Boost Chopper  Discussion
Current (power) flow
High
voltage
Input
Buck
chopper
Output
Low
voltage
Output
High
voltage
(a)
Current (power) flow
Low
voltage
Input
Boost
chopper
(b)
Current (power) flow
(Buck)
(Boost)
High
voltage
Buck/Boost
chopper
Low
voltage
(c)
Figure 49. Current (power) flow in various types of choppers.
A buck/boost chopper built with two electronic switches and two diodes (freewheeling diodes) is shown in Figure 50. This figure shows that the buck/boost
chopper consists of a buck chopper and a boost chopper connected together.
The buck chopper operates when the current flows from the high-voltage side to
the low-voltage side. In this case, the components of the boost chopper could be
removed without disturbing the operation of the buck chopper. Conversely, the
boost chopper operates when the current flows from the low-voltage side to the
high-voltage side. In this case, the components of the buck chopper could be
removed without disturbing the operation of the boost chopper.
100
DC Power Electronics A
Exercise 7 – The Buck/Boost Chopper  Discussion
Buck/Boost chopper
Boost chopper
Buck chopper
Switching
control
signal of the
buck chopper
Low
voltage
Amplitude (V)
High
voltage
5
0
Switching
control
signal of the
boost chopper
Amplitude (V)
Time
5
0
Time
Figure 50. A buck/boost chopper built with two electronic switches and two diodes.
Figure 50 also shows the waveforms of the switching control signals applied to
the electronic switches. These signals are pulse trains whose duty cycles are
complementary. For instance, when the duty cycle of one signal is 60%, the duty
cycle of the other signal is 40%. Therefore, when one electronic switch is
switched on, the other electronic switch is switched off and vice versa.
(high voltage) and
(low voltage) in the buck
The equation relating voltages
(high voltage)
chopper and the equation relating voltages (low voltage) and
in the boost chopper also apply for the buck/boost chopper. The duty cycle of the
switching control signal applied to the buck chopper electronic switch must be
used when using the buck chopper equation. Similarly, the duty cycle of the
switching control signal applied to the boost chopper electronic switch must be
used when using the boost chopper equation.
and
in a buck
Table 12 shows that both the equations relating voltages
chopper and a boost chopper can be used to determine the relationship
and
when the appropriate duty cycle is used. In this example the
between
duty cycle of the buck chopper is 40%, consequently the duty cycle of the
boost chopper is 60%.
A DC Power Electronics
101
Exercise 7 – The Buck/Boost Chopper  Procedure Outline
Table 12. Relationship between voltages
chopper and a boost chopper.
Buck chopper
and
determined using the equation for a buck
Boost chopper
1
0.4
0.4
PROCEDURE OUTLINE
1
0.6
0.4
0.4
The Procedure is divided into the following sections:



Setup and connections
Switching control signals of a buck/boost chopper
Operation of a buck/boost chopper
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Make sure that the batteries in the Lead-Acid Battery Pack are fully charged
by measuring the open-circuit voltage with a voltmeter. If the open-circuit
voltage is lower than 51 V, ask your instructor for assistance as the battery
pack is probably not fully-charged. Appendix C of this manual indicates how
to prepare (charge) the Lead-Acid Battery Pack before each laboratory
period.
Install the required equipment in the Workstation.
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
102
DC Power Electronics A
Exercise 7 – The Buck/Boost Chopper  Procedure
3. Connect the USB port of the Data Acquisition and Control Interface to a
USB port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
Connect Switching Control Inputs 1 and 4 of the IGBT Chopper/Inverter to
Analog Inputs 1 and 2 of the Data Acquisition and Control Interface
using miniature banana plug leads.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
7. Set up the circuit shown in Figure 51. Use the 50 mH inductor of the Filtering
Inductors/Capacitors module to implement .
Use the Lead-Acid Battery Pack as low voltage source.
A DC Power Electronics
103
Exercise 7 – The Buck/Boost Chopper  Procedure
Chopper/Inverter
50 mH
100 V
High
voltage
source
50 V
Low
voltage
source
+
Switching control signals
from digital outputs
on DACI
Figure 51. Buck/Boost chopper circuit.
Switching control signals of a buck/boost chopper
In this part of the exercise, you will observe that the switching control signals of
the electronic switches in a buck/boost chopper are complementary.
8. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:

Select the Voltage Source (+) function.

Set the voltage to 100 V.

Do not start the voltage source now.
9. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:
104

Select the Buck/Boost Chopper function.

Set the switching frequency to 1000 Hz.

Set the Duty Cycle Control to Knob.

Set the duty cycle to 50%.

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.
DC Power Electronics A
Exercise 7 – The Buck/Boost Chopper  Procedure

Make sure that the Q and Q parameters are set to PWM.

Start the buck/boost chopper now.
10. In LVDAC-EMS, open the Oscilloscope window and display the voltage and
the current (inputs E1 and I1) at the high-voltage side, the two switching
control signals (AI-1 and AI-2), as well as the voltage and current (inputs E2
and I2) at the low-voltage side.
Select the Continuous Refresh mode, set the time base to display at least
two complete cycles, and set the trigger controls so that the Oscilloscope
triggers when the rising edge of the switching control signal (AI-1) of
reaches 2 V.
electronic switch
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation of the waveforms.
11. Referring to the waveform of the switching control signals (AI-1 and AI-2),
and .
describe the switching sequence of electronic switches
12. In the Chopper/Inverter Control window, set the duty cycle to 60%.
Referring to the waveform of the switching control signals (AI-1 and AI-2),
does the duty cycle setting that you have made in the Chopper/Inverter
Control window correspond to the duty cycle of the switching control signal of
(AI-1) or to the duty cycle of the switching control signal
electronic switch
(AI-2)?
of electronic switch
13. From what you observed in the previous steps, what will the duty cycles of
the switching control signals of the electronic switches be if the duty cycle in
the Chopper/Inverter Control window is set to 20%?
A DC Power Electronics
Duty cycle of the switching control signal of
: _____
Duty cycle of the switching control signal of
: _____
105
Exercise 7 – The Buck/Boost Chopper  Procedure
Operation of a buck/boost chopper
In this part of the exercise, you will vary the duty cycle to observe the buck/boost
chopper operation in the buck and boost chopper modes.
14. In the Chopper/Inverter Control window, set the duty cycle to 50%.
In the Four-Quadrant Dynamometer/Power Supply window, start the voltage
source, then using the arrow buttons below the control knob, adjust the
voltage so that the current at the high-voltage side and low-voltage side,
measured using inputs I1 and I2, are almost null. This setting is required to
balance the circuit taking account of the state of charge of the batteries in the
Lead-Acid Battery Pack.
15. Successively set the duty cycle
to each of the values shown in Table 13.
For each value, record the average (dc) voltage and current at the highvoltage side (inputs E1 and I1) and low-voltage side (inputs E2 and I2) in the
corresponding cells of Table 13.
Table 13. Average dc Voltages and currents in a buck/boost chopper.
High-voltage side
DC voltage
(V)
DC current
(A)
Duty cycle
(%)
Low-voltage side
DC voltage
(V)
DC current
(A)
45
50
55
16. According to the polarity of the currents measured at the high-voltage side
and low-voltage side when the duty cycle is 45%, in which direction does the
power flow?
17. Does the circuit operate as a buck chopper or boost chopper when the duty
cycle is 45%? Explain why.
106
DC Power Electronics A
Exercise 7 – The Buck/Boost Chopper  Procedure
18. According to the currents measured at the high-voltage side and low-voltage
side, does the circuit operate as a buck chopper or boost chopper when the
duty cycle is set to 50%? Explain why.
19. According to the polarity of the currents measured at the high-voltage side
and low-voltage side, does the circuit operate as a buck chopper or boost
chopper when the duty cycle is set to 55%? Explain why.
20. Using the arrow buttons below the Duty Cycle control knob in the Buck/Boost
Chopper window, slowly decrease the duty cycle from 55% (current value)
to 45% while observing the currents using the Oscilloscope.
Describe what happens to the currents when the duty cycle passes
through 50%. What does this mean?
21. From the observations you carried out in this exercise, which feature
distinguishes the buck/boost chopper from the buck chopper and boost
chopper?
22. Stop the buck/boost chopper and the voltage source.
23. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
A DC Power Electronics
107
Exercise 7 – The Buck/Boost Chopper  Conclusion
CONCLUSION
In this exercise, you verified that a buck/boost chopper can operate either as a
buck chopper or a boost chopper. You observed that the duty cycles of the two
switching control signals in a buck/boost chopper are complementary, that is,
when the duty cycle of one switching control signal is 70% for example, the duty
cycle of the other switching control signal is 30%. You found that the current can
flow in either direction in a buck/boost chopper, and that the direction of the
current determines whether the circuit operates as a buck chopper or a boost
chopper.
REVIEW QUESTIONS
1. Briefly describe the operation of a buck/boost chopper.
2. Refer to the circuit shown in Figure 52. Knowing that the duty cycles of the
and
are 25%
switching control signals applied to electronic switches
and 75%, respectively, is the 12 V battery supplying or receiving power?
40 V
12 V
Figure 52. A buck/boost chopper circuit.
108
DC Power Electronics A
Exercise 7 – The Buck/Boost Chopper  Review Questions
3. Under the conditions stated in review question 2, would it be possible to
from the circuit without disturbing the operation
remove electronic switch
of the buck/boost chopper? Explain.
4. Refer to the circuit shown in Figure 52. Knowing that the duty cycles of the
and
are 60%
switching control signals applied to electronic switches
and 40% respectively, is the circuit operating as a buck chopper or as a
boost chopper?
5. What is the main feature which distinguishes the buck/boost chopper from
the buck chopper and the boost chopper?
A DC Power Electronics
109
Exercise
8
The Four‐Quadrant Chopper
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the operation of
the four-quadrant chopper.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:

DISCUSSION
The Four-Quadrant Chopper
The Four‐Quadrant Chopper
The dc-to-dc converters discussed so far, that is, the buck chopper, the boost
chopper, and the buck/boost chopper, allow a dc voltage to be converted into a
dc voltage of the same polarity and having a lower or higher value. Furthermore,
in the buck and boost choppers, the current can flow in one direction only,
whereas in the buck/boost chopper, it can flow in either direction.
Another type of dc-to-dc converter, the four-quadrant chopper, allows a
dc voltage to be converted into a dc voltage having a lower voltage value as well
as current flow in either direction. Furthermore, in the four-quadrant chopper, the
polarity of the converted dc voltage can be reversed with respect to the polarity of
the original voltage. Therefore, a four-quadrant chopper can provide a positive or
a negative dc voltage regardless of the direction in which current flows. This
feature allows the four-quadrant chopper to operate in any of the four quadrants
of a voltage-current Cartesian plan as shown in Figure 53.
Voltage (V)
Quadrant II
Quadrant I
Current (A)
Quadrant III
Quadrant IV
Figure 53. Operation in any of the four quadrants.
A DC Power Electronics
111
Exercise 8 – The Four-Quadrant Chopper  Discussion
In Figure 53, four dots have been drawn, one in each quadrant. Each of these
dots represents a possible operating point of the four-quadrant chopper. Drawing
straight lines from any one of these dots to the horizontal and vertical
axes (current and voltage axes, respectively) allows the dc current and voltage at
the output of the four-quadrant chopper to be determined. For example, the dot in
Quadrant 3 indicates that the current and voltage are equal to -2 A dc and
-60 V dc, respectively. If the operating point of the four-quadrant chopper passes
from the dot in Quadrant 3 to the dot in Quadrant 4, the current and voltage
change to +2 A dc and -60 V dc, respectively. This indicates that the direction in
which current flows has been reversed while the voltage remains the same.
Figure 54 shows a four-quadrant chopper built with electronic switches and
diodes, and some waveforms related to the circuit. The switching control signals
show that the electronic switches are switched in pairs, that is,
with
and
with , and that when one pair of electronic switches is on, the other pair is off.
Therefore, the input voltage
is alternately applied to the output of the fourquadrant chopper through either one of the two pairs of electronic switches. The
depends on which pair of
instantaneous polarity of the output voltage
and
are on,
electronic switches is on. It is positive when electronic switches
and
are on. The average (dc)
and negative when electronic switches
of the four-quadrant chopper depends on the time each pair of
output voltage
electronic switches is on during each cycle. This, in turn, depends on the duty
and is
cycle of the switching control signals. The equation relating voltages
written below.
2
where
,
1
(8)
is the dc voltage at the four-quadrant chopper output.
is the dc voltage at the four-quadrant chopper input.
,
is the duty cycle expressed as a decimal (50% = 0.5).
is the duty cycle of the switching control signals applied to
In this equation,
,
electronic switches
and
in the four-quadrant chopper shown in Figure 54.
can
Since the duty cycle can vary from approximately 0 to 100%, the voltage
vary from approximately - to + .
112
DC Power Electronics A
Exercise 8 – The Four-Quadrant Chopper  Discussion
Amplitude
and
Switching
control
signals
and
and
on
off
and
and
off
on
and
and
on
off
5
0
and
Switching
control
signals
Amplitude
Time
5
0
Output
Voltage
( )
Amplitude
Time
0
Time
-
Amplitude
I
Output
Current
( )
0
Time
Figure 54. Operation of the four-quadrant chopper.
Varying the frequency of the switching control signals while maintaining the duty
cycle constant does not affect the dc voltage and current at the four-quadrant
chopper output. However, the ripple magnitude decreases as the frequency of
the switching control signals increases.
A DC Power Electronics
113
Exercise 8 – The Four-Quadrant Chopper  Procedure Outline
PROCEDURE OUTLINE
The Procedure is divided into the following sections:





Setup and connections
Switching control signals of a four-quadrant chopper
Operation of a four-quadrant chopper
Partially discharging the batteries in the Lead-Acid Battery Pack
Demonstrating four-quadrant operation
PROCEDURE
High voltages are present in this laboratory exercise. Do not make or modify any
banana jack connections with the power on unless otherwise specified.
Setup and connections
In this part of the exercise, you will set up and connect the equipment.
1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of
equipment required to perform this exercise.
Make sure that the batteries in the Lead-Acid Battery Pack are fully charged
by measuring the open-circuit voltage with a voltmeter. If the open-circuit
voltage is lower than 51 V, ask your instructor for assistance as the battery
pack is probably not fully-charged. Appendix C of this manual indicates how
to prepare (charge) the Lead-Acid Battery Pack before each laboratory
period.
Install the required equipment in the Workstation.
2. Connect the Power Input of the Data Acquisition and Control Interface to
a 24 V ac power supply.
Connect the Low Power Input of the IGBT Chopper/Inverter to the Power
Input of the Data Acquisition and Control Interface. Turn the 24 V ac power
supply on.
3. Connect the USB port of the Data Acquisition and Control Interface to a
USB port of the host computer.
Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to
a USB port of the host computer.
114
DC Power Electronics A
Exercise 8 – The Four-Quadrant Chopper  Procedure
4. Make sure that the main power switch of the Four-Quadrant Dynamometer/
Power Supply is set to O (off), then connect the Power Input to an ac power
outlet.
Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
5. Connect the Digital Outputs of the Data Acquisition and Control
Interface (DACI)
to
the
Switching
Control
Inputs
of
the
IGBT Chopper/Inverter using a DB9 connector cable.
Connect Switching Control Inputs 1, 2, 4, and 5 of the IGBT Chopper/Inverter
to Analog Inputs 1, 2, 4, and 5, respectively, of the Data Acquisition and
Control Interface using miniature banana plug leads.
Connect the common (white) terminal of the Switching Control Inputs on the
IGBT Chopper/Inverter to one of the two analog common (white) terminals on
the Data Acquisition and Control Interface using a miniature banana plug
lead.
6. Turn the host computer on, then start the LVDAC-EMS software.
In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition
and Control Interface and the Four-Quadrant Dynamometer/Power Supply
are detected. Make sure that the Computer-Based Instrumentation and
Chopper/Inverter Control functions for the Data Acquisition and Control
Interface are available, as well as the Standard Functions (C.B. control) for
the Four-Quadrant Dynamometer/Power Supply. Select the network voltage
and frequency that correspond to the voltage and frequency of your local
ac power network, then click the OK button to close the LVDAC-EMS
Start-Up window.
7. Set up the circuit shown in Figure 55. Use the 2 mH inductor of the Filtering
Inductors/Capacitors module to implement .
A DC Power Electronics
115
Exercise 8 – The Four-Quadrant Chopper  Procedure
Chopper/Inverter
2 mH
100 V
171 Ω
Switching control signals
from digital outputs
on DACI
Figure 55. Four-quadrant chopper circuit.
8. Make the necessary connections and switch settings on the Resistive Load
in order to obtain the resistance value required.
Switching control signals of a four‐quadrant chopper
In this part of the exercise, you will observe the switching control signals and see
that the electronic switches in a four-quadrant chopper are switched in pairs (
and
with ), and that when one pair of electronic switches is on, the
with
other pair is off.
9. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply
window and make the following settings:
116

Select the Voltage Source (+) function.

Set the voltage to 100 V.

Start the voltage source.
DC Power Electronics A
Exercise 8 – The Four-Quadrant Chopper  Procedure
10. In LVDAC-EMS, open the Chopper/Inverter Control window and make the
following settings:

Select the Four-Quadrant Chopper function.

Set the switching frequency to 1000 Hz.

Set the Duty Cycle Control to Knob.

Set the duty cycle to 25%.

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.

Make sure that the Q , Q , Q , and Q parameters are set to PWM.

Start the four-quadrant chopper.
11. In LVDAC-EMS, open the Oscilloscope window and display the four
switching control signals (AI-1, AI-2, AI-4, and AI-5).
Select the Continuous Refresh mode, set the time base to display two
complete cycles, and set the trigger controls so that the Oscilloscope triggers
when the rising edge of the switching control signal (AI-1) of electronic
reaches 2 V.
switch
Select convenient vertical scale and position settings in the Oscilloscope to
facilitate observation of the waveforms.
12. Referring to the waveforms of the switching control signals displayed on the
Oscilloscope, describe the switching sequence of the electronic switches in
the four-quadrant chopper.
13. Referring to the waveform of the switching control signals, does the duty
cycle setting that you have made in the Chopper/Inverter Control window
correspond to the duty cycle of the switching control signals of electronic
and
or to the duty cycle of the switching control signals of
switches
and ?
electronic switches
A DC Power Electronics
117
Exercise 8 – The Four-Quadrant Chopper  Procedure
Operation of a four‐quadrant chopper
In this part of the exercise, you will calculate the average (dc) output voltage of a
four-quadrant chopper for various duty cycles and compare the results with the
measured average (dc) output voltages using the circuit in Figure 55. You will
observe that the dc voltage can be either positive or negative and that the current
can flow in both directions at the output of a four-quadrant chopper. You will also
observe the effect of the switching frequency on the voltage conversion.
14. Considering an input voltage
equal to 100 V and using the equation
2 ,
1 , calculate the dc voltage which should appear at the
output of the four-quadrant chopper for the following duty cycles: 25%, 50%,
and 75%.
Table 14. Calculated dc output voltages of a four-quadrant chopper for various duty cycles.
Duty cycle
DC output voltage
(V)
,
(%)
25
50
75
15. In the Oscilloscope window, display the voltage and current (inputs E1
and I1) at the four-quadrant chopper input, the switching control signal (AI-1)
of electronic switch , and the voltage and current (inputs E2 and I2) at the
four-quadrant chopper output.
16. In the Data Acquisition and Control Settings of LVDAC-EMS, set the range of
E4 to High. This corresponds to a 0 V to 800 V range.
17. In the Chopper/Inverter Control window, set the switching frequency
to 15 000 Hz, and then successively set the duty cycle to each of the values
in Table 15. For each value, measure the dc voltage and current at the
input (inputs E1 and I1) and output (inputs E2 and I2) of the four-quadrant
chopper. Record your results in the corresponding cells of Table 15.
Table 15. DC voltages and currents in a four-quadrant chopper.
DC input
voltage
(V)
DC input
current
(A)
Duty cycle
,
(%)
DC output
voltage
(V)
DC output
current
(A)
25
50
75
118
DC Power Electronics A
Exercise 8 – The Four-Quadrant Chopper  Procedure
18. Explain why the dc voltage and current at the four-quadrant chopper output
are almost null when the duty cycle is 50%.
19. Does the polarity of the dc currents measured in step 16 confirm that the
current can flow in both directions in a four-quadrant chopper?
 Yes
 No
20. Does the polarity of the dc voltages at the output of the four-quadrant
chopper measured in step 16 confirm that the voltage can be either positive
or negative?
 Yes
 No
21. Do the voltages calculated in step 14 and measured in step 16 confirm that
2 ,
1 in a four-quadrant chopper?
 Yes
 No
22. Determine the range of the dc output voltage of the four-quadrant chopper by
varying the duty cycle from 0% to 100%.
DC output voltage when the duty cycle
,
is set to 0%: _____
DC output voltage when the duty cycle
,
is set to 100%: _____
23. In the Chopper/Inverter Control window, set the duty cycle to 80%.
Vary the switching frequency from 15 000 Hz to 1000 Hz while observing the
voltage and current waveforms at the output of the four-quadrant chopper.
Does the switching frequency have a significant effect on the dc voltage and
current the four-quadrant chopper supplies? If so, describe this effect.
24. Stop the four-quadrant chopper and the voltage source.
A DC Power Electronics
119
Exercise 8 – The Four-Quadrant Chopper  Procedure
Partially discharging the batteries in the Lead‐Acid Battery Pack
In this part of the exercise, you will partially discharge the batteries in the LeadAcid Battery Pack. The batteries must be partially discharged in the next part of
the exercise to obtain the desired operating point and make the appropriate
observations.
25. Perform procedure step 37 and step 38 in Exercise 5.
Demonstrating four‐quadrant operation
You observed that the dc voltage can be either positive or negative and that the
current can flow in both directions at the output of a four-quadrant chopper.
However, you observed the operation of the four-quadrant chopper in
quadrants 1 and 3 only. In this part of the exercise, you will use the circuit shown
in Figure 56 to observe the operation of the chopper in the four quadrants. You
will use the Oscilloscope in the X-Y mode to observe in which quadrant the fourquadrant chopper operates.
26. Set up the circuit shown in Figure 56.
Chopper/Inverter
2 mH
50 V
100 V
Switching control signals
from digital outputs
on DACI
Figure 56. Circuit used to demonstrate four-quadrant operation.
27. In the Four-Quadrant Dynamometer/Power Supply window, make sure that
the source voltage is set to 100 V, and start the voltage source.
120
DC Power Electronics A
Exercise 8 – The Four-Quadrant Chopper  Procedure
28. In the Chopper/Inverter Control window, make the following settings:

Make sure that the Four-Quadrant Chopper function is selected.

Set the switching frequency to 20 000 Hz.

Set the duty cycle to 75%.

Make sure that the acceleration time is set to 0.0 s.

Make sure that the deceleration time is set to 0.0 s.

Make sure that the Q , Q , Q , and Q parameters are set to PWM.

Start the four-quadrant chopper.
29. In the Oscilloscope window, make the following settings:

Set Channel 1 (X) to display the four-quadrant chopper output
current (input I2) and set the sensitivity to 1 A/div. The input selected
in Channel 1 is used as the X-axis parameter in the X-Y display
mode of the Oscilloscope.

Set Channel 2 (Y) to display the four-quadrant chopper output
voltage (input E2) and set the sensitivity to 20 V/div. The input
selected in Channel 2 is used as the Y-axis parameter in the X-Y
display mode of the Oscilloscope.

Turn the X-Y and X-Y Average parameters on.

Select the Continuous Refresh mode.
30. In LVDAC-EMS, open the Data Table window. Under the Options tab, open
the Record Settings dialog box, select Oscilloscope in the Settings field, then
select Channel 1 AVG and Channel 2 AVG as the parameters to be recorded. Close the Record Settings dialog box.
31. Make sure that the average (dc) output current (input I2) is null. Slightly
readjust the voltage of the voltage source if necessary. In this operating
of the four-quadrant chopper is
condition, the average (dc) output voltage
positive and equal to the battery voltage plus or minus the voltage drop
across inductor .
In the Chopper/Inverter Control window, vary the duty cycle between 70%
and 80% by 1% increments. For each setting, observe the location of the dot
in the Oscilloscope display and record the average (dc) output current
(Channel 2 AVG) of the
(Channel 1 AVG) and average (dc) output voltage
four-quadrant chopper in the data table. Do not exceed 4.5 A at the output of
the four-quadrant chopper. Manually record the duty cycle
,
corresponding to each setting in the Data Table.
A DC Power Electronics
121
Exercise 8 – The Four-Quadrant Chopper  Procedure
32. Stop the four-quadrant chopper and the voltage source.
33. Reverse the battery connections in the circuit as shown in Figure 57.
Chopper/Inverter
2 mH
50 V
100 V
Switching control signals
from digital outputs
on DACI
Figure 57. Circuit used to demonstrate four-quadrant operation (with reversed battery
connections).
34. In the Chopper/Inverter Control window, set the duty cycle to 25% then start
the four-quadrant chopper and the voltage source. Make sure that the
average (dc) output current (input I2) is null. Slightly readjust the voltage of
the voltage source if necessary. In this operating condition, the average
of the four-quadrant chopper is negative and equal to
(dc) output voltage
the battery voltage plus or minus the voltage drop across inductor .
In the Chopper/Inverter Control window, vary the duty cycle between 30%
and 20% by 1% increments. For each setting, observe the location of the dot
in the Oscilloscope display and record the average (dc) output current
(Channel 2 AVG) of the four(Channel 1 AVG) and average (dc) voltage
quadrant chopper in the data table. Make sure not to exceed 4.5 A at the
output of the four-quadrant chopper. Manually record the duty cycle
corresponding to each setting in the data table.
35. Stop the four-quadrant chopper and the voltage source.
122
DC Power Electronics A
Exercise 8 – The Four-Quadrant Chopper  Conclusion
36. Transfer your recorded data into a spreadsheet application, and plot on a
same graph, a curve of the dc output voltage (Y axis) versus dc output
current (X axis) when the duty cycle varies from 70% to 80%, and a curve of
the dc output voltage (Y axis) versus dc output current (X axis) when the duty
cycle varies from 30% to 20%.
37. Do the curves you plotted confirm that the four-quadrant chopper can
operate in any of the four quadrants?
 Yes
 No
38. Referring to the curves you plotted, determine the duty cycle at which the
current changes polarity.
39. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.
CONCLUSION
In this exercise, you observed that the electronic switches are switched in pairs in
a four-quadrant chopper, and that when one pair of electronic switches is on, the
other pair is off. You observed that the voltage at the input of the four-quadrant
chopper is alternately applied to its output through each pair of electronic
switches and that the polarity of the output voltage alternates depending on
which pair of electronic switches is on. You also observed that the four-quadrant
chopper can provide a positive or a negative average (dc) voltage regardless of
the direction in which current flows, thereby allowing four-quadrant operation.
REVIEW QUESTIONS
1. Referring to Figure 54, is the voltage
and
negative when electronic switches
applied to the load positive or
are switched on?
2. Determine the output voltage variation that can be observed when the duty
cycle of a four-quadrant chopper varies from 0% to 100%?
A DC Power Electronics
123
Exercise 8 – The Four-Quadrant Chopper  Review Questions
3. Referring to Figure 54, briefly describe the four-quadrant chopper operation.
4. A four-quadrant chopper, which is supplied by a +200 V dc power supply,
provides -24 V dc at its output. What is the duty cycle
, ?
5. What is the main feature which distinguishes the four-quadrant chopper from
the buck/boost chopper?
124
DC Power Electronics A
Appendix A
Equipment Utilization Chart
Equipment
Model
Exercise
Description
1
2
3
4
5
6
7
8
1
1
8131(1)
Workstation
1
1
1
1
1
1
8311
Resistive Load
1
1
1
1
1
1
8325-A
Filtering Inductors/Capacitors
1
1
1
1
1
8802
Lead-Acid Battery Pack
(2)
1
1
1
1
1
1
1
Connection Leads
1
1
1
1
1
1
1
1
Four-Quadrant Dynamometer/Power Supply
1
1
1
1
1
1
1
1
Host Computer
1
1
1
1
1
1
1
1
9063-C
Data Acquisition and Control Interface
1
1
1
1
1
1
1
1
30004-2
24 V AC Power Supply
1
1
1
1
1
1
1
1
(4)
1
1
Multimeter
8990
1
1
8946-2
8960-E
1
1
IGBT Chopper/Inverter
(3)
1
1
8837-B
8951-L
1
1
1
(1)
Workstation model 8110 or model 8134 can also be used.
The prefix IGBT has been left out in this manualwhen referring to this module.
(3)
Model 8960-E consists of the Four-Quadrant Dynamometer/Power Supply, Model 8960-2, with functions 8968-1,
8968-2, and 8968-4.
(4)
Model 9063-C consists of the Data Acquisition and Control Interface, Model 9063, with functions 9069-1
and 9069-2.
(2)
A DC Power Electronics
125
Appendix B
Resistance Table for the Resistive Load
Module
Table 16 gives resistance values which can be obtained by using the Resistive
Load, model 8311. Other parallel combinations can be used to obtain the same
resistance values listed. Figure 58 shows the load resistors and connection.
Table 16. Combinations of switch positions required to obtain various resistance values.
Resistance
(Ω)
1200
Position of the switches
1
2
4
5
6
I
I
I
I
I
I
I
I
7
8
9
I
600
I
300
I
400
I
240
I
200
171
I
150
I
133
I
I
I
I
I
I
I
120
I
109
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
100
I
92
86
I
80
I
75
I
71
I
67
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
63
I
60
57
A DC Power Electronics
3
I
127
Appendix B
Resistance Table for the Resistive Load Module
Figure 58. Location and connection of the load resistors on the Resistive Load.
128
DC Power Electronics A
Appendix C
Preparation of the Lead‐Acid Battery Pack
This appendix explains how to prepare (fully charge) each Lead-Acid Battery
Pack, Model 8802, before a laboratory period. The Four-Quadrant
Dynamometer/Power Supply, Model 8960, is used to charge either a single
battery pack or several battery packs (connected in parallel) at a time.
Charging procedure
The charging procedure below can be performed overnight so that the equipment
is ready to be used by another group of students the following day.
Perform the following steps:
1. To charge a single Lead-Acid Battery Pack, start the procedure at step 2. To
charge several battery packs (connected in parallel) at the same time, start
the procedure at step 3.
2. Connect the positive (red) terminal of the Lead-Acid Battery Pack to the
yellow Power Supply terminal of the Four-Quadrant Dynamometer/Power
Supply. Connect the negative (black) terminal of the Lead-Acid Battery Pack
to the white Power Supply terminal of the Four-Quadrant Dynamometer/
Power Supply. Go to step 4.
3. Connect the Parallel Charging Input (red) terminal of each Lead-Acid Battery
Pack to the yellow Power Supply terminal of the Four-Quadrant
Dynamometer/Power Supply. Connect the negative (black) terminal of each
Lead-Acid Battery Pack to the white Power Supply terminal of the FourQuadrant Dynamometer/Power Supply. Go to the next step.
4. Make sure the main power switch of the Four-Quadrant Dynamometer/Power
Supply is set to O (off), then connect the Power Input to an ac power outlet.
5. Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power
Supply to Power Supply.
6. Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main
power switch to I (on).
7. On the Four-Quadrant Dynamometer/Power Supply, select the Pb-Acid
Battery Float Charger function using the Function button, and set the
charging voltage to 55.2 V using the Command knob.
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Appendix C
Preparation of the Lead‐Acid Battery Pack
8. Start battery charging by depressing the Start/Stop push button. The display
indicates Started, signifying that battery charging has begun.
9. Once battery charging is completed, stop the Pb-Acid Battery Float Charger
by depressing the Start/Stop push button, then turn the Four-Quadrant
Dynamometer/Power Supply off. Disconnect each Lead-Acid Battery Pack
from the Four-Quadrant Dynamometer/Power Supply. Test each Lead-Acid
Battery Pack for sulfation as described in the Sulfation test section of this
appendix.
Sulfation test
a
This test must be performed on a single Lead-Acid Battery Pack at a time.
Perform the following steps:
1. Connect the positive (red) terminal of the Lead-Acid Battery Pack to the
Power Supply (yellow) terminal of the Four-Quadrant Dynamometer/Power
Supply. Connect the negative (black) terminal of the Lead-Acid Battery Pack
to the Power Supply (white) terminal of the Four-Quadrant
Dynamometer/Power Supply.
2. Make sure the main power switch of the Four-Quadrant Dynamometer/Power
Supply is set to O (off), then connect the Power Input to an ac power outlet.
3. On the Four-Quadrant Dynamometer/Power Supply, make the settings
required so that it operates as a negative current source [Current Source (–)
function].
4. Set the current command to -4 A, then start the negative current source.
5. Observe the battery pack voltage. If the battery pack is unable to maintain its
voltage above 48 V for one minute, it may be sulfated.
6. Stop the negative current source.
7. Repeat the sulfation test if you have other battery packs to test.
8. Turn the Four-Quadrant Dynamometer/Power Supply off.
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DC Power Electronics A
Appendix C
Preparation of the Lead‐Acid Battery Pack
9. Faulty battery packs can be desulfated by using a desulfator, otherwise they
must be replaced.
Battery maintenance
In order to optimize the cycle life of the batteries in the Lead-Acid Battery Pack
and to prevent sulfation, the battery pack should be charged as soon as possible
after a discharge cycle (e.g., after completion of a laboratory exercise). The
Lead-Acid Battery Pack should never be stored with the batteries discharged for
an extended period of time.
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Appendix D
Glossary of New Terms
anode (A)
An anode is an electrode through which electric current flows into a polarized
electrical device like a diode.
capacitance
The ability of a capacitor to hold an electrical charge, and thus to oppose voltage
variations is called capacitance. It is expressed in microfarads (μF).
capacitor
A capacitor basically consists of two plates of a conductive material (generally a
metal) separated by an insulating material.
capacity
The capacity of a battery is a measure of the amount of energy that can be
delivered by the battery when it is fully charged. The capacity of a battery is
usually expressed in ampere-hours (Ah).
cathode (K)
A cathode is an electrode through which electric current flows out of a polarized
electrical device like a diode
cell
The cell is the basic electrochemical unit that produces electric energy in a
battery. A battery consists of a series/parallel arrangement of several cells.
characteristic curve
A characteristic curve is a graphical representation of certain electrical
characteristics of a device or component.
cycle
The complete sequence of values of a periodic quantity that occur during a
period. In the case of a signal made of rectangular pulses, the cycle corresponds
to the duration the signal is at high level plus the duration the signal is at low
level.
diode
Two-terminal semiconductor device that acts similar to a switch which has no
movable parts. The main function of a diode is to allow electric current to flow in
one direction and to block current in the opposite direction.
duty cycle
The ratio of the pulse duration to the duration of one cycle. It can be expressed
as a decimal or a percentage.
electrical energy
Energy used to make electric devices work. Electrical energy can be obtained by
converting the energy produced by various sources: chemical energy from
batteries, mechanical (rotational) energy from rotors, hydraulic energy from the
moving water of a river or fall, thermal energy from burning coal or gas, solar
energy from the sun, etc. In the international (SI) system of units, electrical
energy is measured in joules (J). One joule is equal to one watt-second
(i.e., a power of one watt used during one second).
forward biased
A diode is forward biased when the voltage at its anode is higher than
the voltage at its cathode.
free-wheeling diode
A diode that provides an alternate path for current to continue flowing in
a current-type circuit when the main path is open.
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Appendix D
Glossary of New Terms
frequency
The frequency of a sine wave, expressed in hertz (Hz), indicates the number
of times the sine wave cycle repeats itself in one second. The frequency is
inversely proportional to the period .
IGBT
Insulated Gate Bipolar Transistor. A power transistor that has characteristics of
both MOSFET and bipolar transistors. Introduced in the 1980s, the IGBT
handles high current and enables fast switching. IGBTs are found in home
appliances, electric cars and digital stereo power amplifiers.
inductance
The ability of an inductor to oppose current variations is called inductance. It is
expressed in henries (H).
inductor
An inductor consists basically of a coil of wire wound around an iron core.
knee voltage
The knee voltage of a diode is the voltage drop across the diode when the
current starts to increase very rapidly.
leakage current
The leakage current of a diode is a small undesirable current which flows
through the diode when the diode is reverse biased.
N-type layer
A N type layer is a layer of semiconducting material containing negative charge
carriers (electrons).
P-type layer
A P type layer is a layer of semiconducting material containing positive charge
carriers (holes).
period
The period
of a sine wave, expressed in seconds (s), indicates the time
required for one complete cycle to occur. The period is inversely proportional to
the frequency .
PWM
Pulse Width Modulation. A modulation technique that generates variable-width
pulses to represent the amplitude of an analog input signal. The output switching
transistor is on more of the time for a high-amplitude signal and off more of the
time for a low-amplitude signal.
semiconductor
A semiconductor is a material with electrical conductivity due to electron flow.
The conductivity can be varied between that of a conductor and that of an
insulator by adjusting the magnitude of the electron flow.
reverse biased
A diode is reverse biased when the voltage at its anode is lower than the voltage
at its cathode.
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DC Power Electronics A
Appendix E
Circuit Diagram Symbols
Various symbols are used in the circuit diagrams of this manual. Each symbol is
a functional representation of a particular electrical device that can be
implemented using Lab-Volt equipment. The use of these symbols greatly
simplifies the number of interconnections that need to be shown on the circuit
diagram, and thus, makes it easier to understand the circuit operation.
For each symbol other than those of power sources, resistors, inductors, and
capacitors, this appendix gives the name of the device which the symbol
represents, as well as the equipment and the connections required to properly
connect the device to a circuit. Notice that the terminals of each symbol are
identified using circled letters. The same circled letters identify the corresponding
terminals in the Equipment and Connections diagram. Also notice that the
numbers (when present) in the Equipment and Connections diagrams
correspond to terminal numbering used on the actual equipment.
Symbol
Equipment and Connections
Data Acquisition and
Control Interface (9063)
Voltage
inputs
Current
inputs
Isolated voltage and
current measurement inputs
a
When a current at inputs I1, I2, I3, or I4 exceeds 4 A (either permanently or
momentarily), use the corresponding 40 A input terminal and set the Range
parameter of the corresponding input to High in the Data Acquisition and Control
Settings window of LVDAC-EMS.
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Appendix E
Circuit Diagram Symbols
Symbol
Equipment and Connections
Four-Pole Squirrel Cage Induction
Motor (8221-0)
Induction
machine
Three-phase induction
machine
Three-Phase Induction
Machine (8221-B)
Induction
machine
Three-phase induction
machine
Synchronous
Motor / Generator (8241-2)
Synchronous
motor
Three-phase synchronous
motor
136
DC Power Electronics A
Appendix E
Circuit Diagram Symbols
Symbol
Equipment and Connections
Synchronous
Motor / Generator (8241-2)
Synchronous
generator
Three-phase synchronous
generator
Three-Phase Wound-Rotor
Induction Machine (8231-B)
Woundrotor
induction
machine
Three-phase wound-rotor
induction machine
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Appendix E
Circuit Diagram Symbols
Symbol
Equipment and Connections
Permanent Magnet
Synchronous Machine (8245)
U
PMSM
V
W
Permanent Magnet
Synchronous Machine
Rectifier and Filtering
Capacitors (8842-A)
Power diode three-phase
full-wave rectifier
Power Thyristors
(8841)
Power thyristor
three-phase bridge
138
DC Power Electronics A
Appendix E
Symbol
Circuit Diagram Symbols
Equipment and Connections
IGBT Chopper / Inverter
(8837-B)
Three-phase inverter
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139
Index of New Terms
a
For each term, the bold page number indicates the main entry. Refer to
the Glossary of New Terms for definitions of new terms.
anode (A) ............................................................................................................... 3
capacitance .................................................................................................... 37, 55
capacitor ................................................................................. 37, 38, 40, 55, 89, 90
capacity .............................................................................................. 53, 55, 70, 71
cathode (K)............................................................................................................. 3
cell .................................................................................................................. 69, 71
characteristic curve ................................................................................................ 6
cycle ................................................ 9, 10, 26, 27, 40, 54, 71, 72, 73, 91, 101, 112
diode ..................................................................3, 4, 5, 6, 7, 37, 39, 40, 41, 89, 90
duty cycle .......................................... 10, 11, 26, 27, 71, 72, 73, 91, 101, 112, 113
electrical energy ................................................................................... 1, 54, 55, 69
forward biased.................................................................................................... 5, 6
free-wheeling diode ........................................................................................ 39, 41
frequency ..................................................................... 1, 9, 27, 37, 54, 56, 91, 113
IGBT ....................................................................................................................... 8
inductance ...................................................................................................... 38, 54
inductor ......................................................................38, 39, 40, 41, 54, 56, 89, 91
knee voltage ........................................................................................................... 6
leakage current ...................................................................................................... 6
MOSFET ................................................................................................................ 8
N-type layer ............................................................................................................ 4
period ......................................................................................................... 9, 10, 55
P-type layer ............................................................................................................ 4
PWM .................................................................................................................... 10
reverse biased.................................................................................................... 5, 6
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Acronyms
The following acronyms are used in this manual:
AVG
average
DACI
Data Acquisition and Control Interface
DB9
common type of electrical connector having 9 pins
DC
direct current
DFIG
doubly-fed induction generator
EMF
electromotive force
EMS
Electromechanical System
GHG
greenhouse gas
IGBT
insulated gate bipolar transistor
LVDAC
Lab-Volt Data Acquisition and Control
MOSFET
metal-oxide-semiconductor field-effect transistor
PWM
pulse-width modulation
PI
proportional and integral
USB
universal serial bus
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143
Bibliography
Jackson, Herbert W, Introduction to Electric Circuits, 8th ed. Oxford: Oxford
University Press, 2008, ISBN 0-19-542310-0
Linden, David, and Reddy, Thomas B., Handbook of Batteries, 3rd ed. New York:
McGraw-Hill, 2002, ISBN 0-07-135978-8.
Wildi, Theodore, Electrical Machines, Drives, and Power Systems, 6th ed. New
Jersey: Pearson Prentice Hall, 2006, ISBN 0-13-177691-6
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A DC Power Electronics
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