lu2012

A photovoltaic panel emulator using a buck-boost DC/DC
converter and a low cost micro-controller
Dylan D.C. Lu
⇑
, Quang Ngoc Nguyen
School of Electrical and Information Engineering, The University of Sydney, NSW 2006, Australia
Available online 7 March 2012
Received 5 September 2011; received in revised form 8 February 2012; accepted 9 February 2012
Available online 7 March 2012
Communicated by: Associate Editor Nicola Romeo
Abstract
In order to facilitate the design and testing of photovoltaic (PV) power systems, a PV emulator which models the electrical charac-
teristic of a PV panel or array is needed. Among different approaches to modeling PV characteristic, namely the I–Vcurve, curve-fitting is
a popular approach. Even though a single high-order polynomial equation may accurately represent the I–Vcurve, the process of der-
ivation and implementation is rather complex. This paper hence proposes the use of piecewise linear approach which is easier to derive
and implement in a low-cost micro-controller. A two-switch buck-boost DC/DC converter is selected as the PV emulator and is analyzed.
Experimental results on a hardware prototype of the proposed PV emulator are reported to show the effectiveness of the approach.
Crown Copyright Ó2012 Published by Elsevier Ltd. All rights reserved.
Keywords: DC/DC converter; Photovoltaic; Micro-controller; Emulator
1. Introduction
The demand of photovoltaic (PV) power system installa-
tion has been increased over the past decade due to techno-
logical improvement, better environmental awareness,
lowered system costs, governmental initiatives, rising
electricity bills, etc. While these installed PV systems and
products are operating properly, there are still ongoing
issues to be investigated and solved. For example, reliability
of PV power systems (Petrone et al., 2008), PV power gener-
ation analysis (Ishaque et al., 2011; Paraskevadaki and
Papathanassiou, 2011) and electricity network performance
(van der Borg and Jansen, 2003) due to partial shading,
development of power electronics interfaces (Marsh, 2011,
2010), etc. All these research and development activities
require a stable, repeatable and variable PV source for
design and testing. Hence there is a need of a PV generator
emulator.
The main task for a PV generator emulator is to repro-
duce the I–Vcurve of a practical PV panel. There are differ-
ent approaches to performing this task. In Nagayoshi
(2004), a p–n photodiode is used and a DC power amplifier
increases the power level to match with that of a PV panel.
However, this approach requires a light source and associ-
ated circuit to reproduce the I–Vcurves of a PV panel. In
fact, a power electronics converter can mimic the I–Vcurve
accurately with only a DC input voltage source (Mukerjee
and Dasgupta, 2007). In Khouzam and Hoffman (1996),a
AC/DC buck converter is used as the PV emulator to emu-
late a PV cell circuit model. However this approach requires
the knowledge of the values of the parameters which are
usually difficult to obtain. In fact, to model a PV panel,
one may use the data available from the datasheet of the
PV panel manufacturer and derive an analytical model to
represent the I–Vcurves (Ortiz-Rivera and Peng, 2005).
0038-092X/$ - see front matter Crown Copyright Ó2012 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.solener.2012.02.008
⇑
Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address: [email protected] (D.D.C. Lu).
www.elsevier.com/locate/solener
Available online at www.sciencedirect.com
Solar Energy 86 (2012) 1477–1484

Look-up table and curve fitting are two popular
approaches to implementing I–Vcurves of a PV panel by
the power electronics converters. Look-up table would
require a large memory storage of the micro-controller as
large amount of panel data is stored if many I–Vcurves
at different conditions and with high accuracy are imple-
mented. Hence to implement look-up table in a low cost
micro-controller which has limited memory space is usually
difficult. Curve-fitting approach in general uses one or
more polynomial equations to model an I–Vcurve and
needs a digital controller with fast computational speed
to find the solution. While this method requires less mem-
ory space, the non-linearity of the I–Vcurve requires the
equations to be of higher orders which may increase the
computational time substantially. A powerful DSP control-
ler is usually needed to produce very fast and accurate
results (Zhang and Zhao, 2010). Also the derivation pro-
cess of the polynomial equations for different conditions
such as insolation and temperature is rather troublesome.
In order to use curve-fitting efficiently on a low-cost
micro-controller, this paper introduces a PV emulator
using multiple simple linear equations to mimic an I–V
curve of the PV panel. This approach reduces computa-
tional time while maintaining sufficient accuracy and can
be implemented in a low-cost 8-bit micro-controller. The
paper is organized as follows: Section 2describes the circuit
and operation principle of the proposed PV emulator. Sec-
tion 3reports the experimental results of the emulator
which models a BP Solar SX-10 PV panel. Section 4dis-
cusses the limitations of the emulator and followed by
the conclusions in Section 5.
2. Description of the PV emulator
2.1. System overview
The PV emulator, as shown in Fig. 1, consists of a DC
input source, V
in
, a DC/DC converter for shaping the
output I–Vcurves of the PV panel, a micro-controller for
sensing the output voltage v
pv
and current i
pv
, calculation
and sending duty cycle command, and a gate driver for
amplifying the incoming duty cycle command suitable for
driving the power transistor (MOSFET in this case). The
output load R
L
is modeled as a variable resistor to repre-
sent an equivalent resistance of a maximum power point
tracker (MPPT).
2.2. Mathematical modeling of a PV panel
Apart from measuring an actual PV panel, one can also
use an analytical model to represent the data in the data-
sheet from the manufacturer to obtain the I–Vcurves of
a specific PV panel. In Ortiz-Rivera and Peng (2005), the
authors have generated an analytical model for a PV panel
which is adopted in this paper:
IðVÞ¼aImax si
1exp V
bðacþ1cÞðVmax þsVÞ1 b
ð1Þ
where ais the percentage of effective intensity of the light, b
is the characteristic I–Vcurve constant, cis the shading lin-
ear factor, s
i
is the rate of change with the temperature for
the current (A/°C), s
V
is the rate of change with the temper-
ature for the voltage (V/°C) and I
max
is the ideal maximum
current (when V=1 at STC).
For this paper, a PV panel from BP Solar (Model: SX-
10) is modeled. Assuming no shading and using a= 1 and
others values provided by the datasheet (BP Solar PC SX-
10 data sheet, 2003), a numerical expression of this PV
panel can be found:
IðVÞ¼ 0:65
1e1=b1V b21 1 b
 ð2Þ
Using the maximum power point condition at 16.8 V and
0.59 A, the value of bcan be calculated by (2) as
0.085. At 25 °C, (2) can be further simplified to:
IðVÞ¼0:65½1eðV=1:78511:7647Þð3Þ
Similarly at 75 °C one can get:
IðVÞ¼0:6711½1eðV=1:44511:7647Þð4Þ
Fig. 2 shows the MATLAB plot of the I–Vcharacteristic
curves of SX-10 PV panel Eqs. (3) and (4).
2.3. Two-line and multiple-line fitting approaches
To generate Nnumber of fitting lines, N+ 1 points from
the curve need to be selected. To begin with, a two-line
approach as shown in Fig. 3 is discussed. The two ends points
from the curve are the open-circuit voltage (21 V, 0 A) and
short-circuit current (0 V, 0.65 A). The third point is selected
(16.8 V, 0.62 A) as the maximum power point (MPP) of the
curve where the two lines converge. Therefore the two equa-
tions which represent the two lines are expressed as
IðVÞ¼0:65 0:004Vð5Þ
IðVÞ¼2:94 0:14Vð6Þ
Fig. 1. Block diagram of the PV emulator.
1478 D.D.C. Lu, Q.N. Nguyen / Solar Energy 86 (2012) 1477–1484

To improve the accuracy of the curve-fitting method, five
lines as shown in Fig. 4 are used. Similarly, with six points
selected, the five equations which represent the five lines are
given by
IðVÞ¼2:94 9:2143e4V½for V¼0–14ð7Þ
IðVÞ¼0:8233 0:0133V½for V¼14–16ð8Þ
IðVÞ¼1:2633 0:0408V½for V¼16–18ð9Þ
IðVÞ¼2:1651 0:0909V½for V¼18–19ð10Þ
IðVÞ¼4:599 0:2190V½for V¼19–21ð11Þ
2.4. DC/DC converter
Among the basic converters, buck and buck-boost
converters are able to be implemented as the DC/DC
converter for the PV emulator. The design consideration
is that the converter is able to sweep through the entire
voltage range of the PV panel. For a buck converter, a
DC input voltage which is higher than the panel is required
as it is a step-down converter. The buck-boost converter is
more flexible as it can perform both step-up and step-down
functions. The boost converter which is a step-up con-
verter, however, can only operate when the input voltage
is lower than the output voltage hence it cannot reach
down to 0 V and cannot be used in this case.
3. Experimental setup and results
3.1. Design considerations and hardware description
To verify the proposed PV emulator for the BP SX-10
model, a hardware prototype is built and tested. The sche-
matic of the PV emulator circuit is shown in Fig. 5. The rea-
son to select a two-switch buck-boost converter for this
implementation is twofold. Firstly it can work with lower
input voltage (12–15 V) that reduces the power loss when
the input voltage is used to step down further for the
micro-controller circuit (5 V). If a buck converter is used,
the input voltage has to be higher than 21 V which is the
open-circuit voltage of the PV panel. Secondly, the output
voltage is non-inverting as compared to the single-switch
buck-boost converter with inverting output. The advantage
is that the output ground will be the same for the input
ground and ground of other equipment which is connected
to the output of the PV emulator, e.g. a MPPT switching
converter. Another advantage is that the use of either split-
ting power supply or opto-coupler and associated circuit for
negative voltage feedback sensing, which added complexity
and slowed down the response of the system, is eliminated.
The two-switch buck-boost converter consists of two
power MOSFETs (Q1 and Q2), two diodes (D1 and D2),
an inductor (L1) and an output capacitor (C1), as shown
Fig. 2. I–Vcharacteristics of SX-10 at 25 °C and 75 °C.
Fig. 3. Two-line curve fitting approach.
Fig. 4. Five-line curve fitting approach.
D.D.C. Lu, Q.N. Nguyen / Solar Energy 86 (2012) 1477–1484 1479

in Fig. 5. The two power transistors Q1 and Q2 share the
same driving pulses from the Gate Driver block. When
both transistors are closed, the inductor is charged by the
input voltage. When the transistors are turned off, the
inductor is discharged via two diodes (D1 and D2) to out-
put. By controlling the duty cycle of the transistors as a
result of the linear equations calculation as stated in Sec-
tion 2.2, the I–Vcurve of the PV panel is implemented.
For the micro-controller, a 8-bit PICAXE AXE-08M
chip is selected. It is a modified version of Microchip
PIC12F68 model. It has three ADC inputs and a PWM
output pin which is easily configured by a single-line com-
mand. The chip runs at 4 MHz and therefore it can operate
easily at 50 kHz switching frequency for the converter with
sufficient accuracy and speed.
The inductor (L1) is chosen to operate in continuous
conduction mode (CCM) as it produces less conduction
loss. A ±20% maximum current ripple in the inductor is
selected. At 50 kHz switching frequency, the minimum
inductance L
min
to meet such requirement is at 15 V input
and 21 V output (open-circuit voltage)
Lmin ¼Vin D
fsDi¼15 0:58 50000 0:4¼435 lHð12Þ
where V
in
is the input voltage, Dis the duty cycle of the
power switches, f
s
is the switching frequency, and Diis
the maximum current ripple.
In order to provide at least 12 V and floating gate drive
for the MOSFET, a high-side driver IR2117 is used. Since
the PICAXE chip operates at 5 V but the IR2117 driver
requires at least 9.6 V input, a simple transistor inverter
as a level lifting circuit is implemented. This small transis-
tor (2N7000) requires only 2.5 V to drive. When a high
pulse from PICAXE chip is generated, the transistor is
turned on and pulled the output to ground. When a low
pulse is generated, the transistor is turned off and output
is risen up to Vcc (12–15 V in this design).
The programming for PICAXE is done on a free pro-
gramming editor provided by PICAXE. Once the program
is written it can be downloaded to the PICAXE AXE-08M
micro-controller chip via a USB cable (AXE027) or RS-232
cable (AXE028) connecting the computer and the chip
(Download Socket in Fig. 5). This cable is not a normal
USB or RS-232 connector as it contains some electronic
parts and it is pre-programmed. The programming lan-
guage for PICAXE is similar to the BASIC language.
The program flow chart is shown in Fig. 6. The program
starts with defining the symbols for voltage and current
measurements and for counters. Then it outputs a small
duty cycle to start the buck-boost converter. Once the con-
verter operates, the program can take readings from the
voltage and current (V_sense and I_sense in Fig. 5)to
determine the operating point for the PV panel the con-
verter mimics. Note that we are using fractional open
voltage MPPT algorithm so only a voltage reading is
needed to operate the converter to the desired operating
point. The current measurement is only used for overload-
ing protection. The PICAXE-08M chip can contain maxi-
mum of 8 linear equations so we have used 5 linear
equations for 75 °C and 3 linear equations for 25 °C. The
Fig. 5. Schematic of the proposed PV emulator based on a buck-boost converter.
1480 D.D.C. Lu, Q.N. Nguyen / Solar Energy 86 (2012) 1477–1484

selection of what temperature to use is by providing a high
or low signal to available ADC input of the PICAXE (Pin
4). Here we selected signal high for 75 °C and low for
25 °C. After the temperature is selected, the program will
use the voltage reading to locate the nearest linear equation
to find out the operating point. A new duty cycle is then
determined and produced from the PWM pin of the PIC-
AXE and the converter duty cycle is updated. After that
the program goes back to the voltage and current checking
process and repeats the procedure. The full program code
for implementing the five-line approach is shown in Appen-
dix A.
3.2. Results
Fig. 7 shows the output voltage ripple of the PV emula-
tor is less than 450 mV at 50 kHz switching frequency. The
“On”and “Off”labels in the figure indicate the turn-on
instant and turn-off instant of the power transistors Q
1
and Q
2
for each switching period. The duration of “On”
period indicates the duty cycle of the transistors. And from
the waveform it can be observed that the converter is oper-
ating in continuous conduction mode (CCM) as the ripple
has only two stages, confirming our design in Section 3.1
and Eq. (12). Note that the voltage spikes during the
turn-on and turn-off instants and appeared beyond the hor-
izontal cursors are due to the pick-up of electromagnetic
noise from the voltage probe. Minimizing the ground loop
of the voltage probe will greatly reduce the pick-up. Fig. 8
shows the measured results on the prototype using the two-
line approach. The results are very close to the two operat-
ing lines. Fig. 9 shows further results on the five-line
approach at both 25 °C and 75 °C. Due to the memory
limitation of the micro-controller, only eight lines can be
implemented in a single program. Therefore Fig. 9 shows
5 lines for 25 °C and 3 lines 75 °C. Nevertheless, the mea-
sured results are closely matched with the theoretical
designed curves.
As shown in Fig. 10, the PV emulator reaches a maxi-
mum efficiency of around 80% near maximum power point
voltage for both temperature settings. The low efficiency
occurs at lower voltages because the output power of the
power converter is small and the switching losses of power
transistors and diodes are dominant. When PV emulator
Fig. 6. Program flow chart for implementation of 5-line for 75 °C and 3-
line for 25 °C of the BP SX-10 PV panel.
Fig. 7. Output voltage ripple of the PV emulator (Y-axis: 200 mV/div;
time scale: 10 ls/div).
Fig. 8. Comparison between experimental and theoretical results on two-
line approach.
D.D.C. Lu, Q.N. Nguyen / Solar Energy 86 (2012) 1477–1484 1481