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6. CURRENT FED TWO-INDUCTOR BOOST CONVERTER
Parts of this chapter have been published in the Australian Journal of Electrical &
Electronic Engineering in 2004 and in the Proceedings of AUPEC 2003, 2004 and
2005, APEC 2005 and PESC 2005.
Chapter 2 has shown that MIC implementations with an unfolding stage are able to
avoid the complex circuit design and the high switching losses associated with the
PWM control technique in the dc-ac inversion stage. It has been shown in Chapter 3
that a buck conversion stage must be used as the current source for the two-inductor
boost converter so that the rectified sinusoidal waveforms can be generated at the
output and an unfolder can be employed in the dc-ac inversion stage. This chapter
studies the current fed two-inductor boost converter in detail and provides the
experimental results of a 100-W converter with both the hard-switched and the soft-
switched topologies. While this approach does result in a rather long power train, it
is still possible to achieve adequate conversion efficiencies. One advantage is that
the boost cell can operate at fixed duty ratio and be optimised better as the buck
stage can perform most or all of the required voltage variations for the control.
6.1 Buck Conversion Stage
It has been shown earlier in the thesis that under the voltage source input, a variable
output voltage can be produced by varying the switching duty ratios in the hard-

247
switched two-inductor boost converter or by varying the switching frequency in the
soft-switched two-inductor boost converter. However, the two-inductor boost
converter is a boost derived converter and zero output voltage cannot be reached in
either the hard-switched or the soft-switched forms. In order to generate the
rectified sinusoidal waveforms at the output of the two-inductor boost converter, a
buck conversion stage must be added. Therefore the converters in Figures 3.10 and
3.11 can be developed.
Recently, multi-phase converter arrangements have been widely adopted as an
efficient approach to parallel multiple converters to provide high current output
[168]. Under multi-phase operation, the individual converter input and output
currents with an equal phase shift, which is the quotient of 360º divided by the
number of phases, are added together and the equivalent input and output current
ripple frequencies will be multiplied by the number of the phases. The converter
also has a smaller input or output current ripple magnitude as the current ripples in
the individual phases cancel [169]. This eases the requirement on bulky input and
output filter components such as inductors and capacitors. A two-phase buck
converter will be employed as the current source for the two-inductor boost
converter.
In order to feed the output from the two-phase buck converter to the input of the
two-inductor boost converter and make use of the two existing inductors in the boost
converter, an interphase transformer (IPT) is utilised. The IPT is a tapped inductor,
which has 1:1 turns ratio. The IPT has been previously used in the dc-dc converter

248
applications [170] and more widely in mains frequency, high pulse number rectifiers
[101]. The employment of the IPT enables the equivalent switching frequency of
the buck converter to be doubled without higher switching losses. The hard-
switched and the soft-switched two-inductor boost converters with a two-phase buck
converter are respectively shown in Figures 6.1 and 6.2.
E
L
1
L
2
D
4
D
3
T
2
C
O2
D
1
D
2
Q
1
Q
2
T
1
+
−
S
1
S
2
S
3
S
4
C
O1
+
−
v
O
Q
3
Q
4
T
2
v
C
Figure 6.1 Hard-Switched Two-Inductor Boost Converter with a Two-Phase Buck
Converter
E
D
4
D
3
C
O2
D
1
D
2
Q
1
Q
2
T
1
+
−
S
1
S
2
S
3
S
4
C
O1
+
−
v
O
T
2
v
C
L
1
L
2
C
1
C
2
L
r
Q
3
Q
4
T
2
D
Q2
D
Q1
Figure 6.2 Soft-Switched Two-Inductor Boost Converter with a Two-Phase Buck
Converter
The two-phase buck topology shown in Figures 6.1 and 6.2 can be further improved
by using the concept of the synchronous rectifier, where the diodes are replaced by
the MOSFETs. In a conventional converter which uses a diode in the load current

249
conduction path, the minimisation of the conduction power losses in the diode is
difficult as the reduction of the diode forward voltage drop below a certain level
presents a great challenge [171]. The synchronous rectifier is able to largely
improve the converter efficiency by replacing the diode with a MOSFET, as the
forward resistance of the synchronous MOSFET can be very low [172]. If the
synchronous rectifier is used, dead time must be applied between the turn-on of the
control and the synchronous MOSFETs to prevent “shoot-through”. A Schottky
diode is placed in reverse parallel with the synchronous MOSFET in the standard
design to stop the load current from flowing through the MOSFET body diode,
which normally has a higher voltage drop and inferior reverse recovery
characteristic.
The hard-switched and the soft-switched two-inductor boost converters, which are
fed from a sinusoidally modulated two-phase synchronous buck converter, will be
respectively analysed in detail in the following sections.
6.2 Hard-Switched Current Fed Two-Inductor Boost Converter
This section provides a detailed analysis of the hard-switched current fed two-
inductor boost converter.
6.2.1 Circuit Diagram
Figure 6.3 shows the hard-switched two-inductor boost converter with a two-phase

250
synchronous buck converter, where a resistive load is used.
E
L
1
L
2
D
4
D
3
C
O2
D
1
D
2
Q
1
Q
2
T
1
+
−
C
O1
v
C
+
−
v
H
+
−
+
−
v
2
+
−
T
2
T
2
Q
3
Q
4
Q
6
Q
5
v
1
v
O
R
S
1
S
2
S
3
S
4
v
T2p
+
−
Figure 6.3 Hard-Switched Two-Inductor Boost Converter with a Two-Phase
Synchronous Buck Converter
The converter in Figure 6.3 is a three stage converter including the buck, the boost
and the unfolding stages. The transfer functions of the individual stages can be
respectively found as:
EDv buckavgH
=
, (6.1)
avgH
boost
T
Cv
D
n
v,
2
12 −
= (6.2)
⎩
⎨
⎧
−
=onSandSv
onSandSv
v
C
C
O42
31
,
, (6.3)
where Dbuck and Dboost are respectively the duty ratios of the buck stage MOSFETs
Q1 and Q2 and the boost stage MOSFETs Q3 and Q4, nT2 is the transformer T2 turns
ratio, E is the converter input voltage, vH,avg is the boost stage average input voltage
over one equivalent buck stage switching period, vC is the boost stage output voltage
and vO is the converter output voltage.