IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 4, APRIL 2009
979
Family of Soft-Switching PWM Converters With
Current Sharing in Switches
Ehsan Adib, Student Member, IEEE, and Hosein Farzanehfard, Member, IEEE
Abstract—In this paper, a new family of soft-switching
pulsewidth modulation (PWM) converters is introduced. In
this family of converters, two switches operate out of phase and
share the output current while providing soft-switching condition
for each other. A buck converter, from this family of converters,
is analyzed and its operating modes are discussed. The adoption
of regular PWM control circuit to the proposed converters is presented. A prototype converter is implemented and its experimental
results are illustrated.
Index Terms—DC–DC power conversion, zero-current (ZC)
switching, zero-voltage switching.
I. INTRODUCTION
N ORDER to increase the efficiency and power conversion
density, soft-switching techniques are vastly applied to
dc–dc converters. Resonant and quasi-resonant converters are
a family of soft-switching converters. In these converters, a
resonant tank is added to the converter. Thus, resonances occur
in the switch current or in the voltage across the switch. During
these resonances when the switch current or voltage reaches
zero, the switch can be turned on or off under soft-switching
condition. Since the switch-on time or switch-off time is limited by the resonance period, so the converter output power is
usually controlled by variation of switching frequency. In order
to improve these converters, zero-voltage transition (ZVT)
and zero-current transition (ZCT) converters are developed. In
these converters, the resonances are limited only to switching
instances, and therefore the converter operates like a regular
pulsewidth modulation (PWM) converter. In these converters,
an auxiliary circuit that provides soft switching is connected
to the converter by an auxiliary switch at switching instances.
In ZVT converters, by turning the auxiliary switch on, the
output capacitor of the main switch is discharged to provide
zero-voltage switching condition for switch turn-on. In ZCT
converters, by turning the auxiliary switch on, the main switch
current is reduced to zero for switch turn-off. In ZVT converters, soft-switching condition for switch turn-off is provided
by adding a capacitor across the main switch, and in ZCT
converters, a series inductor provides soft-switching condition
for switch turn-on. ZVT and ZCT converters have the advantages of resonant and quasi-resonant converters suchas soft
I
Manuscript received July 30, 2008; revised September 24, 2008. First published January 23, 2009; current version published nulldate. Recommended for
publication by Associate Editor F. Z. Peng.
The authors are with the Department of Electrical and Computer Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran (e-mail:
adib.ehsan@gmail.com; hosein@cc.iut.ac.ir).
Digital Object Identifier 10.1109/TPEL.2008.2008022
switching and low electromagnetic interference (EMI), while
the converter output power is still controlled with variation of
duty cycle like PWM converters.
In ZVT and ZCT converters, an auxiliary circuit containing
resonant elements and an auxiliary switch is used that provide
soft switching at switching instances and is usually incapable of
transferring energy from an input source to output [1]–[20]. In
some of these converters or some members of converter family,
the auxiliary circuit can boost the effective duty cycle, but the
amount of energy that is transferred through the auxiliary circuit
cannot be controlled once the converter is designed [14]–[18]. In
the ZVT converter family introduced in [19], the output current
can be shared between main and auxiliary switches even though
the authors did not have the intention of current sharing for these
converters. Nevertheless, in these converters, the current stress
of the auxiliary switch in current sharing condition is very high.
Besides, in this converter family, the auxiliary switch turn-off is
not soft. In ZCT converters introduced in [20], the output current
is shared between the switches; however, the switches do not
turn off under soft-switching condition.
This paper introduces a new family of soft-switched PWM
converters. In this converter family, two switches share the
output current while providing soft-switching condition for
each other. The buck converter from this converter family is
analyzed and its operating modes are discussed in the second
section. In the third section, the design considerations are
discussed. In the fourth section, adopting conventional PWM
controllers to proposed converters is presented. Experimental
results are illustrated in the fifth section. Other proposed converter family members are introduced in the sixth section.
II. CIRCUIT DESCRIPTION AND OPERATION
The proposed soft-switching switch cell is shown in Fig. 1(a)
and is applied to a buck converter, as shown in Fig. 1(b). The
and
proposed buck converter is composed of two switches
, two diodes
and
, two coupled inductors
and
with turns ratio of 1: , filter inductor , and filter capacitor
. The snubber capacitor of
is
. The converter has seven
different operating intervals in a switching cycle. To simplify
is large
the converter analysis, it is assumed that inductor
enough so that its current is almost constant in a switching cycle
and is equal to . Also, the input voltage is assumed constant
and is equal to
in a switching cycle. The main theoretical
waveforms of the proposed buck converter are shown in Fig. 2,
and the equivalent circuit for each operating interval is shown in
is charged
Fig. 3. Before the first interval, it is assumed that
to
, diode
is conducting, and all other semiconductor
devices are OFF.
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 4, APRIL 2009
Interval 1
: This interval starts by turning
on,
current
and thus input voltage is placed across . Inductor
equation during this interval is
(1)
According to (1), zero-current (ZC) switching condition is
provided for turn-on.
voltage stress during this interval is
(2)
This interval ends when
current reaches and
turns
off under ZC condition.
: In this interval, a resonance starts
Interval 2
between
and , and this capacitor is discharged until its
voltage and
current during this
voltage reaches zero.
interval are
Fig. 1. (a) Proposed soft-switching switch cell. (b) Proposed soft-switching
buck converter.
(3)
(4)
where
(5)
(6)
: In this interval, either
or the body
Interval 3
may start to conduct. If the semiconductor devices
diode of
are assumed ideal, this interval cannot be analyzed. In practice,
the body diode of starts to conduct only if the voltage across
is reduced to
where
is the conducting
and
is the conducting voltage of
body
voltage of
diode. At this condition, the voltage across
is
and the voltage across
is
, which
, and therefore
is already foris equal
must be conducting. Since is large
ward biased, and thus
(i.e., > 5), once
is conducting, the voltage across
body
diode is very small to be forward biased for any reasonable circuit elements. The experimental results presented in Section V
approve this fact. It is important to notice that large value of
is desirable as discussed in Section III. Therefore, in practice,
always starts to conduct. Since the total ampere turns of
and
is constant and also
current should be equal to sum
and current, the relevant equations for
and
curof
rents during this interval are
(7)
Fig. 2. Main theoretical waveforms of the proposed buck converter.
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(8)
ADIB AND FARZANEHFARD: FAMILY OF SOFT-SWITCHING PWM CONVERTERS WITH CURRENT SHARING IN SWITCHES
Fig. 3. Equivalent circuit for each operating interval of the proposed circuit (only semiconductor devices that carry current are shown). (a) [ t
(c) [t
t ]. (d) [ t
t ]. (e) [ t
t ]. (f) [t
t ]. (g) [ t
t
T ].
0
0
0
0
0 +
In this interval,
is ON and energy is transferred from the
input voltage source to output. Any time during this interval,
can be turned on under zero-voltage zero-current (ZVZC)
since its current
conditions. The ZC condition is due to
.
remains constant and no current flows through
: This interval begins with turning
Interval 4
off and since
and
are ON, this switch is turned off under
zero-voltage (ZV) condition. Since the total ampere turns of
and
should remain constant,
and
currents during this
interval are
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0t
]. (b) [ t
is charged with
current until its voltage reaches
fore, the duration of this interval is
0t
].
. There-
(11)
Interval 6
: In this interval,
begins to conduct and
is placed across
till its current reduces to zero. Therefore,
the duration of this interval is
(12)
(9)
voltage during this interval is
(10)
is small,
and
have a very small
In practice, since
off, the energy of this leakage
leakage inductor. By turning
inductance is absorbed by
output capacitor and a small
voltage will occur across this switch. Therefore, S1 turns off
under almost ZV condition. This effect can be observed in the
experimental results. During this interval, the energy is still
transferred from the input source to output.
: This interval begins by turning
off
Interval 5
starts charging. Since the duration of this interval is
and
current can be assumed almost constant, and thus
small,
(13)
Interval 7
:
is conducting during this interval
and the converter operates like a regular buck converter.
III. DESIGN CONSIDERATIONS
The filter inductor and filter capacitor are designed like a regular PWM buck converter. Therefore, it is important to select
,
, , and semiconductor devices.
is the snubber caand its value can be calculated like any turn-off
pacitor of
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 4, APRIL 2009
Fig. 4. Schematic of the converter controller.
snubber [21].
is the turn-on snubber of
and its value can
be calculated like any turn-on snubber too [21]. When and
are ON, an additional circulating current stress is applied to these
switches that can be calculated from (7), (9), and (10). As it can
be observed from these equations, this additional current stress
can be reduced to any extent with selection of large values for
and . If necessary, in order to increase ,
can be overdesigned. Large value of will also decrease the voltage stress
of , which can be calculated from (14). However, this will inthat is calculated from (2), which
crease the voltage stress of
is a minor concern. Therefore, can be selected between 5 and
current should be
10 or even higher. In the seventh interval,
decreased to zero
(14)
is the converter maximum duty cycle and is the
where
switching period. The previous equation can be simplified as
follows:
(15)
has a limitation that can be calculated from
Therefore,
the previous equation. Also, the converter minimum duty cycle
is limited to the duration of first and second intervals. Therefore
(16)
IV. ADOPTING CONVENTIONAL PWM CONTROLLERS WITH
THE PROPOSED CONVERTER
The schematic of the controller for the proposed converter
is shown in Fig. 4 . The output gate pulse of the conventional
PWM controller is applied to a derivative circuit, and then to
a Schmitt trigger buffer (like ICL7667). By tuning the derivative elements, the output of the Schmitt trigger buffer is a pulse
where
is conwith maximum duration of
verter maximum operating duty cycle that occurs at nominal
load. This pulse is applied to . The output pulse of the conventional PWM converter is also applied to an integrator cir-
Fig. 5. Schematic of the implemented circuit.
cuit, and then to a Schmitt trigger buffer. By tuning the integrator elements, the output of this buffer is a pulse with maxand delay of
. This
imum duration of
. With this circuit, at converter
pulse is a proper pulse for
nominal duty cycle, two pulses with equal duration are applied
to the switches and output current is equally shared between
the switches. At lower operating duty cycles, the duration of
pulse is decreased while duration of
pulse remains equal
to
. With this circuit, the conventional PWM controllers can be simply adopted for controlling the proposed con, only
verter. If the duty cycle decreases to less than
S1 turns on. In this condition, turn-off losses are less than reg, and this switch turn-off is under
ular buck converters due to
almost ZV condition.
V. DESIGN EXAMPLE AND EXPERIMENTAL RESULTS
A 200-W laboratory prototype operating at 100 kHz is implemented. The converter input voltage is around 100 V and its
output voltage is 40 V. According to [21] and considering 2-A
current ripple for , the value of this inductor is calculated as
100 H. Also, a 50- F capacitor is used as the output filter capacitor to have less than 0.2-V output voltage ripple. Since the
voltage stress of switches is approximately 100 V, IRF640 is
used for switches. By substituting the specifications of IRF640
from its datasheet in the equations presented in [21], the minand
are calculated as 0.8 H and 1.8 nF,
imum value for
respectively. However, in order to clearly verify the achieved
and a
soft-switching condition, a 10-nF capacitor is used for
. In an ideal buck converter with
10- H inductor is used for
aforementioned input and output voltage levels and switching
frequency, the switch is ON for 4 s and is OFF for 6 s. Since
0.5 s of the duty cycle is lost due to
in the first interval, so
the switch-on time should be 4.5 s. Also, considering 90% efficiency for the converter at the worst case condition and input
voltage ripple, the maximum switch-on time is approximately
5 s. Therefore, according to (15) , with the selected value of
, is limited to 7. The complete implemented circuit and its
parameters are shown in Fig. 5. In order to implement couple
and , EE-19 ferrite core with five turns winding
inductors
for
and a very small air gap is used. Also, an EE-30 fer-
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ADIB AND FARZANEHFARD: FAMILY OF SOFT-SWITCHING PWM CONVERTERS WITH CURRENT SHARING IN SWITCHES
983
Fig. 6. Waveforms: (top) voltage waveform and (bottom) current waveform.
(a) S (vertical scale is 80 V/division or 5 A/division, time scale is 1 s/division). (b) S (vertical scale is 80 V/division or 5 A/division, time scale is 1
s/division). (c) D (vertical scale is 80 V/division or 5 A/division, time scale
is 1 s/division). (d) D (vertical scale is 200 V/division or 2 A/division, time
scale is 1 s/division).
Fig. 7. Efficiency of the proposed soft-switching buck converter (continuous
line) in comparison with the regular buck converter (broken line).
Fig. 8. Other basic soft-switching dc–dc converters. (a) Boost. (b) Buck–boost.
(c) Cuk. (d) SEPIC. (e) Zeta.
rite core with 30 turns winding and 1 mm air gap is used for
implementation of . A high-voltage diode (BYV26E) is used
for . Usually, high-voltage diodes have high reverse recovery
time, but since this diode is in series with a large inductor ( ),
its reverse recovery time is not so important. The experimental
results are presented in Fig. 6 that justifies the theoretical analysis. The converter efficiency curve is presented in Fig. 7. The
efficiency of the hard switching converter is for a buck converter
with same parameters using IRF640 for its switch and BYV32
for its diode. In theoretical analysis, it was predicted that current remains zero until is turned off. However, in practice due
current has increased before is
to conducting voltage,
current does not remain constant as specified
turned off and
in the third interval. This is a desirable effect since it decreases
the converter circulating current and also reduces the leakage
inductance energy.
VI. OTHER SOFT-SWITCHED CONVERTERS
The proposed switch cell can be used instead of converter
switch in any basic dc–dc converter such as buck, boost,
buck–boost, Cuk, SEPIC, and zeta. Also, the proposed switch
cell can be applied to single-switch isolated converters such as
forward, flyback, isolated Cuk, and isolated SEPIC converters.
The operation of this auxiliary circuit in these converters is
similar to its operation in the buck converter. These converters
are shown in Figs. 8 and 9.
VII. CONCLUSION
In this paper, a new soft-switching switch cell is introduced
that can be applied in dc–dc converters instead of their switch.
This switch cell is composed of two switches that provide soft-
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 4, APRIL 2009
Fig. 9. Isolated soft-switching converters. (a) Forward. (b) Flyback. (c) Isolated
Cuk. (d) Isolated SEPIC.
switching condition for each other. Furthermore, the converter
output current can be shared between the switches. The proposed soft-switching buck converter is analyzed and the presented experimental results confirm the validity of the solution.
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Ehsan Adib (S’08) was born in Isfahan, Iran, in
1982. He received the B.S. and M.S. degrees in electrical engineering in 2003 and 2006, respectively,
from the Isfahan University of Technology, Isfahan,
Iran, where he is currently working toward the Ph.D.
degree in electrical engineering.
His current research interests include softswitching techniques in dc–dc converters.
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ADIB AND FARZANEHFARD: FAMILY OF SOFT-SWITCHING PWM CONVERTERS WITH CURRENT SHARING IN SWITCHES
Hosein Farzanehfard (M’08) was born in Isfahan,
Iran, in 1961. He received the B.S. and M.S. degrees
in electrical engineering from the University of Missouri, Columbia, in 1983 and 1985, respectively, and
the Ph.D. degree from Virginia Polytechnic Institute
and State University, Blacksburg, in 1992.
Since 1993, he has been a faculty member in the
Department of Electrical and Computer Engineering,
Isfahan University of Technology, Isfahan, Iran,
where he is currently an Associate Professor and the
President of the Information and Communication
985
Technology Institute. His current research interests include high-frequency
soft-switching converters, pulse power applications, power factor correction,
active power filters, and high-frequency electronic ballasts. He is the author or
coauthor of more than 70 technical papers published in journals and conference
proceedings.
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