International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
Analysis and Comparison of various
Soft-Switching Topologies for PSFB
DC-DC Converter with Additional
Auxiliary Circuits
Sudha Bansala, Lalit Mohan Sainib
bansal.sudha@gmail.com , lmsaini@gmail.com
I. 
Abstract— The phase-shifted full bridge (PSFB) Soft switched
-PWM converter is widely used in medium to high power
applications. These converters have many limitations like
reduced range of soft switching, conduction losses etc. To
overcome these limitations an additional auxiliary circuit is
used. The placement of this auxiliary circuit results in variation
in the converter’s performance. In this paper a detailed review
for these topologies is presented. The merits and limitations of
these topologies have been analyzed and their key features and
characteristics have been compared.
Lf2 Current doubler inductance (in henrys).
Llk Primary leakage inductance
m Turn-ratio of auxiliary winding
nA Turn- ratio of auxiliary transformer
td Dead time between MOSFET gate signals (in seconds).
Treset Primary current reset time (in second)
TS switching time period (in second)
Vd Input dc voltage (in volts).
VLlk
Voltage across leakage inductance of the transformer
(in volts).
Zr Impedance of the resonant circuit
Index Terms— Phase-shifted; resonant tank, reverse recovery;
III. INTRODUCTION
synchronous rectifier (SR); Adaptable soft switching;
zero-voltage switching (ZVS); zero-current switching (ZCS);
full-bridge converter; lagging leg; leading leg.
The operation of the Full- Bridge (FB) dc/dc converter at
high frequency is preferred as it reduces the size of the
magnetic circuit and hence reduces the overall size of the
converter, thus improving actual efficiency, achieving higher
performances as high quality waveforms and quicker
responses. But, as the switching frequency of pulse width
modulated (PWM) power converters increases, switching loss
becomes the dominant part of the total power dissipation. To
reduce the switching loss, soft switching techniques have been
used [1]-[9]. Zero-voltage transition (ZVT), zero-current
transition (ZCT), and active clamp techniques can be applied
to regular pulse width modulation (PWM) dc–dc converters,
especially isolated converters, to improve the efficiency and
overcome the mentioned problems caused by leakage
inductance. In these techniques, an auxiliary switch is added
to regular PWM converters to provide soft switching
condition. These techniques require a large circulating current
to maintain soft switching over wide variations in line voltage
and load resistance. These topologies have low switching loss
characteristics; but, the disadvantage is that they circulate
reactive energy during each switching cycle, and the
circulated energy can be as large as the converted energy.
This results in higher conduction loss that can offset the
reduction in switching loss.
II. NOMENCLATURE
C1 Leading-leg snubber capacitance (in farads)
Ca Auxiliary Capacitor (in farads)
Cc Coupling Capacitor (in farads)
Cf Output filter capacitance (in farads).
Ch Holding Capacitor (in farads)
Cp Parallel capacitor (C1║C2)
Cr Resonant Capacitor (in farads)
Csb1 Leading-leg snubber capacitance (in farads).
Csb2 Lagging-leg snubber capacitance (in farads).
D duty Cycle
fs Switching frequency (in hertz).
ILr resonant inductor current
IO Load Current (in amperes).
Ip,min Minimum current level of transformer primary side
(in amperes).
LAUX1 Leading-leg auxiliary inductance (in henrys).
LAUX2 Lagging-leg auxiliary inductance (in henrys).
1255
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
In addition to this, the advantages of full bridge pulse
width modulated (FBPWM) dc/dc converter at high
frequency are: Large reduction of electromagnetic
interference (EMI) and radio frequency (RF) noises;
Reduction of peaky voltage surge spikes, current ringing
caused by parasitic parameters and high di dt and dv dt
dynamic stresses in the power semiconductor switches and
disadvantages are : High component stress of voltage and
current and high switching losses.
To overcome the above mentioned problems, the
phase-shifted full-bridge (PSFB) soft switched PWM
techniques [10]-[14] are used for many applications; because,
it permits all switching devices to operate under soft
switching with a constant switching frequency by using circuit
parasitics such as transformer leakage inductance and power
device junction capacitance. In this configuration as shown in
fig. 1, switches in one leg of the full bridge connected in the
primary of the transformer conduct with a phase delay with
respect to the switches in the other leg. However, due to
phase-shifted PWM control, the converter has a disadvantage
that circulating current which is the sum of the reflected
output current and transformer primary magnetizing current
flows through the power transformer and switching devices
during freewheeling intervals. Due to circulating current, root
mean square (RMS) current stresses of the transformer and
switching devices are still high compared with those of the
conventional hard-switching PWM FB converter.
IO
Q1
Q3
D1
C1
D3
C3
L0
DR1
DR3
A
capacitance can be derived from the data sheet parameters.
Both energy sources are functions of load current, which
makes it difficult to sustain soft switching over a wide load
range. The major limitation of the these converters is that the
lagging switches will lose ZVS under light load condition,
since the energy stored in the leakage inductor is insufficient
to charge and discharge the switch intrinsic capacitors. Hard
switching operation and poor EMI performance are inevitable
in this case. If the large leakage inductor is used to achieve the
soft switching of lagging leg over wide load ranges, it causes
several serious problems such as large circulating energy,
effective duty cycle loss, and serious parasitic ringing across
the output rectifiers. A high leakage inductance also increases
the crossover conduction time of the output rectifiers, which
reduces the effective duty ratio on the secondary. Therefore,
to overcome these problems, several methods have been
proposed for the PSFB [15]–[38]. The zero-voltage
zero-current switching (ZV/ZCS) pulse width modulation
(PWM) converters are derived from the full-bridge
phase-shifted zero-voltage (FB–PS–ZVS) PWM converters,
can reduce the turn on and turn off switching losses and
circulating energy during the freewheeling interval [39]–[44].
The ZCS condition can be obtained by introducing an
auxiliary circuit into the primary or secondary side [45]–[49].
To increase the range of soft switching, an auxiliary circuit is
used to place in the converter’s circuit [50]–[68]. On the basis
of that these converters can be classified into various
categories. This classification has been discussed in section
III. The effect of these techniques on the conduction loss,
duty cycle loss, soft switching range etc., has been discussed
and compared in this paper.
Vin
C0
R0
B
Q2
Q4
D2 C2
D4 C4
DR2
DR4
Fig. 1 (a). Conventional PSFB converter
Fig. 1 (b). Phase-shifted waveform of PSFB converter
The mechanism for soft switching involves displacing
charge in the drain-to-source capacitances of the MOSFETs,
and it occurs in two distinct ways in the converter. The
MOSFETs internal diode conducts the primary current during
the delay after all the charge is displaced. The energy required
to displace the charge on the MOSFETs' nonlinear output
IV. SOFT SWITCHING CONVERTERS
In the soft-switched topologies, a high-frequency resonant
network is added to the conventional hard-switching PWM
dc/dc converters [69]-[71]. These soft-switched converters
have switching waveforms similar to those of conventional
PWM converters except that the rising and falling edges of the
waveforms are ‘smoothed’ and no transient spikes exist. The
soft switching PWM converter is the combination of
converter topologies and switching strategies that result in
zero–voltage and/or zero–current switching (ZVS and/or
ZCS). As a result, the switch voltage or current swings and
crosses zero points and, thus, create the soft-switching
conditions for the power devices [72]-[79]. The important
points to create the soft-switching conditions (ZVS or ZCS)
are: i) Resonance circulating energy be as minimum as
possible and it is completely decoupled from the main power
transfer to the load, ii) It should be enough to create the
soft-switching conditions (ZVS or ZCS), irrespective of the
variations in the load, and iii) When switching transition is
completed, the converter should revert back to the familiar
PWM mode of operation, so that the circulatory energy can be
minimized.
1256
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
Advantages of soft switching are: i) The switching losses
can be minimized, ii) The switch stresses can be reduced, and
iii) EMI can be prevented.
The output voltage of the converter is usually controlled by
PWM with constant switching frequency. Therefore,
depending on the chosen resonant circuit, different shapes of
voltage and current waveforms in the converter can be
obtained. This can lead to a different way of topology
classification. There can be many ways to classify soft
switching techniques, but here only PSFB topologies have
been considered.
Hence soft switching PSFB PWM
converters can be classified (Fig.1) as follows:
I. ZVS PWM converters, 2. ZCS PWM converters, 3.
ZV/ZCS PWM converters
a. Passive auxiliary circuit [86]-[92]
b. Active Auxiliary Circuit [93]-[96]
2. Secondary-side-assisted converters: In these converters an
auxiliary circuit is placed in the secondary of the
converter. In secondary-side-assisted ZV/ZCS converters
the auxiliary circuit prepares ZV/ZCS by suppressing the
load current from the isolation transformer, and
bypassing the load current through them. These
converters can be further classified as:
a. Active auxiliary circuit [97]-[100]
b. Passive Auxiliary Circuit [101]-[104]
A comparison of these techniques on the basis of
conduction loss, the duty cycle loss, the soft switching range,
the circuit complexity etc., is presented in this section.
A. Primary-Side-Assisted Converters
The circuits of conventional PSFB converter is given in
figure 1. The Primary of the converter circuit is shown in fig.
3(a). For the discussion of the working of various C0topologies
only circuit up to point A-B is taken. L0The secondary of the
circuit is shown in fig. 3(b) and it remains same for these
topologies and hence is not shown for every topology.
IO
Q1
D1
C1
Vin
B
C4
V. AUXILIARY CIRCUITS AT DIFFERENT POSITIONS
To extend the soft switching range and to minimize the
problems mentioned above auxiliary circuit is added into the
converter [79]–[85]. The function of the auxiliary circuit is to
control the auxiliary inductor current to realize soft switching
for the lagging leg according to the load current, since
switches lose their soft switching at low load. For obtaining
soft switching for wide load range, different auxiliary circuit
is added with main full bridge circuit. Hence, the converters
can be classified into various categories on the basis of
different type of auxiliary circuit used i.e. active auxiliary
circuit or passive auxiliary circuit and at different places i.e.
auxiliary circuit in the primary of the converter or the
secondary of the converter, as follows:
1. Primary-side-assisted converters: In these converters an
auxiliary circuit is placed in the primary of the converter.
In primary-side-assisted soft switched converters, the
primary current of the main transformer is reset to zero at
every half cycle, hence possibility of magnetic saturation
due to asymmetricity of circuits or transient phenomena
is reduced, which is a very attractive feature in dc–dc
converters with transformer isolation. These converters
can be further classified as:
A
Q2
Cr
A
Fig.2. Classification of soft switching converters
D 2 C2
Q4
D4
L0
DR3
A
C3
Q3
Lr
DR1
D3
C0 Ro R0
BB
DR2
DR4
Fig. 3(a): Primary of the converter b) Secondary of the converter
D2
D
D
2 Circuit:
1) Passive Auxiliary
2
In this, a passive auxiliary circuit is placed in the primary
side of the conventional PSFB converter. Various topologies
are discussed and the comparison of all the topologies has
been discussed here is given in Table I.
Topology A1 [86]: In this full-bridge converter is controlled
by phase-shift switching control method under heavy-load
condition (as shown in fig. 4). PWM switching is used under
light-load and burst PWM mode is used under standby
condition to further reducing the switching losses. In PWM
switching mode the circulating current is eliminated and
hence switching loss is reduced. Disadvantages of this circuit
are complex control circuit; dead time requires is a quarter of
the resonant period.
1257
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
A
B
il1
Q1
D1
Q3
C1
D3
C3
L1
Q1
Csb3
La
Q3
D1
D3
C
La2
R0
Lr
Vin
Csb1
CA1
Vin
Q3
Q1
La1
La1
Csb2
Csb4
CB1
Q2
Q4
D 2 C2
D4
Q2
C4
Q4
Topology A2 [87]: In this converter a passive regenerative
snubber is used. It is composed of a FB converter with a high
frequency linked transformer and passive snubbers (fig.5)
Q5
configured with energy regenerative circuit to prevent
Vin main
freewheeling current. The leakage inductance of the
transformer (Tm) helps to achieve ZCS and the passive
lossless snubber capacitor helps to achieve ZVS turn-off.
Conduction losses are more at low load & small at 50% to full
load.
Cs1
C1
Q3
Cs3
Dr1
Q2
D1
LP
D
2
C0
CB1
N2
N1
Q2
Q4
Q3
TRA
Dr2
Cs2
Q1
A
Ta
Ta
C2
D3
C1 this Q3
Topology A4 [89]: In
PSFB ZVSC3converter auxiliary
D
circuit consists of1 a Vin
low-power auxiliary transformer T RA
shown in Fig. 7. This auxiliary transformer T RA is used to
adaptively store a relatively small amount of energy into
primary inductor that is required for ZVS. Due to this, ZVS of
C4 load range with
the primary switches is obtained over a wide
Q4
greatly reducedD no-load
circulating
C
2
D4 energy and with
2
significantly reduced secondary-side duty cycle loss. Since
L0
the size of primary inductor is reduced, parasitic
ringing is
reduced but the cost of the circuit is more.
i1
1:n2:n2
D4
R0
Dr3
1:n1:n1
Vin
Q4
B
A
Fig. 6. ZVS full bridge DC-DC converter
Fig. 4. Full-bridge converter with current doubler
Q1
D2
L2
B
Dr4
Cs4
D2
Q1
Q4
Vin
CB2
A
Np/2
Ns
Np/2
B
Q2
Fig. 5. DC-DC converter with energy recovery transformer
Topology A3 [88] : The auxiliary circuit in this converter
comprises of (i) eight passive devices (Fig. 6), four drain-tosource snubber capacitors, each connected across one
switch, (ii) a capacitor voltage divider, and (iii) two
auxiliary inductors. With this auxiliary circuit, the full bridge
converter can achieve soft switching independent of line and
load conditions. The power ratings of inductors are ¼ of the
transformer for 500 W prototype, and this makes the
proposed topology seemingly less advantageous while for
higher power level up to 3 kW, the power transformer
significantly increases the size but the auxiliary inductor can
almost use the same core with a larger air gap.; Therefore,
for higher power level applications the size ratio will
become much lower.
Fig.7. A New PWM ZVS Full-Bridge Converter
Topology A5 [90]: In this converter auxiliary circuit
comprises of two capacitors which forms the capacitor
voltage divider, two magnetic components viz. 1:1 auxiliary
transformer and auxiliary energy storage inductor as shown in
Fig. 8. This circuit adaptively stores the energy in the
converter i.e. when the load current is low; the energy stored
is maximum and vice-versa. The capacitors placed on the
input dc bus allow low-impedance path for high-frequency
circulating current. Therefore, soft switching operation over
the entire conversion range is achieved without significantly
increasing the conduction loss.
1258
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
Q1
L0
Q3
Q1
Q3
Ca1
iLa
Va
V- La
La
Vin
Ca2
D1
R0
A
CS
C0
A
Io
Q4
Q2
B
IO
Q2
Q3
Tr
A
Vin
Ca1
ip
+ V1-
Ca2
B
- V2+
Q4
Q2
La VLa
Fig.9. An improved ZVS full-bridge DC-DC converter
Topology A7 [92]: In this circuit the resonant inductor is
replaced with a linear variable inductor (LVI) as shown in fig.
+ V1- current i.e.
10. This variable inductor is controlled with output
inductor has high value of inductance at low load and has low
value at high load. Thus, the required energy to obtain soft
switching operation at low load value is increased due to the
increased value of inductance. The soft switching operation
range is extended and dependency of soft switching operation
to the load current is decreased. By selecting the range of the
LVI properly, dead time control between gate drive signals of
the IGBTs in the same leg is not required. With proper
selection of the minimum and the maximum values of LVI,
nearly constant dead time (≈1μs) is obtained in the converter.
Dead time required is large in this converter.
C4
D4
Fig. 10. LVI controlled PSPWM converter
Topology A6 [91]: Two capacitors Ca1 and Ca 2 , the
Q1
Q4
C2
D2
Fig. 8. FBZVS converter with auxiliary circuit
auxiliary transformer Tr and auxiliary inductor La form the
auxiliary circuit for the PSFB converter (Fig. 9). The auxiliary
circuit is used to store energy for the ZVS operation and this
energy depends on the input voltage and the load current.
Hence, stored energy is minimum under full load condition
and progressively increases as the load current decreases.
Hence, the circulating energy, conduction losses, the duty
cycle loss and voltage ringing across the output rectifiers are
substantially reduced.
LS CS
C3
D3
LS
Vin
B
Ta
C1
After comparing all the topology in the Table I as shown
in the appendix for the passive auxiliary circuit, it is
observed that for higher power level Topology A3 is
showing best result as it is less costly and efficiency is more
than 97%, second best topology is topology A7
performance wise but it is costlier as two auxiliary
transformers are required.
2) Active Auxiliary Circuit
In these converters, an active auxiliary circuit is placed in
L0
the primary side of the PSFB. The auxiliary energy is
provided by employing a passive circuit in the primary circuit,
to help achieve soft
R0 switching, and is independent of the load
current. The topologies discussed here are compared and
compared in Table
II.
C0
Topology B1 [93] : In this converter, the energy stored in the
auxiliary circuit is adjustedIoby the load current to achieve soft
switching for the lagging switches in the entire full load range
and achieves a high efficiency. The auxiliary circuit is
composed of one inductor La and two auxiliary switches Q5
and Q6 as shown in fig. 11. The main switches are phase
shifted controlled, and Q1 and Q2 form the leading leg while
V1-Q4 form the lagging leg. The two auxiliary switches
Q3 +and
and the lagging switches form an auxiliary FB circuit which is
also phase shifted controlled. Q5 and Q6 form the lagging leg
in respect to Q3 and Q4. The shifted phase of the auxiliary FB
circuit is controlled by the load current, which determines the
peak current of the auxiliary inductor. The efficiency of the
proposed converter is slightly lower than the FB converter
without auxiliary circuit.
Q1
Q3
Q1
Q3
Q1
C1
D1
D5
Ip
Q4
Va
C5
Ia
Vin
Q2
La
Q4
LSIa
CS
Lr
Q2
Q5
Q5
C3
D3
Q6
Q4
C2
D2
A
C6
C4
D4
Q6
D6
IO
B
+ V1+ V1 -
1259
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
Fig.11. A PSFB Converter with Controlled Auxiliary Circuit & Switching
sequence of all switches
Topology B2 [94]: A PWM auxiliary switch is inserted
between the DC source and the full-bridge power stage to
regulate the output voltage. With the help of the auxiliary
switch (shown in fig. 12), soft switching operation of the four
main switches can be achieved easily over full line and load
ranges. These reduced switching losses are compensated for
the auxiliary switch’s losses and hence its efficiency
approximates to that of the PSFB converter. Two switching
frequencies are employed, one for the auxiliary switch and
other for the four main switches.
Q1
Q3
C1
D1
C3
D3
Ip
A
Vin
Q3
D2
D4
Fig. 12. A novel soft- switching converter
Q3
C1
Qa
C3
Q3
Q1
Q3
Da1
Dt2
C1
Q4
D1
D4
St
Fig. 14. PWM-bridge converter using fixed –edge gating scheme
Dt1
St
On comparing
allDt2abovementioned
topology in the Table
II as shown inL0the appendix for the active auxiliary circuit, it is
observed that for higher power level Topology B1 is showing VinC3
Q5
best result as its efficiency is around 94.5%, second best
R0
topology is B3 , having
efficiency 92.2%.
Q3
Lt
B. Secondary-Side-Assisted Converters
Topology B3 [95]: By adding a saturable inductor, auxiliary
capacitors, and auxiliary diodes to the conventional circuit,
the proposed circuit can effectively eliminate the turn-on and
turn-off switching losses of the auxiliary switches as shown in
fig. 13. Also, soft switching in wide load range is achieved
using this auxiliary circuit, which contains resonant
components out of the main power flow path without adding
the circulating energy. Auxiliary components used are large in
L0
numbers.
Da1
Q1
B
Dt1
Q1
C4
Q4
C2
D2
A
Lk
Lt
C0
Q2
Q5
D3
Ip
Vin
Lk
B
D1
Q2
D3
In these converters, an auxiliary circuit is placed in the
secondary side of Q4the conventional PSFB converter. In
secondary-side-assisted ZV/ZCS converters the auxiliary
circuit prepares ZCS by suppressing the load current from the
isolation transformer, and bypassing the load current through
them. A snubber circuit or an active clamp circuit can be used
Vas
in an auxiliary circuit.
These converters can be further classified on the basis of
auxiliary circuit used i.e. active auxiliary circuit or passive
auxiliary circuit. For the discussion of the working of various
topologies only the circuit up to point A-B is taken. The
primary circuit of the converter is shown in fig.3(a) and it
remains same for these topologies and hence it is not shown
for every topology. Only the circuit beyond point A-B is
shown and discussed.
R0
Ip
Q3
Ia
Vin
Ca
La
Db1
Llk
Q2
Q4
D2
C2
A
1) Active auxiliary circuit
SL
C4
D4
LSIa
Q4
Q2
Q4
CS
Qb
Qb
Cb
Qa
IO
B
Fig. 13. (a) FB-ZVT PWM dc/dc converter circuit (b) Gating sequence of
Io
all switches
+ V1 + V1-
Topology B4 [96] : A complementary fixed –edge gating
control scheme is used for the control of PWM bridge
converter. This gating scheme together with an optimum
design ensures soft switching for switches Q2, Q3 and Q4. But
soft switching range for the switch Q1 is 0% of rated load. To
ensure soft switching for switches Q1 an auxiliary circuit is
added as shown in fig. 14. The auxiliary switch has hard
turn-off but the current at the instant of turn-off is small.
In these converters, an active auxiliary circuit is placed in
the secondary side of the PSFB converter. Various topologies
are discussed below and compared in Table III in the
Appendix.
Topology C1 [97]: In this topology an active switch in series
with the capacitor is inserted in the rectifier circuit as shown
in fig. 15. By controlling this active switch moderately, ZVS
(for leading-leg switches) and ZCS (for lagging-leg switches)
are achieved without adding any lossy components or the
saturable reactor. Due to this circuit low duty-cycle loss and
small Treset is obtained. Required turn-on time of the
auxiliary active switch is given by
TS c 
n 2 Llk
I o ,max
Vc
1260
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
C0
L0
Q1
Fig.17. ZVS Converter with synchronousC0rectifier
Lo
DR1
A
V
DR3
L0
Ip
Dc
Sc
Vin
a
Co
Ro
B
Cc
DR2
Q2
DR4
Q3
Q1
Fig.15. FB PWM converter using secondary active clamp
Vin
Topology C2 [98] : In this topology, soft Cswitching for all
power switches is achieved by using
controlled output
Q2
SR1
rectifier with new lossless energy recovery turn-off
snubber
Q4
on the secondary side of the converter as shown
in fig. 16.
C
Active secondary
switches T5, L
Tr6 are used to reset
secondary
B
D A
Cr
and primary circulating
current and hence circulating current
Lmturn-off snubber is
is minimal. The purpose of the secondary
to transfer the leakage inductance energy
SR1 to the load.
1
Q1
Ia
Topology C4 [100] : In this topology, the auxiliary resonant
circuit consists of a switch and a capacitor
as shown in fig. 18,
La
Llk ZCS conditions to the primary lagging-leg
to provide
Q4 circuit set up a freewheeling path for
switches. This auxiliary
the filter inductor current during a short period and auxiliary
Qb
switch softly turns on and turns off, reduces
circulating energy
Sc
but high voltage stress appears at the auxiliary
switches. Most
problems are solved at the cheap cost of an auxiliary switch
and a capacitor.
+V
Cc
Lf
1-
Sa
2
2
Q2
Qa
Q3
A
Ca
Ro
Co
B
B
T5
io
Lo
Do Co
Cc5
A
Lss
Lss
B
Dcs
Dss
Fig. 18. ZVZCS-FB-PWM converter
Q1
While
comparing the data in Table III for this type of the
topologies it is found that most efficient
Lr and lessAcostly system
Vin
Cr
for
medium power is topology
C4 having efficiency 95%
while for higher power the preferred topology
Lm in this category
will be topology C1 its efficiency is 94%.
Ro
Dss
Cc6
Q2
B
2) A Passive Auxiliary Circuit
T6
Fig.16. ZVZCS converter with controlled output rectifier
Topology C3 [99]: In this two active switches are used in the
secondary side of the transformer as shown in fig. 17. The
gate pulses given to these synchronous rectifier are phaseshifted to the pulses of the primary inverter circuit and the
degree of phase-shift depends on the value
Q1 of load. Because of
the use of synchronous rectifiers in the secondary side of the
high-frequency transformer, it is possible to reduce
conduction losses and also reverse output current and so assist
soft switching operation under light or zero loads. Also soft
commutation of the output rectifier diodes is achieved. The
circulation energy and current stress is reduced dramatically.
SR2 in
In these converters, a passive auxiliary circuit is placed
the secondary side of the PSFB converter. Various topologies
have been discussed and compared in Table IV.
Topology D1 [101] : The passive auxiliary circuit of this
topology consists of one small capacitor and two small diodes
as shown in fig. 19 to provide ZVZCS conditions to primary
switches as well as to clamp secondary rectifier
C0 voltage
L0
without any additional passive and active clamp circuits. It
can achieve soft switching in wide load and line ranges, small
duty-cycle loss, low rectifier voltage and current stress and
low cost. The secondary side duty cycle should not below 0.5.
L0
Dc2
A
Dc3
Dc1
D5
Cf
Cc
B
MT5
C0
Lf
C5
Sc
Dr1
A
Co
MT6
Q1
D6
D1
Dr2
N1
Q3
n1
n2
TRA
N2
Q2
Fig. 19. ZVZCS FB-PWM converter
Ro
B
LP
Ro
C
c
Topology D2 [102]: The main problem associated
with the
conventional PSFB converter is the voltage stress of the
secondary side rectifier diodes. To reduce this, an auxiliary
Lau
C6
D3
CB1
D
2
Dcau
Vau
Q4
n3
D2
Q4
D3
D3
Q1
Sand
R1 Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Sudha et. al., Analysis
Vin
CB2
T5
A
Cr
Lr
1261
C0
L0
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
Q3
SR2
http://www.ieejournal.com/
Lr
Q1
SR1
Vin
Cr
rectifier circuit is added as shown in fig. 20
Q2 to achieve an
auxiliary voltage source Vau . Current of primary side of theQ4
transformer can be reset by this voltage source when the diode
Dco conducts. But it is more costly.
ID1
D1
A
Lau
Dco
n1
n2
D3
Dcau
D3
D2
Vau
Cau
n3
n4
Co
VI. APPLICATION SPECIFIC COMPARISON
Various topologies are grouped and compared in section
IV. These topologies are compared for different application
on the basis of their cost and performance, efficiency. The
comparison is given in table-V.
These topologies mainly use MOSFET or IGBT as
switches for the inverter circuit and the auxiliary circuit.
Switches used in the topologies under section IV are shown in
C0
L0
the Table VI.
Q1
VII.
B
Vin
Lo
Ro
OVERALL COMPARISON
Lr
A
Cr of the lagging switches,
In order to realize soft switching
Q3
Ro
Fig. 20. ZVZCS converter with an auxiliary voltage source
Lm
the exciting current can be used or additional auxiliary
circuit
MT5
which uses the auxiliary current in it is used. Soft switching of
Q2
Topology D3 [103]: In this circuit an auxiliary circuit
the primary switches is achieved by employing
theBtwo
Sc
comprises of an auxiliary transformer, capacitor and two
magnetic
components
whose
volt-second
product
changes
in
Co
diodes. This auxiliary circuit is placed in between
the
opposition
to
the
change
of
the
shifted-phase
angle
between
Ro
Rectifier Bridge and load as shown in fig. 21. The outcomes
the two bridge legs, which reduces the unnecessary
loss in the
Cc
MT6
of this circuit are as follows:
auxiliary circuit; but, the two additional magnetic components
i) No change in the voltage stress of the secondary rectifier
make the converter too complex.
diode in comparison to that of the conventional
The auxiliary circuit used in the above discussion is either
FB-PWM converter, soft commutation of diodes.
Q4reduce
active or passive auxiliary circuits. Active circuits can
ii) The circulating current is self-adjusted in accordance
circulating current; but, have the drawbacks of increased cost
T5
with the load condition, low reverse
SR1 recovery.
io
(additional semiconductor devices and
drivers) and limited
D
D3
iii) Magnetic
circuit
is
more
costly.
n
V are
switching
frequency.
Passive
circuits
cheaper to
N3
Lf
n
implement; but, have higher circulating currents and therefore
more conduction losses.
Drec
N4
Dd
Q1
Llks
Auxiliary
circuit used is connected either at primary
Ro
Dc
Df
Co
A
inverter circuit in Primary-side-assistedLr converters Aor at the
Df Vin
Cr in Secondary-side-assisted
secondary
rectifier circuit
Lm compared as
Drec
converters. These two configurations can be
B
follows:
Q2
Fig. 21. PWM converter using coupled output inductor
1) Since the edge resonance of the lagging phase B
switches
D
Topology D4 [104] : In this topology for achieving the ZCS
D3
depends on the inverter ncirculating
current,Vthe soft-switching
of lagging leg switches, an auxiliary circuit consists of a
n
operation may not be achieved
by the primary-side-assistedSR2
transformer auxiliary winding and a simple auxiliary circuit as
converters under the light load condition.
as shown in fig. 22 in the secondary side. No large circulating
Dcs
2) The idling power inherent to the phase-shifting modulation
energy is generated and all the active and passive devices are
Cc
in the primary-side inverter can be reduced sufficiently by
operated under the
Cc5 voltage and current stresses.
Lss
Lss minimum
introducing the Secondary-side-assisted converters scheme.
Io
Q3
3) The current ripple of the load current in the
L0
Secondary-side-assisted converters is larger than one in the
D
D
Dh
Primary-side-assisted converters counterpart because of the
A
smoothing inductor-less circuit configuration.
C0
d
d
Primary-side-assisted ZVZCS converters provide the ZCS
R
o
ic
B
condition by introducing the resetting voltage into the primary
N1 : N2
side, which absorbs reactive energy trapped in the leakage
: N3
D
D
C
h
inductor. In primary-side-assisted ZVZCS converters, the
d
d
primary current of the main transformer is reset to zero at
Fig. 22. PWM converter using transformer auxiliary winding
every half cycle; hence possibility of magnetic saturation due
Q4
to asymmetricity of circuits or transient phenomena is
On the basis of the data given in the Table IV, it is reduced, which is a very attractive feature in dc–dc converters
concluded that topology D4 and D3 are showing efficiency with transformer isolation. In secondary-side-assisted ZVZCS
94.5% but it is costly due to the use of auxiliary transformer in converters the auxiliary circuit prepares ZCS by suppressing
the circuit.
Vin
cau
2
au
3
2
cau
au
3
1
4
1
3
4
2
3
2
Q1
1262
L
Vin
r
A Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Sudha et. al., Analysis and Comparison of various
Soft-Switching
Cr
Lm
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
the load current from the isolation transformer, and bypassing
the load current through them. A snubber circuit or an active
clamp circuit can be used as an auxiliary circuit.
VIII. CONCLUSION
The PSFB converters are used for medium and high power
applications. Generally these converters loose soft switching
at low value of load current. Different auxiliary circuits have
been discussed to achieve soft switching at wide load range.
The impacts of these circuits on the performance of the
converters have also been discussed. It is concluded that the
active auxiliary at the secondary gives soft switching even at
no load and are more efficient.
APPENDIX
Table I Performance Comparison Of Topologies With Passive Auxiliary Circuit At Primary Side
Performance parameter Topology A1
Topology A2 Topology A3 Topology A4 Topology A5 Topology A6
Conduction loss
Low
High
Medium
Low
Low
Low
Duty cycle loss
Low
Medium
Medium
Reduced by Low
Low
13.7%
Circulating energy
Very Low
Low
High
Low
Low
Low
Soft switching range
Even at no load
Wide
Up to 10% of 50% to full Entire load Entire range
rated load
load
range
of load
Magnetic core loss
Low
Large
Large
Medium
Large
Large
Control
Simple
Simple
Simple
Complex
Complex
Simple
Extra magnetic core
02
09
02
02
03
03
Rectifier snubber
No
No
No
No
No
No
Secondary side control
No
No
No
No
No
No
Output voltage ringing
Low
Low
Medium
Low
Small
No.
of
auxiliary 05
08
04
04
04
03
component
Regenerative
Type of circulating Load dependent
Load
Adaptive
Adaptive
Adaptive
snubber
energy
dependent
Dead time (ns)
120
400
820
300
Experimental condition 400 W, 400/12V, 3kW,
500
W, 2
kW, 500W, 50A, 1kW,
180 kHz
300/350V,
350-400/55
380/48V,
100 kHz
300-400/54
20 kHz
V, 100 kHz
40A,
120
V, 100 kHz
kHz
Efficiency
26% increased 94.51%
97%
1.6%
94.8%
under light load
increased
Applicable
power Low power
High Power
Low power
Medium
Low power
Medium
range
Power
Power
Auxiliary
Circuit
Design
Parameter
(Inductance)
I Lr2 C a
2Vd2
Cost
Cheap
Costlier
circuit

ts2 /  2 .CS
but
control
costlier


td  1

 t d 
8C sb1  2 f s

2C Sb1Vd2( MAX )
Less costly
Less costly
 IO 
 
 nA 
VdTs 1 D / 8.I p 
VdTs 1 D / 8.I p 
Topology A7
Low
Low
Low
Wide
Medium
Simple
02
No
No
Low
01
Load
dependent
1000
160A, 630V
High
Current, high
Power
2
Vd C p
I p ,min
2
costly
costly
2
Less Cheap
Table II Performance Comparison of Topologies with Active Auxiliary Circuit at Primary Side
Performance
Topology
Topology
TopologyB
Topology
parameter
B1
B2
3
B4
Conduction
Medium
Medium
Medium
Medium
loss
Duty
cycle Low
Medium
Low
Medium
loss
Circulating
Low
Medium
Medium
Medium
energy
Soft
50% to full wide line
Wide line Wide line
switching
load
and load
and
load and load
range
range
range
Magnetic core Low
Low
Medium
Low
loss
Control
Complex
Complex
Complex
Complex
Extra
1
No
1
1
magnetic core
1263
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
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Rectifier
snubber
Secondary
side control
Dead
time
(ns)
Experimental
condition
Efficiency
Applicable
power range
Auxiliary
Circuit
Design
Parameter
(Inductance)
Cost
No
No
No
No
No
No
No
No
300
-
1200
1250
1kW,
270±10%/5
4V,
100
kHz
94.5
high-voltag
e
and
medium-po
wer
300 W ,
300-400/1
2V, 100
kHz,
91
Low
Power
1-kW,200/
152
V,
83-kHz
500
W,
300/48 V,
100 kHz
92.2%
high-voltag
e
and
medium
-power
90.2%
Low
power
I Lr2 C a
2Vd2
2Ca .Vd2 / I Lr2
Vb tb
Ib
Less
costly
More
Costly)
Less
costly
TS
 1
C
4.
 r
td
 2.Z r



More Costly
Table III Performance Comparison of Topologies With Active Auxiliary Circuit at Secondary Side
Performance
Topology C1
Topology C2
Topology C3
Topology C4
parameter
Conduction loss
Medium
High
Medium
High
Duty cycle loss
0.1 µs
Low
Low
Medium
Circulating energy
Low
Medium
Low
Medium
Soft switching range 20% to full load
full load range
entire load
Wide line and load
range
range
Magnetic core loss
Low
Medium
Low
Low
Control
Simple
Simple
Simple
Simple
Extra magnetic core
1
2
No
No
Rectifier snubber
Secondary
side
control
Experimental
condition
No
Complex
Yes
Complex
No
Complex
No
Complex
1.8-kW 100-kHz
1.2kW, 300V, 50 kHz
1kW,
kHz
Efficiency
94%
91.5%
2.8KW,
400/200V,
200KHz
92%
higher power ( 10
kW) applications
Medium power
higher power
Medium power
Auxiliary
Circuit
Design Parameter
(Inductance)
TresetVLlk
nI O
 2V
C a  d
 IO
td
8C sb1 f S
 2V
C a  d
 IO
Cost
Less costly (one
active switch)
Applicable
range
power




2
300/50V,
50
95%




2
More Costly (Two Costly
(Two Less costly (one active
active switches & active switches)
switch)
auxiliary transformer)
Soft switching wide
3%load to Entire
range
load
full load
load
Table IV Performance Comparison of Topologies With Passive Auxiliary
and line
range
Circuit at Secondary Side
ranges
Magnetic core Low
Medium
Large
Performance
Topolog
Topology
Topolo
Topolog
loss
parameter
y D1
D2
gy D3 y D4
Control
Simple
Simple
Simple
Conduction
High
High
Low
Low
Extra magnetic No
01
02
loss
core
Duty cycle loss Low
Low
Low
small
Rectifier
Yes
Yes
No
Circulating
High
High
Low
Low
snubber
energy
wide but
limited
at light
load
Large
Simple
01
No
1264
Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
Secondary side
control
Output voltage
ringing
No. of auxiliary
component
Experimental
condition
No
No
No
No
Low
Medium
Low
Medium
3
7
4
6
2kW,
220/500V,
20 kHz
1kW,
220350/50V,
82kHz
4kW,
220350/50V
80kHz
2.5 kW, 100
kHz
Efficiency
Applicable
power range
Auxiliary
Circuit Design
Parameter
(Inductance)
high
power
94.2%
high input
voltage
(1  D) DTS k
Vd I O (n1  n2 )
Where,
n2
k
n1  n2
94.4%
high
power
94.5%
high
power
Cost
Cheaper
Less
cheap
Less
cheap
n2 Cc ZO2
2 2
2
.n 
1
 DTS   I O1

 . 2.m.V  . (C  C )
d
sb1
sb2

2
.

C


h
Moderat
e cost
Table V Application Specific comparison of various topologies
Power Range
Good
Better
Best
High Power
C3
A7
Medium
Power
Low Power
C2, D1
A4, B3, D2
A2, C1, D3,
D4
A6, B1,C4
A5, B2,
B4
A1
A3
Table VI Device Specific Comparison
Device Used
MOSFET
IGBT
Topologies
A1, A3, A4, B2, B4, C1, C2, C3
A7, A5, A6, A2, B1, B3, C4,
D1, D3, D4
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