soft-switching topologies for psfb dc-dc … and comparison of...zero-current switching (zv/zcs)...
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International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1255 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
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.
Index Terms— Phase-shifted; resonant tank, reverse recovery;
synchronous rectifier (SR); Adaptable soft switching;
zero-voltage switching (ZVS); zero-current switching (ZCS);
full-bridge converter; lagging leg; leading leg.
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).
Lf2 Current doubler inductance (in henrys).
lkL 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
III. INTRODUCTION
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.
Analysis and Comparison of various
Soft-Switching Topologies for PSFB
DC-DC Converter with Additional
Auxiliary Circuits
Sudha Bansala, Lalit Mohan Saini
b
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1256 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
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 dtdi and dt
dv
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.
Q1Q3
Q4
L0
Vin
Q2
C0 R0
A
B
IO
DR2
DR3
DR4
DR1C1D1
D2C2
D3
D4
C3
C4
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
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.
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.
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1257 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
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
Fig.2. Classification of soft switching converters
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. 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 topologies
only circuit up to point A-B is taken. The 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.
Q1 Q3
Q4
L0
Vin
Q2
C0
A
A
B
Lr
Cr Ro
B
C1D1
D2D2
D2
D2C2
D3
D4
C3
C4
L0
C0 R0
A
B
IO
DR2
DR3
DR4
DR1
Fig. 3(a): Primary of the converter b) Secondary of the converter
1) Passive Auxiliary Circuit:
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.
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1258 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Q1 Q3
Q4 Q2
C
R0
AB
Q5
Vin
Lr
Vin
il1
Q3
Q4
C1D1
D2 C2
D3
D4
C3
C4
L1
L2
Fig. 4. Full-bridge converter with current doubler
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)
configured with energy regenerative circuit to prevent
freewheeling current. The leakage inductance of the main
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.
Q1 Q3
Q4
L0
Vin
Q2
C0
R0
A
B
Io
C1
C2
Cs3Cs1
Cs2
Cs4
Dr1
Dr2
Dr3
Dr4
1:n1:n1 1:n2:n2
TaTa
i1
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-to-
source 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.
Q1 Q3
Q4
Vin
Q2
A
Lr
Cr Ro
B
B
CA1
CB1
Csb1
Csb2
Csb3
Csb4
Q3
Q4
C1
D1
D2
C2
D3
D4
C3
C4
Q3
Q4
C1D1
D2C2
D3
D4
C3
C4
La1
La2
La2
La1
Fig. 6. ZVS full bridge DC-DC converter
Topology A4 [89]: In this PSFB ZVS converter auxiliary
circuit consists of a low-power auxiliary transformer TRA
shown in Fig. 7. This auxiliary transformer TRA is used to
adaptively store a relatively small amount of energy into
primary inductor that is required for ZVS. Due to this, ZVS of
the primary switches is obtained over a wide load range with
greatly reduced no-load circulating energy and with
significantly reduced secondary-side duty cycle loss. Since
the size of primary inductor is reduced, parasitic ringing is
reduced but the cost of the circuit is more.
Q1
Q3
Q4
L0
Vin
Q2
C0
A
A
B
Lr
Cr Ro
B
Lr
Cr
Lm
Vin
Q2
Q1
LP
D1
D
2
D2
CB1
CB2
TRA
N2
N1
Np/2
Np/2
Ns
Q1
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.
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1259 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Q1 Q3
Q4
L0
Vin
Q2
C0
R0
A
B
Io
-
Ca1
Ca2
VLaVa
La Ta
iLa
Fig. 8. FBZVS converter with auxiliary circuit
Topology A6 [91]: Two capacitors 1aC and
2aC , the
auxiliary transformer Tr and auxiliary inductor aL 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.
Q1 Q3
Q4
L0
Vin
Q2
C0
R0
A B
Io
Ca1
Ca2
VLa
Va
La
Tr
ip
+ V1-
+ V1-
+ V1-
- V2+
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.
10. This variable inductor is controlled with output current i.e.
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.
Q1 Q3
Q4
L0
Vin
Q2
C0
R0
A
BIo
Q3
Q4
C1D1
D2
C2
D3
D4
C3
C4
LS CS
LS
CS
IO
Fig. 10. LVI controlled PSPWM converter
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
the primary side of the PSFB. The auxiliary energy is
provided by employing a passive circuit in the primary circuit,
to help achieve soft switching, and is independent of the load
current. The topologies discussed here are compared and
compared in Table II.
Topology B1 [93] : In this converter, the energy stored in the
auxiliary circuit is adjusted by 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 aL 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
Q3 and Q4 form the lagging leg. The two auxiliary switches
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.
Q5 Q6
Q1 Q3 Q1
Q2 Q4Q4
Q1 Q3
Q4
L0
Vin
Q2
Lr
R0
A B
Io
Va
+ V1-+ V1-
Ip
Q5
Q6
La
Ia
Ia
Q3
C1D1
C2
D3
D4
C3
C4
LS
CS
IOD2
D5
D6
C5
C6
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1260 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
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.
Q1Q3
Q4
L0
Q2
C0
R0
A
B
Q5
VinLk
Vin
Ip
Q3
Q4
C1D1
D2C2
D3
D4
C3
C4
Fig. 12. A novel soft- switching converter
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
numbers.
Q1 Q3
Q4
L0
Vin
Q2
Llk
R0
A B
Io
Va
+ V1-+ V1-
Ip
Qa
Qb
La
Ia
Ia
Q3
C1D1
C2
D3
D4
C3
C4
LS
CS
IOD2
Da1
Db1
Cb
Ca
Da1
SL
Qb Qa
Q3 Q1 Q3
Q2 Q4Q4
Fig. 13. (a) FB-ZVT PWM dc/dc converter circuit (b) Gating sequence of
all switches
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.
Q2Q3
Q4
L0
Q1
C0
R0
A
B
Q5
VinLk
Vin
Ip
Q3
Q4
D2
D1
D3
D4
C3 C1
Lt Dt1 Dt2 St
Lt
Dt2
Dt1
St
C1
Fig. 14. PWM-bridge converter using fixed –edge gating scheme
On comparing all abovementioned topology in the Table
II as shown in the appendix for the active auxiliary circuit, it is
observed that for higher power level Topology B1 is showing
best result as its efficiency is around 94.5%, second best
topology is B3 , having efficiency 92.2%.
B. Secondary-Side-Assisted Converters
In these converters, an auxiliary circuit is placed in the
secondary side of the 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
as 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.
1) Active auxiliary circuit
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 resetT is obtained. Required turn-on time of the
auxiliary active switch is given by
max,
2
o
c
lkSc I
V
LnT
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1261 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Q1 Q3
Q4
L0
Vin
Q2
C0
A
A
B
Lo
Co Ro
B
Lr
Cr
Lm
Vin
Q2
Q1
SR1
SR2
Cr Ro
Cc
Sc
B
SR1
C1
Dc
C2
D2
DR2
DR3
DR4
DR1
Fig.15. FB PWM converter using secondary active clamp
Topology C2 [98] : In this topology, soft switching for all
power switches is achieved by using controlled output
rectifier with new lossless energy recovery turn-off snubber
on the secondary side of the converter as shown in fig. 16.
Active secondary switches T5, T6 are used to reset secondary
and primary circulating current and hence circulating current
is minimal. The purpose of the secondary turn-off snubber is
to transfer the leakage inductance energy to the load.
Q1 Q3
Q4
L0
Vin
Q2
C0
A A
B
Lo
Co
Ro
B
Lr
Cr
Lm
Vin
Q2
Q1
SR1
SR2
T5
io
Cc
Sc
Lss
Lss
Dss
Dss
Dcs
Cc5
Cc6
T6
Do
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 phase-
shifted to the pulses of the primary inverter circuit and the
degree of phase-shift depends on the value 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.
MT5
Q3
Q4
L0
Vin
MT6
C0
A
A
B
Lo
Co
Ro
B
Lr
Cr
Lm
Vin
Q2
Q1
SR1
SR2
T5
io
Cc
Sc
Lss
Lss
Dss
Dss
Dcs
Cc5
Cc6
T6
DoQ3
Q4
Ip
La
Ia
C5D5
C6
D3
D4
C3
C4
Dr2
Da
Db
Dr1
D6
Fig.17. ZVS Converter with synchronous rectifier
Topology C4 [100] : In this topology, the auxiliary resonant
circuit consists of a switch and a capacitor as shown in fig. 18,
to provide ZCS conditions to the primary lagging-leg
switches. This auxiliary circuit set up a freewheeling path for
the filter inductor current during a short period and auxiliary
switch softly turns on and turns off, reduces circulating energy
but high voltage stress appears at the auxiliary switches. Most
problems are solved at the cheap cost of an auxiliary switch
and a capacitor.
Q1 Q3
Q4
L0
Vin
Q2
Llk
R0
A
B
Io
Va
+ V1-
Ip
Qa
Qb
La
Ia
Ia
Lf
Co
Ro
Ca
Sa
Fig. 18. ZVZCS-FB-PWM converter
While comparing the data in Table III for this type of the
topologies it is found that most efficient and less costly system
for medium power is topology C4 having efficiency 95%
while for higher power the preferred topology in this category
will be topology C1 its efficiency is 94%.
2) A Passive Auxiliary Circuit
In these converters, a passive auxiliary circuit is placed in
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 voltage
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.
Q1
Q3
Q4
L0
Q2
C0
A
A
B
Lf
Cf
Ro
B
Lr
Cr
Lm
Vin
Q2
Q1
LP
D1
D
2
D2
CB1
CB2
TRAN2
N1
Np/2
Np/2
Ns
Q1
Cc
Dc1
Dc2
Dc3
n1
n2
n3
Lau
Dcau
D3
D3Vau
Fig. 19. ZVZCS FB-PWM converter
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
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
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1262 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
rectifier circuit is added as shown in fig. 20 to achieve an
auxiliary voltage sourceauV . Current of primary side of the
transformer can be reset by this voltage source when the diode
Dco conducts. But it is more costly.
Q1 Q3
Q4
L0
Vin
Q2
C0
A
A
B
Lr
Cr Ro
B Lr
Cr
Lm
Vin
Q2
Q1
SR1
SR2
n1
Ro
Co
Lo
Caun2
n3
n4
Lau Dco
Dcau
D1
D3
D3
D2
Vau
ID1
Fig. 20. ZVZCS converter with an auxiliary voltage source
Topology D3 [103]: In this circuit an auxiliary circuit
comprises of an auxiliary transformer, capacitor and two
diodes. This auxiliary circuit is placed in between the
Rectifier Bridge and load as shown in fig. 21. The outcomes
of this circuit are as follows:
i) No change in the voltage stress of the secondary rectifier
diode in comparison to that of the conventional
FB-PWM converter, soft commutation of diodes.
ii) The circulating current is self-adjusted in accordance
with the load condition, low reverse recovery.
iii) Magnetic circuit is more costly.
MT5
Q3
Q4
L0
Vin
MT6
C0
A A
B
Lf
Co
Ro
B
Lr
Cr
Lm
Vin
Q2
Q1
SR1
SR2
T5io
Cc
Sc
LssLss
Drec
Dcs
Cc5
Co
N3
Dd
Drec
RoDf
Df
Dc
Llks
N4
n2
n3
DcauD3Vau
n2
n3
DcauD3Vau
Fig. 21. PWM converter using coupled output inductor
Topology D4 [104] : In this topology for achieving the ZCS
of lagging leg switches, an auxiliary circuit consists of a
transformer auxiliary winding and a simple auxiliary circuit as
as shown in fig. 22 in the secondary side. No large circulating
energy is generated and all the active and passive devices are
operated under the minimum voltage and current stresses.
MT5
Q3
Q4
L0
Vin
MT6
C0
A
A
B
Lf
Co
Ro B
Lr
Cr
Lm
Vin
Q2
Q1
SR1
SR2
T5
ic
Cc
LssLss
Drec
Dcs
Cc5
Co
N3
Dh
Drec
Ro
D1
Df
Dc
LlksN4
Ch
N1 : N2
: N3
Io
D2D3
D4
d1
d2d3
d4
D4
Fig. 22. PWM converter using transformer auxiliary winding
On the basis of the data given in the Table IV, it is
concluded that topology D4 and D3 are showing efficiency
94.5% but it is costly due to the use of auxiliary transformer in
the circuit.
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
the Table VI.
VII. OVERALL COMPARISON
In order to realize soft switching of the lagging switches,
the exciting current can be used or additional auxiliary circuit
which uses the auxiliary current in it is used. Soft switching of
the primary switches is achieved by employing the two
magnetic components whose volt-second product changes in
opposition to the change of the shifted-phase angle between
the two bridge legs, which reduces the unnecessary loss in the
auxiliary circuit; but, the two additional magnetic components
make the converter too complex.
The auxiliary circuit used in the above discussion is either
active or passive auxiliary circuits. Active circuits can reduce
circulating current; but, have the drawbacks of increased cost
(additional semiconductor devices and drivers) and limited
switching frequency. Passive circuits are cheaper to
implement; but, have higher circulating currents and therefore
more conduction losses.
Auxiliary circuit used is connected either at primary
inverter circuit in Primary-side-assisted converters or at the
secondary rectifier circuit in Secondary-side-assisted
converters. These two configurations can be compared as
follows:
1) Since the edge resonance of the lagging phase switches
depends on the inverter circulating current, the soft-switching
operation may not be achieved by the primary-side-assisted
converters under the light load condition.
2) The idling power inherent to the phase-shifting modulation
in the primary-side inverter can be reduced sufficiently by
introducing the Secondary-side-assisted converters scheme.
3) The current ripple of the load current in the
Secondary-side-assisted converters is larger than one in the
Primary-side-assisted converters counterpart because of the
smoothing inductor-less circuit configuration.
Primary-side-assisted ZVZCS converters provide the ZCS
condition by introducing the resetting voltage into the primary
side, which absorbs reactive energy trapped in the leakage
inductor. In primary-side-assisted ZVZCS 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. In secondary-side-assisted ZVZCS
converters the auxiliary circuit prepares ZCS by suppressing
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
ISSN 2078-2365
http://www.ieejournal.com/
1263 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
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 Topology A7
Conduction loss Low High Medium Low Low Low Low
Duty cycle loss Low Medium Medium Reduced by
13.7%
Low Low Low
Circulating energy Very Low Low High Low Low Low Low
Soft switching range Even at no load Wide Up to 10% of
rated load
50% to full
load
Entire load
range
Entire range
of load
Wide
Magnetic core loss Low Large Large Medium Large Large Medium
Control Simple Simple Simple Complex Complex Simple Simple
Extra magnetic core 02 09 02 02 03 03 02
Rectifier snubber No No No No No No No
Secondary side control No No No No No No No
Output voltage ringing Low Low Medium Low - Small Low
No. of auxiliary
component
05 08 04 04 04 03 01
Type of circulating
energy
Load dependent Regenerative
snubber Load
dependent
Adaptive Adaptive Adaptive Load
dependent
Dead time (ns) 120 - 400 820 - 300 1000
Experimental condition 400 W, 400/12V,
180 kHz
3kW,
300/350V,
20 kHz
500 W,
350-400/55
V, 100 kHz
2 kW,
380/48V,
40A, 120
kHz
500W, 50A,
100 kHz
1kW,
300-400/54
V, 100 kHz
160A, 630V
Efficiency 26% increased
under light load
94.51% 97% 1.6%
increased
- 94.8% -
Applicable power
range
Low power High Power Low power Medium
Power
Low power Medium
Power
High
Current, high
Power
Auxiliary Circuit
Design Parameter
(Inductance) 2
2
2 d
aLr
V
CI Ss Ct ./ 22
d
ssb
d tfC
t
2
1
8 1
2
2
)(12
A
O
MAXdSb
n
I
VC
psd IDTV .8/1 psd IDTV .8/1
2
min,
2
p
pd
I
CV
Cost Cheap but
Costlier control
circuit
costlier Less costly Less costly
costly costly Less Cheap
Table II Performance Comparison of Topologies with Active Auxiliary Circuit at Primary Side
Performance
parameter
Topology
B1
Topology
B2
TopologyB
3
Topology
B4
Conduction
loss
Medium Medium Medium Medium
Duty cycle
loss
Low Medium Low Medium
Circulating
energy
Low Medium Medium Medium
Soft
switching
range
50% to full
load
wide line
and load
Wide line
and load
range
Wide line
and load
range
Magnetic core
loss
Low Low Medium Low
Control Complex Complex Complex Complex
Extra
magnetic core
1 No 1 1
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
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1264 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Rectifier
snubber
No No No No
Secondary
side control
No No No No
Dead time
(ns)
300 - 1200 1250
Experimental
condition
1kW,
270±10%/5
4V, 100
kHz
300 W ,
300-400/1
2V, 100
kHz,
1-kW,200/
152 V,
83-kHz
500 W,
300/48 V,
100 kHz
Efficiency 94.5 91 92.2% 90.2%
Applicable
power range
high-voltag
e and
medium-po
wer
Low
Power
high-voltag
e and
medium
-power
Low
power
Auxiliary
Circuit
Design
Parameter
(Inductance)
d
r
r
S
t
C
Z
T
.2
1.4
2
2
2 d
aLr
V
CI 22 /.2 Lrda IVC
b
bb
I
tV
Cost More Costly Less
costly
More
Costly)
Less
costly
Table III Performance Comparison of Topologies With Active Auxiliary Circuit at Secondary Side
Performance
parameter
Topology C1 Topology C2 Topology C3 Topology C4
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
range
Wide line and load
range
Magnetic core loss Low Medium Low Low
Control Simple Simple Simple Simple
Extra magnetic core 1 2 No No
Rectifier snubber No Yes No No
Secondary side
control
Complex Complex Complex Complex
Experimental
condition
1.8-kW 100-kHz 1.2kW, 300V, 50 kHz 2.8KW,
400/200V,
200KHz
1kW, 300/50V, 50
kHz
Efficiency 94% 91.5% 92% 95%
Applicable power
range
higher power ( 10
kW) applications
Medium power higher power Medium power
Auxiliary Circuit
Design Parameter
(Inductance) O
Llkreset
nI
VT 2
2
O
d
aI
VC
Ssb
d
fC
t
18
2
2
O
d
aI
VC
Cost Less costly (one
active switch)
More Costly (Two
active switches &
auxiliary transformer)
Costly (Two
active switches)
Less costly (one active
switch)
Table IV Performance Comparison of Topologies With Passive Auxiliary
Circuit at Secondary Side
Performance
parameter
Topolog
y D1
Topology
D2
Topolo
gy D3
Topolog
y D4
Conduction
loss
High High Low Low
Duty cycle loss Low Low Low small
Circulating
energy
High High Low Low
Soft switching
range
wide
load
and line
ranges
3%load to
full load
Entire
load
range
wide but
limited
at light
load
Magnetic core
loss
Low Medium Large Large
Control Simple Simple Simple Simple
Extra magnetic
core
No 01 02 01
Rectifier
snubber
Yes Yes No No
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.2, pp. 1255-1268
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1265 Sudha et. al., Analysis and Comparison of various Soft-Switching Topologies for PSFB DC-DC Converter with Additional Auxiliary Circuits
Secondary side
control
No No No No
Output voltage
ringing
Low Medium Low Medium
No. of auxiliary
component
3 7 4 6
Experimental
condition
2kW,
220/500V,
20 kHz
1kW, 220-
350/50V,
82kHz
4kW,
220-
350/50V
80kHz
2.5 kW, 100
kHz
Efficiency - 94.2% 94.4% 94.5%
Applicable
power range
high
power
high input
voltage
high
power
high
power
Auxiliary
Circuit Design
Parameter
(Inductance)
n2 Cc ZO2 )(
)1(
21 nnIV
kDTD
Od
S
Where,
21
2
nn
nk
h
S
C
DT 1.
.2
2
)(
1.
..2
.
21
22
sbsbd
O
CCVm
nI
Cost Cheaper Less
cheap
Less
cheap
Moderat
e cost
Table V Application Specific comparison of various topologies
Power Range Good Better Best
High Power C3 A7 A2, C1, D3,
D4
Medium
Power
C2, D1 A4, B3, D2 A6, B1,C4
Low Power A5, B2,
B4
A1 A3
Table VI Device Specific Comparison
Device Used Topologies
MOSFET A1, A3, A4, B2, B4, C1, C2, C3
IGBT A7, A5, A6, A2, B1, B3, C4,
D1, D3, D4
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