Soft-Switching Topologies for PSFB DC-DC … and Comparison of...zero-current switching (ZV/ZCS)...

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

[email protected] , [email protected]



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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.



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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.



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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.



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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



Vol. 5 (2014) No.2, pp. 1255-1268

ISSN 2078-2365



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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Soft-Switching Topologies for PSFB DC-DC … and Comparison of...zero-current switching (ZV/ZCS)...

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