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Circuit Properties of Zero-Voltage-Transition PWM Converters 35

JPE 8-1-4

Circuit Properties of Zero-Voltage-Transition PWM Converters

Amir Ostadi*, Xing Gao* and Gerry Moschopoulos†

†*Dept. of Electrical and Electronics Eng., University of Western Ontario, London, Ontario, Canada

ABSTRACT

A zero-voltage-transition (ZVT) pulse width modulated (PWM) converter is a PWM converter with a single main power

switch that has an auxiliary circuit to help it turn on with zero-voltage switching (ZVS). There have been many

ZVT-PWM converters proposed in the literature as they are the most popular type of ZVS-PWM converters. In this paper,

the properties and characteristics of several types of ZVT-PWM converters are reviewed. A new type of ZVT-PWM

converter is then introduced, and the operation of a sample converter of this type is explained and analyzed in detail. A

procedure for the design of the converter is presented and demonstrated experimentally. The feasibility of the new

converter is confirmed with results obtained from an experimental prototype. Conclusions on the performance of

ZVT-PWM converters in general are made based on the efficiency results obtained from the experimental prototypes of

various ZVT-PWM converters of different types.

Keywords: Zero-voltage transition, PWM converters, Switch-mode power supplies, High-frequency converters

1. Introduction

High switching frequencies are used in power

converters to reduce the size and weight of their magnetic

and filter components, thus reducing overall converter size

and weight. Operating at higher switching frequencies,

however, increases switching losses, which reduces

converter efficiency. Converters operating with high

switching frequencies are, therefore, typically implemented

with zero-voltage switching (ZVS) to minimize these

problems. With ZVS, converter switches are made to

operate with a zero-voltage turn-on and turn-off.

In recent years, the most widely used single-switch

pulse-width modulated (PWM) ZVS power converters

have been so called zero-voltage-transition (ZVT) PWM

converters. There have been many previously proposed

ZVT-PWM converters (i.e.

certain common properties. These converters have an

auxiliary circuit connected in parallel to the main switch to

help it turn on with ZVS and a snubber capacitor to help

the switch turn off with ZVS, as shown in Fig. 1. They

operate in the same manner as regular PWM converters,

but with reduced switching losses. This reduction is due

to the fact that the auxiliary circuit operates for only a

small portion of the switching cycle and is activated just

before the main converter switch is about to be turned on.

Since the auxiliary circuit is on for such a short time, a

device with better switching characteristics than that used

as the main switch can be chosen, as conduction losses are

not an issue.

[1]-[14]) and they all share

Manuscript received Oct. 1, 2007; revised Nov. 19, 2007

†Corresponding Author: gmoschopoulos@eng.uwo.ca

Tel: 519-661-2111, Fax: 519-850-2436, Univ. of Western Ontario

*Dept. of Elec. and Comp. Eng. Univ. of Western Ontario

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36 Journal of Power Electronics, Vol. 8, No. 1, January 2008

The objectives of this paper are as follows:

(i) To review the basic circuit properties of ZVT-PWM

converters for design engineers who may not be

familiar with them;

(ii) To determine if it is possible to maximize the

efficiency of these converters using these circuit

properties.

In this paper, the properties and characteristics of

several types of ZVT-PWM converters are reviewed. A

new type of ZVT-PWM converter is then introduced and

the operation of an experimental converter of this type is

then demonstrated. The feasibility of the new converter is

confirmed with results obtained from an experimental

prototype. Conclusions on the performance of the

converter and on the performance of ZVT-PWM

converters in general are made based on efficiency results

obtained from experimental prototypes

ZVT-PWM converters that are representative of different

types.

of other

2. Non-Resonant and Resonant Auxiliary

Circuits in ZVT-PWM Converters

There have been many auxiliary circuits that have been

previously proposed for use in ZVT-PWM converters [14].

With few exceptions, auxiliary circuits in ZVT-PWM

converters are generally one of three types: non-resonant

circuits, resonant circuits that have an LC resonant

network placed in series with the auxiliary circuit switch,

or dual circuits that are a combination of the first two

types. The operation of a ZVT-PWM boost converter with

an experimental non-resonant and an experimental resonant

auxiliary circuit is reviewed in this section of the paper.

The operation of a ZVT-PWM boost converter with an

experimental dual circuit will be reviewed in the next section.

2.1 Non-Resonant Auxiliary Circuit

Consider the converter shown in Fig. 2 which is an

example of a ZVT-PWM converter with a non-resonant

auxiliary circuit [1]. The auxiliary circuit consists of a switch,

S2, an inductor, Lr, a capacitor, Cr, and two diodes, D2 and

D3. The circuit is a non-resonant circuit because there is no

capacitor in series with the auxiliary circuit inductor.

V in

C s1

C o

R

L in

S 1

D 1

Auxiliary

Circuit

a

b

c

Fig. 1 General structure of a ZVT PWM boost converter

Vin

Cs1

Lr

Co R

Lin

S1

S2

D1

D2

Cr

D3

Fig. 2 ZVT-PWM boost converter with non-resonant auxiliary

circuit

Iin

Vo

D1

Lr1

S2

Iin

C s1

S 2

L r1

I in

S2

Lr1

S1

(a) [t0-t1] (b) [t1-t2] (c) [t2-t3]

Iin

D2

Lr1

S1

Cr

I in

S 1

Iin

Cs1

(d) [t3-t4] (e) [t4-t5] (f) [t5-t6]

Vo

D3

Cr

Iin

Cs1

Iin

V o

D 1

(g) [t6-t7] (i) t > t7

Fig. 3 Modes of operation of a ZVT-PWM boost converter with

a non-resonant auxiliary circuit

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Circuit Properties of Zero-Voltage-Transition PWM Converters 37

Fig. 3 shows circuit diagrams of the modes of operation

that the converter shown in Fig. 2 goes through during a

switching cycle. For these diagrams, the input inductor, Lin,

is assumed large enough to be considered as a constant

current source, Iin, and the output capacitor, Co, is large

enough to be considered as a voltage source, Vo.. Typical

waveforms that illustrate the converter's operation are

shown in Fig. 4.

The converter works as follows: Before the auxiliary

switch S2 is turned on to help the main switch S1 turn on

with ZVS, current flows through the main power boost

diode D1. At some time t = t0, the auxiliary switch S2 is

turned on and current begins to be diverted from D1 to the

auxiliary circuit. Since there is an inductor in series with

the switch, it is turned on with zero-current switching

(ZCS) as the inductor slows down the rate of current rise

in the switch. At t = t1, there is no current flowing in D1

and capacitor Cs1 begins to discharge as the voltage across

it is now not clamped to the output voltage. Cs1 is totally

discharged at some time t = t2 and the body diode of S1

conducts current. S1 can be turned on with ZVS as the

voltage across Cs1 is almost zero.

Once S1 has been turned on, S2 can be turned off at

some time t = t3. When this happens, the current through

Lr is diverted to D2 and charges capacitor Cr and the

current in S1 stops flowing through the body diode and

instead flows through the switch. When the current

through Lr becomes zero at some time t = t4, the converter

then operates like a conventional PFC boost converter. S1

is turned off at t = t5 and capacitor Cs1 is charged until D3

begins to conduct at t = t6. Cr is eventually discharged

through D3 and current then flows through D1 at t = t7 until

S1 is turned on again to start a new switching cycle.

The following facts, which are true for all ZVT-PWM

converters with non-resonant auxiliary circuits, should be

noted:

(i) The current flowing through the auxiliary switch is

interrupted when the switch is turned off. Although

the switch has a hard turn-off that somewhat offsets

the gain in efficiency that is derived by having the

auxiliary circuit in the circuit, these turn-off losses are

still less than the turn-on losses of the main power

switch in a conventional PWM converter.

(ii) The operation of the auxiliary circuit in the converter

does not affect the voltage or current stress of the

main power switch or the main power boost diode.

Other non-resonant auxiliary circuits that can be used

in ZVT-PWM converters were proposed in [2], [4], [8], [9];

some of these are shown in Fig. 5. Regardless of how

these circuits may look, the fundamental circuit properties

of all non-resonant circuits are the same. The only real

difference is in the way that energy is transferred out of

the auxiliary circuit after the auxiliary switch is turned off.

2.2 ZVT-PWM Converter with Resonant Auxiliary

Circuit

Consider the converter shown in Fig. 6, which is an

example of a ZVT-PWM converter with a resonant

auxiliary circuit [5], [6]. The auxiliary circuit consists

S 1

S 2

V s1

I s1

I Lr

I in

V Cr

t 5 t 7

t 6

V o

t 0 t 1 t 2 t 3 t 4

Fig. 4 Typical waveforms of a ZVT-PWM boost converter

with a non-resonant auxiliary circuit

b

c

a

b

c

a

Fig. 5 Other non-resonant auxiliary circuits

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38 Journal of Power Electronics, Vol. 8, No. 1, January 2008

of a switch, S2, an inductor, Lr, a capacitor, Cr, and three

diodes, D2, D3 and D4. The purpose of D3 is to keep

current from flowing through the body diode of S2 so that

current can flow through D4, which is a faster device. The

circuit is a resonant circuit because there is a capacitor in

series with the auxiliary circuit inductor.

Fig. 7 shows the circuit diagrams of the modes of

operation that the converter shown in Fig. 6 goes through

during a switching cycle, while Fig. 8 shows typical

waveforms that illustrate the converter’s operation. The

converter works as follows: The auxiliary switch S2 is

turned on with ZCS at t = t0 and current begins to be

diverted from D1 to the auxiliary circuit. At t = t1,

capacitor Cs1 begins to discharge as there is no current

flowing in D1 and is totally discharged at some time t = t2

when the body diode of S1 conducts current. S1 can then

be turned on with ZVS.

Initially, during the time interval from t0 to t2 when

current is flowing through S2, the current through S2 rises,

but then begins to drop as capacitor Cr is being charged -

especially as Cs1 is being discharged and the net voltage

across Lr1 is negative. At t = t3, auxiliary circuit current

ILr2 becomes less than the input current Iin and, thus, the

current through S1 changes direction and stops flowing

through the body diode. ILr2 continues decreasing until it

becomes zero at t = t4 then reverses direction and flows

through D4 and S1, so that Cr can discharge; switch S2 can

be turned off softly while this is happening. At t = t5,

current stops flowing in the auxiliary circuit as the voltage

across Cr has become negative and there is no path for Cr

to discharge. The operation of the converter becomes

Vin

Cs1

Co

R

Lin

S1

D1

Cr

Lr

D2

S2

D4

D3

Fig. 6 ZVT-PWM boost converter with a resonant auxiliary

circuit

D 1

D 3

L r1

S 2

C r

I in

V o

Cs1

D 3

L r1

S2

C r

I in

S1

S2

D3

Lr1

Cr

I in

(a) [t0-t1] (b) [t1-t2] (c) [t2-t3]

C r

(d)[t3-t4] (e) [t4-t5] (f) [t5-t6]

S 1

S 2

D 3

L r1

I in

D 4

S1

L r1

C r

I in

I in

S1

(g)[t6-t7] (h) [t7-t8] (i) t > t8

Fig. 7 Modes of operation of a ZVT-PWM boost converter

with a resonant auxiliary circuit

I in

C s1

V o

D2

C r

I in

C s1

Vo

D1

I in

S1

S2

Vs1

Is1

ILr

Iin

Vo

VCr

t0t1t2t3

t 6

t4

t8

t7

t5

Fig. 8 Typical waveforms of a ZVT-PWM boost converter

with a resonant auxiliary circuit

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Circuit Properties of Zero-Voltage-Transition PWM Converters 39

identical to that of the converter shown in Fig. 2 until the

start of the next switching cycle.

The following facts, which are true for all ZVT-PWM

converters with resonant auxiliary circuits, should be

noted:

(i) The series inductor-capacitor components in the

auxiliary circuit force the auxiliary switch current to

drop to zero naturally so that there is no current

flowing through the auxiliary switch when the switch

is turned off. The switch is turned off softly while

current is flowing through diode D4, which is

anti-parallel to it.

(ii) The operation of the auxiliary circuit in the converter

results in an increase in the peak current stress of the

main power switch S1 due to the auxiliary circuit

current that flows through D4 when it is in its negative

resonant half-cycle. This circulating current increases

both the peak current stress of the main switch and

conduction losses.

Other resonant auxiliary circuits that can be used in

ZVT-PWM converters were proposed in [3], [7], and [10];

some of these are shown in Fig. 9. Regardless of how

these circuits may look, the fundamental circuit properties

of all resonant circuits are the same. The only real

difference is in the way that energy is transferred out of

the auxiliary circuit to the load. In general, the more

sophisticated the auxiliary circuit, the more efficiently this

process will occur.

3. Dual Auxiliary Circuits in ZVT-PWM

Converters

In recent years, dual auxiliary circuits that combine the

advantages of resonant and non-resonant converters while

reducing the drawbacks of each have been proposed (i.e.

[11-13]). The auxiliary switch in these circuits has the soft

turn-off of switches in resonant auxiliary circuits, but

without the increase in main switch peak current stress and

conduction losses as is the case with non-resonant circuits.

An example of a boost converter with a dual auxiliary

circuit is shown in Fig. 10 [13]. This auxiliary circuit can be

considered to be dual since it has two parallel branches: a

non-resonant branch consisting of components: Lr1, Lr2, D3,

D4 and a resonant branch consisting of components: Lr2, Cr

D4. In general, a dual auxiliary circuit can be formed by

combining any one non-resonant auxiliary circuit with any

one resonant auxiliary circuit then eliminating all

redundant components; a procedure for doing so was

presented in [13]. In the dual circuit shown in Fig. 10, the

circuit has been formed by combining the non-resonant

auxiliary circuit presented in Section II.A with the

resonant circuit presented in Section II.B then eliminating

all redundant components.

The equivalent circuit for each mode is shown in Fig.

11 and the converter waveforms are shown in Fig. 12. The

converter works as follows: At t0, the auxiliary switch S2

is turned on with ZCS. The current through Lr1, Lr2 and Cr

increases as current is being diverted away from the boost

diode, D1. The mode ends at t = t1 when the current

flowing through D1 is zero. At t1, the capacitor across the

main switch, Cs, begins to be discharged through the

auxiliary circuit. Current ILr1, and voltage VCs continue to

increase. ILr2 reaches its maximum value and begins to

decrease when VCs drops below VCr. Cs. is still being

discharged until it is totally discharged at t= t2, when the

main switch body-diode begins to conduct.

At t2, the body-diode of the main switch begins to

conduct as ILr2 is larger than Iin; this allows the main

b

c

a

b

c

a

Fig. 9 Other resonant auxiliary circuits

Vin

Cs1

Co R

Lin

S1

D1

Cr

D3

Lr1

Lr2

D4

S2

D2

Fig. 10 ZVT-PWM boost converter with a dual auxiliary

circuit