Circuit Properties of Zero-Voltage-Transition PWM Converters 35
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
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
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.
-) and they all share
Manuscript received Oct. 1, 2007; revised Nov. 19, 2007
†Corresponding Author: email@example.com
Tel: 519-661-2111, Fax: 519-850-2436, Univ. of Western Ontario
*Dept. of Elec. and Comp. Eng. Univ. of Western Ontario
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
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
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 .
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 . 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.
Fig. 1 General structure of a ZVT PWM boost converter
Fig. 2 ZVT-PWM boost converter with non-resonant auxiliary
(a) [t0-t1] (b) [t1-t2] (c) [t2-t3]
(d) [t3-t4] (e) [t4-t5] (f) [t5-t6]
(g) [t6-t7] (i) t > t7
Fig. 3 Modes of operation of a ZVT-PWM boost converter with
a non-resonant auxiliary circuit
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
(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 , , , ;
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
Consider the converter shown in Fig. 6, which is an
example of a ZVT-PWM converter with a resonant
auxiliary circuit , . The auxiliary circuit consists
t 5 t 7
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
Fig. 5 Other non-resonant auxiliary circuits
Circuit Properties of Zero-Voltage-Transition PWM Converters 49
without the main switch peak current stress and with less
circulating current. At the present time, dual circuits are
the most efficient type of auxiliary circuits.
In this paper, the fundamental circuit principle of
"off-tuning" was presented as a way of maximizing the
efficiency of auxiliary circuits. The circulating current in
dual auxiliary circuits can be reduced even further by
"off-tuning" the resonant branch so that there are some
auxiliary switch turn-off losses, but less than those found
in non-resonant circuits. The paper reviewed the operation
of auxiliary circuits in ZVT-PWM converters, and then
discussed the properties and characteristics of the new
off-tuned type of auxiliary circuits. There are many
possible off-tuned auxiliary circuits as there are presently
many previously proposed non-resonant and resonant
Experimental results obtained from 500W, 100 kHz
prototype converters confirmed the feasibility of a
ZVT-PWM boost converter
experimental off-tuned auxiliary circuit and a comparison
was made between the experimental circuit and the
original non-resonant, resonant, and dual circuits from
which it was derived. Based on this comparison, several
general circuit properties were determined. It was
concluded that an off-tuned circuit will be the most
efficient circuit of the set of non-resonant, resonant, and
dual circuits from which it was derived, but it also has the
most components. It is up to a circuit designer to
determine the cost benefits of one type of converter over
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methods for PWM
Amir Ostadi received a B.Sc. degree from
Sharif University of Technology, Tehran,
Iran, in Electrical Engineering. Currently, he
is working towards his M.Sc. degree in
Electrical Engineering, at the University of
Western Ontario, London, Ontario, Canada.
His research interests include wind power generation, application
of power electronics in power systems, power system
restructuring, and power system stability.
50 Journal of Power Electronics, Vol. 8, No. 1, January 2008
Xing Gao received the M.E.Sc degree in
Electrical & Computer Engineering from
University of Western Ontario, London,
Canada in 2006. He also received the M.E.Sc
degree and the B. Eng degree with distinction
from University of Inner Mongolia, China in
1985 and 1982 respectively. He has been a power electronics
design engineer for many years in Canada and in the United
States. He is currently working in Boston, US as a senior design
Gerry Moschopoulos received the Bachelor
of Engineering, Master's of Applied Science
and Ph.D degrees from Concordia University
in Montreal, Quebec, Canada in 1989, 1992,
and 1997 respectively. From 1996 to 1998,
he was a design engineer in the Advanced
Power Systems division of Nortel Networks in Lachine, Quebec,
Canada, working on developing power supplies and systems for
telecom applications. From 1998 to 2000, he was a research
engineer at Concordia University working on power converter
operating with soft-switching and active power factor correction.
Since 2000, he has been with the Department of Electrical and
Computer Engineering at the University of Western Ontario in
London, Ontario, Canada, where he is presently an associate
professor. He is also a member of the Professional Engineers of