IEEE Vehicle Power and Propulsion Conference (VPPC), September 3-5, 2008, Harbin, China
Review of Soft-Switching Techniques for HighFrequency Switched-Mode Power Converters
T.W. Ching * and K.U. Chan**
*
**
University of Macau, Macau, China. Email: twching@umac.mo
University of Macau, Macau, China. Email: kauchan@umac.mo
Abstract—Given the increasing demand to increase the
switching frequency and efficiency of switched-mode power
converters, pulse-width-modulated (PWM) soft-switching
techniques have been proposed to combine the desirable
features of PWM and resonant-mode converters. This
paper presents the evolution of several soft-switching
techniques,
operating
principles,
performance
characteristics, experimental results, merits and limitations
are also discussed. Experimental results of three classes of
zero-voltage-switched converters and one class of zerocurrent-switched converter are prototyped and presented.
(a)
Keywords—Power Converter; Soft-switching Technique
I.
INTRODUCTION
In the area of switched-mode power conversion, there
has been an ever-increasing demand to increase the
switching frequency [1] to allow the use of smaller filters
and energy-storage elements, thus achieving higher
power density and better dynamic performance. The
higher switching frequency results in increased switching
losses.
The switching losses at turn-on are mainly by an abrupt
change of the energy stored in the parasitic capacitances
of the solid-state devices. When a transistor turns on, the
energy stored in its output capacitance is dissipated in the
device and the junction capacitance of a rectifier is
dissipatively charged through the switch. At turn-off,
switching loss is mainly caused by the leakage inductance.
When an active switch is turned off, a voltage spike is
induced by the sharp di / dt across the leakage inductance
[2].
Furthermore, the conventional hard switching
converters exhibit high dv / dt and di / dt in their
operation, which cause excessive electromagnetic
interference (EMI). These EMI noise can easily affect the
operation of other equipment if it is not adequately
controlled. Turn-off transient for both conventional hardswitching and soft-switching (SS) converters are shown in
Fig. 1.
II.
SOFT-SWITCHING TECHNIQUES
In the past two decade, numerous SS converters have
been successfully developed to allow power
semiconductor devices in switch-mode-power-supplies
(SMPS) operating at favorable conditions, leading to
achieve high power density and high efficiency [1]. SS
converters, being developed for SMPS, are generally
classified as the resonant converter (RC), quasi-resonant
converter (QRC) [2], multi-resonant converter (MRC) [3]
and zero-transition converter (ZTC) [4]. Except the ZTC
C 2008 IEEE.
978-1-4244-1849-7/08/$25.00○
(b)
Figure 1. Turn-off transient of power electronic converters:
(a) conventional hard-switching; (b) soft-switching
inherently operates at constant switching frequency, the
others originally operate at variable switching frequency
and have recently been extended to constant-frequency
(CF) operation, so-called the CF-RC, CF-QRC and CFMRC [5]–[7]. CF operation takes definite advantages
over variable-frequency (VF) because it favors the
optimization of reactive components and closed-loop
bandwidth as well as the filtering of EMI and noise.
Instead of using VF modulation, the control of CF
converters is generally accomplished by means of pulsewidth-modulation (PWM).
The common feature of these SS converters is the
existence of a resonant tank or network which is used to
shape the current and voltage waveforms of power
devices to achieve either zero-voltage-switching (ZVS) or
zero-current-switching (ZCS). Since power metal-oxidesemiconductor-field-effect-transistor (MOSFET) is the
most suitable power device for high-frequency low power
applications such as SMPS and low-inductance dc servos,
ZVS is usually preferred to ZCS because it can eliminate
the major switching losses due to the discharging of its
inherent junction capacitance.
At present, those ZVS-CF versions of SS converters
include the ZVS-CF-RC, ZVS-CF-QRC, ZVS-CF-MRC
and ZVS-ZTC (usually called ZVT-PWM converter).
Focusing on those single-ended converter types to
minimize the hardware count and control complexity, the
ZVS-CF-RC will not be considered because it usually