U-138
APPLICATION NOTE
Zero Voltage Switching
Resonant Power Conversion
Bill Andreycak
ing zero current, hence zero power switching.
And while true, two obvious concerns can
impede the quest for high efficiency operation
with high voltage inputs.
Abstract
The technique of zero voltage switching in
modern power conversion is explored. Several
ZVS topologies and applications, limitations of
the ZVS technique, and a generalized design
procedure are featured. Two design examples
are presented: a 50 Watt DC/DC converter,
and an off-line 300 Watt multiple output power
supply. This topic concludes with a perfor-
mance comparison of ZVS converters to their
square wave counterparts, and a summary of
typical applications.
By nature of the resonant tank and zero
current switching limitation, the peak switch
current is significantly higher than its square
wave counterpart. In fact, the peak of the full
load switch current is a minimum of twice that
of its square wave kin. In its off state, the
switch returns to a blocking a high voltage
every cycle. When activated by the next drive
pulse, the MOSFET output capacitance
is discharged by the FET, contributing a signifi-
cant power loss at high frequencies and high
voltages. Instead, both of these losses are
avoided by implementing a zero voltage switch-
ing technique [9,lO].
Introduction
Advances in resonant and quasi-resonant
power conversion technology propose alterna-
tive solutions to a conflicting set of square
wave conversion design goals; obtaining high
efficiency operation at a high switching fre-
quency from a high voltage source. Currently,
the conventional approaches are by far, still in
the production mainstream. However, an
increasing challenge can be witnessed by the
emerging resonant technologies, primarily due
to their lossless switching merits. The intent of
this presentation is to unravel the details of
zero voltage switching via a comprehensive
analysis of the timing intervals and relevant
voltage and current waveforms.
Zero Voltage Switching Overview
Zero voltage switching can best be defined
as conventional square wave power conversion
during the switch’s on-time with “resonant”
switching transitions. For the most part, it can
be considered as square wave power utilizing a
constant off-time control which varies the
conversion frequency, or on-time to maintain
regulation of the output voltage. For a given
unit of time, this method is similar to fixed
frequency conversion which uses an adjustable
duty cycle, as shown in Fig. 1.
The concept of quasi-resonant, “lossless”
switching is not new, most noticeably patented
by one individual [1] and publicized by another
at various power conferences [2,3]. Numerous
efforts focusing on zero current switching
ensued, first perceived as the likely candidate
for tomorrow’s generation of high frequency
power converters [4,5,6,7,8]. In theory, the on-
off transitions occur at a time in the resonant
cycle where the switch current is zero, facilitat-
Regulation of the output voltage is accomp-
lished by adjusting the effective duty cycle,
performed by varying the conversion frequency.
This changes the effective on-time in a ZVS
design. The foundation of this conversion is
simply the volt-second product equating of the
input and output. It is virtually identical to that
of square wave power conversion, and vastly
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