SLUA159 Datasheet, PDF (2/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
U-138
Fig. 1 - Zero Voltage Switching vs. Conventional Square Wave
unlike the energy transfer system of its electri-
cal dual, the zero current switched converter.
During the ZVS switch off-time, the L-C
tank circuit resonates. This traverses the volt-
age across the switch from zero to its peak,
and back down again to zero. At this point the
switch can be reactivated, and lossless zero
voltage switching facilitated. Since the output
capacitance of the MOSFET switch (Co& has
been discharged by the resonant tank, it does
not contribute to power loss or dissipation in
the switch. Therefore, the MOSFET transition
losses go to zero - regardless of operating
frequency and input voltage. This could repre-
sent a significant savings in power, and result in
a substantial improvement in efficiency. Obvi-
ously, this attribute makes zero voltage switch-
ing a suitable candidate for high frequency,
high voltage converter designs. Additionally, the
gate drive requirements are somewhat reduced
in a ZVS design due to the lack of the gate to
drain (Miller) charge, which is deleted when
V& equals zero.
The technique of zero voltage switching is
applicable to all switching topologies; the buck
regulator and its derivatives (forward, half and
full bridge), the flyback, and boost converters,
to name a few. This presentation will focus on
the continuous output current, buck derived
topologies, however a list of references describ-
ing the others has been included in the appen-
dix.
Fig. 2 - Resonant Switch Implementation
Fig. 3 - General Waveforms
ZVS Benefits
n Zero power “Lossless” switching transitions
n Reduced EMI / RFI at transitions
n No power loss due to discharging Goss
n No higher peak currents, (ie. ZCS) same as
square wave systems
n High efficiency with high voltage inputs at
any frequency
n Can incorporate parasitic circuit and compo-
nent L & C
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