SLUA159 Datasheet, PDF (8/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
ZVS Converter Limitations:
In a ZVS converter operating under ideal
conditions, the on-time of the switch (fZ+tJ4)
approaches zero, and the converter will operate
at maximum frequency and deliver zero output
voltage. In a practical design , however, the
switch on-time cannot go to zero for several
reasons.
First of all, the resonant tank components
are selected based on the maximum input
voltage TJJN- and minimum output current
IOmin for the circuit to remain resonant over all
operating conditions of line and load. If the
circuit is to remain zero voltage switched, then
the resonant tank current cannot be allowed to
go to zero. It can, however, reach IO,,,,.,, .
There is a finite switch on-time associated
with the inductor charging interval tu where the
resonant inductor current linearly increases
from - I, to + I,. As the on-time in the power
transfer interval tJr approaches zero, so will the
converter output voltage. Therefore, the mini-
mum on-time and the maximum conversion
frequency can be calculated based upon the
limitation of Iomin and zero output voltage.
The limits of the four zero voltage switched
time intervals will be analyzed when IO goes to
IO minimum. Each solution will be retained in
terms of the resonant tank frequency wR for
generalization.
U-138
Both the minimum on-time and maximum
off-time have been described in terms of the
resonant tank frequency , wR. Taking this one
step further will result in the maximum conver-
sion frequency fCoti, also as a function of
the resonant tank frequency.
Maximum Off-Time:
The maximum conversion frequency corre-
sponds to the minimum conversion period,
TcoW,, , which is the sum of the minimum on-
time and maximum off-time:
The maximum conversion frequency, fcowm
= l/Tcowti,, , equals
The ratio of the maximum conversion fre-
quency to that of the resonant tank frequency
can be expressed as a topology coefficient, Kr.
For this zero voltage switched Buck regulator
and its derivatives, KTmax equals:
3-336