SLUA159 Datasheet, PDF (4/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
U-138
Table II - CAPACITOR CHARGING: b - tl
Fig. 5 - Simplified Model
Fig. 6 - Resonant Capacitor Waveforms
it is off, the current is diverted around the
switch through the resonant capacitor CR. The
constant output current will linearly increase
the voltage across the resonant capacitor until
it reaches the input voltage (VCR = VI,,,). Since
the current is not changing, neither is the
voltage across resonant inductor L,.
At time t, the switch current IO “instantly”
drops from IO to zero. Simultaneously, the
resonant capacitor current IcR snaps from zero
to I,, while the resonant inductor current ILR
and output inductor current IL0 are constant
and also equal to I, during interval toI. Voltage
across output inductor Lo and output catch
diode D, linearly decreases during this interval
due to the linearly increasing voltage across
resonant capacitor CR. At time tl , VCR equals
and D, starts to conduct.
Resonant State: t, - t,
The resonant portion of the conversion cycle
begins at tI when the voltage across resonant
capacitor VCR equals the input voltage VIN, and
the output catch diode begins conducting. At
t, , current through the resonant components
IcR and ILR equals the output current I,.
The stimulus for this series resonant L-C
circuit is output current I, flowing through the
resonant inductor prior to time t,. The ensuing
resonant tank current follows a cosine function
beginning at time t,, and ending at time t2. At
the natural resonant frequency OR9 each of the
L-C tank components exhibit an impedance
equal to the tank impedance, ZR. Therefore,
the peak voltage across CR and switch Q, are a
function of ZR and I, .
The instantaneous voltage across CR and Q,
can be evaluated over the resonant time inter-
val using the following relationships:
Of greater importance is the ability to solve
the equations for the precise off-time of the
switch. This off-time will vary with line and
load changes and the control circuit must
respond in order to facilitate true zero voltage
switching. While some allowance does exist for
a fixed off time technique, the degree of lati-
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