SLUA159 Datasheet, PDF (6/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
Inductor Charging State: fi - t3
To facilitate zero voltage switching, switch
Q, is activated once the voltage I& across Q,
and resonant capacitor VCR has reached zero,
occurring at time tZ. During this inductor
charging interval tu resonant inductor current
ILR is linearly returned from its negative peak
of minus I, to its positive level of plus I,.
The output catch diode D, conducts during
the tu interval. It continues to freewheel the
full output current I,, clamping one end of the
resonant inductor to ground through Do. There
is a constant voltage, V’,N - I& , across the
resonant inductor. As a result, ILR rises linearly,
I,, decreases linearly. Energy stored in output
inductor L, continues to be delivered to the
load during this time period.
A noteworthy peculiarity during this time-
span can be seen in the switch dram current
waveform. At time f2, when the switch is turned
on, current is actually returning from the
resonant tank to the input source, I&. This
indicates the requirement for a reverse polarity
diode across the switch to accommodate the bi-
directional current. An interesting result is that
the switch can be turned on at any time during
the first half of the fB interval without affecting
normal operation. A separate time interval
could be used to identify this region if desired.
Table IV - INDUCTOR CHARGING: t2 - t3
COMP. STATUS CIRCUIT VALUES
the conversion period, most of the pertinent
waveforms approach DC conditions.
Assuming ideal components, with Q, closed,
the input source supplies output current , and
the output filter inductor voltage VLo equals VIN
- Vo. The switch current and resonant inductor
current are both equal to IO, and their respec-
tive voltage drops are zero (V& = V’,=O).
Catch diode voltage Vm equals VIN, and Im = 0.
In closed loop operation where the output
voltage is in regulation, the control circuit
essentially varies the on-time of the switch
during the tJ4 interval. Variable frequency
operation is actually the result of modulating
the on-time as dictated by line and load condi-
tions. Increasing the time duration, or lowering
the conversion frequency has the same effect as
widening the duty cycle in a traditional square
wave converter. For example, if the output
voltage were to drop in response to an
increased load, the conversion frequency would
decrease in order to raise the effective ON
period. Conversely, at light loads where little
energy is drawn from the output capacitor, the
control circuit would adjust to minimize the lJ,
duration by increasing the conversion frequen-
cy. In summary, the conversion frequency is
inversely proportional to the power delivered to
the load.
Power Transfer State: t3 - tr
Once the resonant inductor current ILR has
reached I, at time tJ, the zero voltage switched
converter resembles a conventional square
wave power processor. During the remainder of
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