SLUA159 Datasheet, PDF (9/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
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conversion period where tJ4 equals zero. Topol-
ogy coefficient K-r ^{will be incorporated to}
define the ratio of the maximum conversion
frequency (minimum conversion period) to that
of the resonant tank frequency, OR.
Fig. IO -- Waveforms at Fcow = KT l fR
In a realistic application, the output voltage
of the power supply is held in regulation at VO
which stipulates that the on-time in the power
processing state, tJ4 , cannot go to zero as in
the example above. The volt-second product re-
quirements of the output must be satisfied
during this period, just as in any square wave
converter design. Analogous to minimum duty
cycle, the minimum on-time for a given design
will be a function of V’&, VO and the resonant
tank frequency, oR.
Although small, a specific amount of energy
is transferred from the input to the output
during the capacitor charging interval t,. The
voltage into the output filter section linearly
decreases from & at time t, to zero at tl,
equal to an average value of V,,,,/2. In addition,
a constant current equal to the output current
I, was being supplied from the input source.
The average energy transferred during this
interval is defined as:
The equation can be reorganized in terms of
CR and f+ as:
This minimum energy can be equated to
minimum output watts by dividing it by its
This demonstrates that a zero power output
is unobtainable in reality. The same is true for
the ability to obtain zero output voltage.
The equation can be rewritten as:
Solving for the highest minimum output
voltage, the worst case for occurs when I,
equals Iomin and VIN is at its maximum, VlN-.
Under normal circumstances the circuit will
be operating far above this minimum require-
ment. In most applications, the amount of
power transferred during the capacitor charging
interval t, can be neglected as it represents
less than seven percent (7%) of the minimum
input power. This corresponds to less than one
percent of the total input power assuming a
10:1 load range.
ZVS Effective Duty Cycles:
A valid assumption is that a negligible
amount of power is delivered to the load
during the capacitor charging interval to,. Also,
no power is transferred during the resonant
period from t,,. Although the switch is on
during period tw, it is only recharging the
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