SLUA159 Datasheet, PDF (17/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
Multiresonant ZVS Conversion
Another technique to avoid the parasitic
resonance involving the catch diode capacitance
is to shunt it with a capacitor much larger than
the junction capacitance. Labelled CD, this
element introduces favorable switching charac-
teristics for both the switch and catch diode.
The general circuit diagram and associated
waveforms are shown below, but will not be
explored further in this presentation [14,15].
U-138
Fig. 21 -- Multiresonant ZVS Circuit
Current Mode Controlled
ZVS Conversion
Variable frequency power converters can
also benefit from the use of current mode
control. Two loops are used to determine the
precise ON time of the power switch -- an
“outer” voltage feedback loop, and an “inner”
current sensing loop. The advantage to this
approach is making the power stage operate as
a voltage controlled current source. This elimi-
nates the two pole output inductor characteris-
tics in addition to providing enhanced dynamic
transient response.
Principles of operation. Two control ICS are
utilized in this design example. The UC3843A
PWM performs the current mode control by
providing an output pulse width determined by
the two control loop inputs. This pulse width,
or repetition rate is used to set the conversion
period of the UC3864 ZVS resonant controller.
Rather than utilize its voltage controlled oscil-
lator to generate the conversion period, it is
Fig. 22 -- Multiresonant Waveforms
determined by the UC3843A output pulse
width.
Zero voltage switching is performed by the
UC3864 one-shot timer and zero crossing
detection circuitry. When the resonant capaci-
tor voltage crosses zero, the UC3864 output
goes high. This turns ON the power switch and
recycles the UC3843A to initiate the next
current mode controlled period. The UC3864
fault circuitry functions, but its error amplifier
and VCO are not used.
3-345