SLUA159 Datasheet, PDF (20/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
U-138
lated range of conversion frequencies spans 87
to 310 kHz. These values will be used for this
“first cut” draft of the control circuit pro-
gramming. Due to the numerous circuit specif-
ics omitted from the computer program for
simplicity, the actual range of conversion fre-
quencies will probably be somewhat wider than
planned. Later, the actual timing component
values can be adjusted to accommodate these
differences.
First, a minimum fc of 75 kHz has been
selected and programmed according to the
following equation:
Fig. 26 -- CR Volts & Off-time vs. Line & Load
time. Initially, the one-shot is programmed for
the maximum off-time, and modulated via the
ZERO detection circuitry. The switch drain-
source voltage is sensed and scaled to initiate
turn-on when the precision 0.5V threshold is
crossed. This offset was selected to accommo-
date propogation delays between the instant the
threshold is sensed and the instant that the
switch is actually turned on. Although brief,
these delays can become significant in high
frequency applications, and if left unaccounted,
can cause NONZERO switching transitions.
Referring to Fig. 26, in this design, the off-
time varies between 1.11 and 1.80 microsec-
onds, using ideal components and neglecting
temperature effects on the resonant compo-
nents. Since the ZERO detect logic will facili-
tate “true” zero voltage switching, the off-time
can be set for a much greater period.
The one-shot has a 3:1 range capability
and will be programmed for 2.2 US
( m a x ) , c o n t r o l l a b l e d o w n t o 0 . 7 5 U S.
Programming of the one-shot requires
a single R-C time constant, and is
straightforward using the design infor-
mation and equations from the data
sheet. Implementation of this feature is
shown in the control circuit schematic.
The maximumfc of 350 kHZ is programmed
by:
Numerous values of Rti,, and C,, will satisfy
the equations. The procedure can be simplified
by letting Rtin equal 100K.
The VCO gain in frequency per volt from
the error amplifier output is approximated by:
with an approximate 3.6 volt delta from the
error amplifier.
VOLTAGE CONTROLLED OSCILLATOR
Programming the VCO. The calcu-
Fig. 27 -- E/A - VCO Block Diagram
3-348