SLUA159 Datasheet, PDF (16/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
ZVS Design Procedure
Buck Derived Topologies -- Continuous
output Current:
1. List all input/output specs and ranges.
VrN min & max ; Vo ; I, min & max
2. Estimate the maximum switch voltages. For
unclamped applications (buck and forward):
V_{DSmru = J%hltLz(l + (lomru/lomin)}
Note: Increase IO,,,,.,, if V’s= is too high if
possible).
For clamped applications (bridges):
V_{DSnw = bvnuu}
3. Select a resonant tank frequency, OR
(HINT: OR = 2x&).
4. Calculate the resonant tank impedance and
component values.
5. Calculate each of the interval durations (toI
thru fM) and their ranges as a function of all
line and load combinations.
(See Appendix for a sample computer
program written in BASIC)
Additionally, summarize the results to estab-
lish the range of conversion frequencies,
peak voltages and currents, etc.
6. Analyze the results. Determine if the fre-
quency range is suitable for the application.
If not, a recommendation is to limit the load
range by raising rod,, and start the design
procedure again. Verify also that the design
is feasible with existing technology and
components.
7. Finalize the circuit specifics and details.
0 Derive the transformer turns ratio. (non-
buck applications)
0 Design the output filter section based upon
the lowest conversion frequency and output
ripple current&(
0 Select applicable components; diode,
MOSFET etc.
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8. Breadboard the circuit carefully using RF
techniques wherever possible. Remember --
parasitic inductances and capacitances prefer
to resonate upon stimulation, and quite
often, unfavorably.
9. Debug and modify the circuit as required to
accommodate component parasitics, layout
concerns or packaging considerations.
Avoiding Parasitics
Ringing of the catch diode junction capaci-
tance with circuit inductance (and package
leads) will significantly degrade the circuit
performance. Probably the most common
solution to this everyday occurrance in square
wave converters is to shunt the diode with an
R-C snubber. Although somewhat dissipative,
a compromise can be established between
snubber losses and parasitic overshoot caused
by the ringing. Unsnubbed examples of various
applicable diodes are shown in Fig. 20 below.
Fig. 20 -- Catch Diode Ringing
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