SLUA159 Datasheet, PDF (10/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
resonant and output inductors to maintain the
minimum output current, Iolrdn. In summary,
NO output power is derived from V”‘N during
interval tm.
The power required to support V0 at its
current of I0 is obtained from the input source
during the power transfer period tJ4. Therefore,
an effective “duty cycle” can be used to de-
scribe the power transfer interval fJ4 to that of
the entire switching period, tw, or Tconrv.
ZVS - Effective Duty Cycle Calculations:
U-138
could optionally be evaluated.
A computer program to calculate the numer-
ous time intervals and conversion frequencies
as a function of line and load can simplify the
design process, if not prove to be indispensable.
Listed in the Appendix of this section is a
BASIC language program which can be used to
initiate the design procedure.
To summarize: When the switch is on, re-
place with = v;N
KWv&(On))
(I/IN-IO l RDS(on))*
When the free-wheeling diode is on, replace V.
with (Vo+VF).
And can be analyzed over line and load
ranges using previous equations for each inter-
val.
Accommodating Losses in the Design
Equations:
Equations for zero voltage switching using
ideal components and circuit parameters have
been generated, primarily to understand each
of the intervals in addition to computer model-
ing purposes. The next logical progression is to
modify the equations to accommodate voltage
drops across the components due to series
impedance, like RDSo, and the catch diode
forward voltage drop. These two represent the
most significant loss contributions in the buck
regulator model. Later, the same equations will
be adapted for the buck derived topologies
which incorporate a transformer in the power
stage.
The procedure to modify the equations is
straightforward. Wherever V,N appears in the
equations while the switch is on it will be
replaced by V,N-V~~~on~ , the latter being a
function of the load current I,. The equations
can be further adjusted to accept changes of
R_{DS(on) and vF ?} etc. with the device junction
temperatures. Resonant component initial
tolerances, and temperature variations likewise
Transformer Coupled Circuit Equa-
tions:
The general design equations for the Buck
topology also apply for its derivates; namely the
forward, half-bridge, full-bridge and push-pull
converters. Listed below are the modifications
and circuit specifics to apply the previous
equations to transformer coupled circuits.
General Transformer Coupled Circuits. Maint-
aining the resonant tank components on the
primary side of the transformer isolation boun-
dary is probably the most common and sim-
plest of configurations. The design procedure
begins by transforming the output voltage and
current to the primary side through the turns
ratio, N. The prime (') designator will be used
to signify the translated variables as seen by the
primary side circuitry.
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