SLUA159 Datasheet, PDF (11/28 Pages) Texas Instruments – Zero Voltage Switching Resonant Power Conversion

APPLICATION NOTE
N=
Primary Turns
Secondary Turns
To satisfy the condition for resonance, IR c Iof
The resonant tank component equations now
become:
I
Note: the calculated resonant inductance
value does not include any series inductance,
typical of the transformer leakage and wiring
inductances.
Note: the calculated resonant capacitor value
does not include any parallel capacitance,
typical of a MOSFET output capacitance, Goss,
in shunt. Multi-transistor variations of the buck
topology should accommodate all switch capaci-
tances in the analysis.
Timing Equations (including N):
U-138
Determining Transformer Turns Ratio (N):
The transformer turns ratio is derived from the
equations used to define the power transfer
interval tM in addition to the maximum off-
time, tcu. While this may first seem like an
iterative process, it simplifies to the volt-second
product relationship described. The general
equations are listed below.
The turns ratio N is derived by substituting
NW0 for the output voltage V. in the power
transfer interval tJ4 equation. Solving for N
results in the relationship:
The transformer magnetizing and leakage
inductance is part of the resonant inductance.
This requires adjustment of the resonant induc-
tor value, or both the resonant tank impedance
zR and frequency wR will be off-target. One
Fig. 11 -- Transformer Inductance “Shim ”
option is to design the transformer inductance
to be exactly the required resonant inductance,
thus eliminating one component. For precision
applications, the transformer inductance should
be made slightly smaller than required, and
“shimmed” up with a small inductor.
3-339