AN1673 APPLICATION NOTE

12/24

Figure 14. Simplified real converter

Consistent with the above consideration, we can see the circuit as a multiple resonant path.

E.g. in the family of curves depicted in fig.15, it can be observed the trend of the admittance as a function

of the frequency, for different resistive load.

Figure 15. Multiple Resonance

We have two separate peak of resonance:

The first peak (F01, lower in frequency) is inherent to the magnetizing inductance (in series with the L.Ext.

if one is connected) it is the case with the load open. The second resonance (F02) depends on the series

Both the resonance play a role in the energy transferred to the load and its control.

its current is negligible for the frequency range of our interest.

In this hypothesis, it is evident that the maximum current will occurs at the peak of F02 resonance; it should

not be reasonable to operate at frequency lower than F02 in order do not loosing the zero-voltage switch-

ing and also because the energy transferred to the load should not longer be increased reducing the fre-

quency.

In the practice, for the real transformer, the magnetizing inductance can't be negligible (its value will not

be so high). Anyway it should be possible to control its value making so it possible to operate at switching

frequency lower than the F02 (See fig16).

SECONDARY SIDE

PRIMARY SIDE

TRANSFORMER

L-SER

FULL BRIDGE

RECTIFICATION

N2

N1

LOAD

D03IN1433

Z

1

log

D03IN1446

F01

F02

operating locus

()