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AN1673 APPLICATION NOTE

The control of magnetizing inductance (in the LCL topology) can play an important role in the energy man-

Assuming the load is disconnected, the maximum current occurs at the peak of F01 resonance that con-

The value of the magnetizing inductance will be (usually) significantly higher than the series inductance,

consistently; the "lowest" resonance (F01) is mainly dependant on its values.

We can define F01 "lowest" resonance to well remind that the switching frequency must not be lower than

its value.

In fact, forcing the half bridge working at lower frequency (F01), the series circuit represents a capacitive

load. This condition cannot be accepted in the application for the following reasons:

– In capacitive load the "zero voltage switch operation" is lost with resulting significant power dissipation

(hard switching).

– In closed loop, the transfer function should be reversed (when going in below resonance) and the con-

trol should be lost.

To summarize:

It is needed to operate in the "inductive region" of the curve. In this way, the higher frequency applied to

the resonant net, the lower is the resulting power.

It is remarkable that, working in the deep inductive mode (far from the peak of resonance), the current

moves from sinusoidal shape to triangular and wide frequency change is requested to control the regula-

tion.

On the contrary, working as close as possible to the resonance, for a given load change, the frequency

variation will be minimized.

If the application is designed to operate between to the "lowest" resonance (F01) and the second reso-

nance (F02), it will be verified that the primary current (at the switch off time of one of the two Powermos),

should not be too low in order to ensure the energy needed for soft switching in all condition.

The L6598 function and the resonance's effects in the regulation have been discussed. Now we can start

to describe some design criteria.

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D03IN1449