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April 2004

AN436

APPLICATION NOTE

TRIAC CONTROL BY PULSE TRANSFORMER

Rev. D2A - 3575

INTRODUCTION

Among the many ways to drive a triac the pulse transformer is one of the easiest. By applying some simple

rules it can be used to design an efficient triac triggering circuit without reduction of the commutation ca-

pability of the triac.

WHY USE A PULSE TRANSFORMER?

The use of pulse transformers in triac triggering circuits offers many advantages:

– Galvanic insulation between the power and gate drive circuit (a few kV).

– Gate drive circuit with a few components.

– Optimization of gate signal (single pulse or train of pulses).

– Possibility to drive several triacs with only one drive circuit

THE PULSE TRANSFORMER

To optimize the triac and the pulse transformer in the application it is necessary to know the main charac-

teristics of the transformer:

The transformer ratio

It is the N2/N1 ratio, where N1 corresponds to the primary winding and N2 to the secondary.

The primary winding inductance measured at a given frequency.

The primary winding resistance.

The area of the output pulse

For a given magnetic material the voltage.time product Vo.to of the output pulse is constant. For each type

of transformer the manufacturer gives the maximum voltage.time product under no load operation which

corresponds to the Figure 1.