AN436 Datasheet, PDF (1/9 Pages) STMicroelectronics – TRIAC CONTROL BY PULSE TRANSFORMER

AN436
APPLICATION NOTE
TRIAC CONTROL BY PULSE TRANSFORMER
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.
– Choice of the gate current polarity (triggering in the 2^nd and 3^rd quadrants for SNUBBERLESS triacs).
– 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 L_P inductance
The primary winding inductance measured at a given frequency.
The R_P resistance
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.
April 2004
Rev. D2A - 3575
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