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AN436 APPLICATION NOTE

a safety coefficient of 2:

Duration

The Vo.to product defines the maximum pulse duration at the output of the transformer. The anode current

For drives with a pulse train we can sometimes use very short pulses ( for example tp =10µs with a 15µs

cycle).

For proper triac triggering the gate current rise time is very important in a circuit with very high di/dt (>20

A/µs): case of resistive load.

THE COMMUTATION

The use of a triac with a pulse transformer needs some precautions in order not to decrease the commu-

tation capability.

The commutation

Review: during the conduction a certain quantity of charges is injected into the triac.

During the fall of the current most of them disappear by recombination. If the current decreases too fast

the charges do not have time to recombine and some charge stays in the gate area. This can provoke a

spurious firing.

The parameter which characterizes the commutation is the anode current slope (di/dt)c, that is to say the

slope of current before zero crossing.

The specified value in the data sheet is the critical (di/dt)c. Above this value the triac is liable to fire spuri-

ously. Figure 5 shows the spurious firing due to (di/dt)c.

Case of a triac triggered by a transformer

When the triac is on, a voltage of about 0.6V appears across the gate and cathode. This voltage is either

positive or negative depending on the anode current polarity. A current i can flow through the secondary

winding of the transformer (see Figure 4).

Figure 4. Use of a triac with a pulse transformeur: when the Triac is on a current flows through the

gate

Due to the inductance of the transformer, at the end of the half wave the current i continues to flow in the

gate and increases the risk of spurious firing at the next cycle (see Figure 5).

i

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