Snubber circuit functions and drawback

AN437

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1

Snubber circuit functions and drawback

1.1

Turn-off improvement

1.1.1

TRIAC turn-off reminder

When a TRIAC switches from on-state to off-state, the current passes through zero and the

supply voltage is reapplied instantaneously across the structure. In certain conditions, the

component is not able to block this voltage and then turns on spontaneously.

Indeed, a TRIAC can be compared to two Thyristors mounted in back-to-back and coupled

with a single control area. To trigger the two Thyristors, the control area overlaps the two

conduction areas.

During the conduction time, a certain quantity of charges is injected into the structure.

These charges disappear by recombination during the current decrease and by extraction

after the turn-off with the reverse recovery current (refer to Figure 1). Nonetheless, an

excess of charge remains, particularly in the neighboring regions of the gate, which can

induce the triggering of the other conduction area when the mains voltage is reapplied

across the TRIAC (refer to Figure 2).

A spurious triggering depends on:

●

determines the quantity of charges which remains, when the current drops to zero, and

which could be injected in the gate area or in the opposite Thyristor.

●

defines the capacitive current which could be injected through the gate.

Figure 1.

TRIAC turn-off on inductive

load - suitable turn-off

Figure 2.

TRIAC turn-off on inductive

load - spurious triggering

V

I

V

Recovery current

V

I

dV/dt

OFF

dI/dt

OFF