TRIAC turn-off description

AN439

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1

TRIAC turn-off description

1.1

Definition

The TRIAC can be compared to two thyristors mounted in back-to-back and coupled with a

control area which allows the triggering of this Alternating Current Switch with only one gate

(see Figure 1).

Looking at the TRIAC silicon structure (see Figure 2), it can be noted that the conduction

areas, corresponding to these two thyristors, narrowly overlap each other on the control

area.

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

biggest part of this charge disappears by recombination during the current decrease, while

another part is extracted after the turn-off by the reverse recovery current. Nonetheless, an

excess 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. This is the problem of commutation.

For a given structure at a determined junction temperature, the turn-off behavior depends

on:

1.

The quantity of charge which remains when the current drops to zero. The

quantity of the charge is linked to the value of the current which was circulating in the

TRIAC approximately 100 µs, about two or three times the minority carriers’ life time,

before the turn-off. Thus, the parameter to consider is the slope of the decreasing

2.

The slope of the reapplied voltage during turn-off. This parameter is the

injected and added to those coming from the previous conduction.

Figure 1.

Simplified equivalent

schematic of TRIAC circuit

Figure 2.

Example of TRIAC silicon

structure

A1

A2

G

Gates

Ctrl.

N3

P2

N2

P1

N1

P2

N2

P1

N4

A2

A1

G

N4

Gates

Ctrl