DocID025296 Rev 1

5/20

AN4363

Current rating selection

20

Figure 3. Maximum power dissipation versus rms on-state current for a T1635T-8FP

1.2

Turn-off limitation for specific applications

One particularity of an AC switch is that it may remain ON when the gate current is

removed, even after that the current has reached zero. This kind of behavior could only

occur if the Triac operates with conditions beyond its datasheet guaranteed operation, and

particularly when the rate of load current decrease, or the rate of reapplied voltage after

turn-off are higher than the specified (dI/dt)c and (dV/dt)c rates.

An AC switch can be compared to two Thyristors mounted 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 charge is injected into the structure. This

charge disappears by recombination during the current decrease and by extraction after the

turn-off with the reverse recovery current (refer to Figure 4.a). Nonetheless, if an excess of

charge remains, particularly in the neighboring regions of the gate, this can induce the

triggering of the other conduction area when the mains voltage is reapplied across the Triac

(refer to Figure Figure 4.b).

P(W)

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12

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02

46

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I(A)

T(RMS)

180°

RMS load

current