AN437

Snubber circuit functions and drawback

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1.1.2

Snubber circuit benefit at TRIAC turn-off

The TRIAC turn-off behavior is characterized by the datasheet curve between the critical

rate of decrease of commutating on-state current ((dI/dt)c) and the critical rate of rise of

commutation off-state voltage ((dV/dt)c) (refer to Figure 3). These parameters are specified

for the maximum operating junction temperature (worst case).

In practice, the current waveform, and thus the slope of the decreasing current, is imposed

by the load. The user can then only limit the slope of the reapplied voltage. Indeed, by

adding an snubber circuit across the TRIAC, the circuit time response is increased and thus,

Figure 3.

(dI/dt)c versus (dV/dt)c curve for Z01 standard TRIACs and snubber

circuit impact

An RC snubber circuit must be used when there is a risk of TRIAC spurious triggering, i.e.

datasheet values, (dI/dt)c at a given (dV/dt)c.

Figure 4 shows the turn-off behavior of a Z0103 standard TRIAC which controls a 26 W

drain pump. Without snubber circuit and for the maximum junction temperature (110° C), a

values, equal respectively to 0.13 A/ms and 10 V/µs, are higher than the guarantee (dI/dt)c -

(dV/dt)c point (only 7 V/µs @ 0.13 A/ms, see Figure 3).

Thanks to an RC snubber circuit (10 nF and 2.7 k

Ω), the slope of the reapplied voltage can

be limited to 1.5 V/µs and thus spurious triggering at turn-off can be avoided (see Figure 3

and Figure 4).

V

T

I

T

Load

V

Mains

R

C

Area of spurious

firing at

commutation

Safe area

Operating point

with RC snubber

Operating point