AN437

Snubber circuit functions and drawback

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1.3

Immunity to fast voltage transient improvement

Electrical noise may appear on the mains and generates across the TRIAC fast voltage

variations, as described in IEC 61000-4-4 standard.

between A2 and the gate, and could trigger the TRIAC. The maximum rate of rise of off-

state voltage that a TRIAC is able to withstand without turning on is called the static dV/dt. A

spurious triggering due to static dV/dt is not dangerous for a component. The aim of the

snubber circuit is to reduce the static dV/dt at a lower level than the dV/dt specified in the

datasheet to avoid spurious triggering.

An RC snubber circuit improves the TRIAC immunity against fast voltage transients. For

example, regarding to the standard IEC 61000-4-4, a Z0109 standard TRIAC has a typical

immunity level of about 0.7 kV, without any snubber circuit. With a snubber circuit (1 nF and

47

Ω), the Z0109 immunity level can reach 4.0 kV.

Designers must manage the following trade-off to choose the suitable RC snubber circuit:

●

Reduce dV/dt rates: the snubber capacitance must be high and the snubber resistance

must be low;

●

Reduce dI/dt rate at turn-on (refer to Section 1.4): the snubber capacitance must be low

and the snubber resistance must be high.

1.4

Turn-on stress due to snubber circuit discharge

The snubber circuit design can lead to low resistance value. However, the snubber resistor

The rate of current rise is directly proportional to the initial capacitance voltage and inversely

proportional to the series inductances of the board and the snubber resistor. The rate of

current rise depends also on the turn-on speed of the TRIAC, the triggering quadrants and

the gate current amplitude. So, there is no simple way to predict the rate of current rise.

Usually, the inductance of the circuit layout is very low, in the range of few nH. Indeed, to

optimize the snubber circuit efficiency, the snubber circuit must be located very close to the

TRIAC (tracks length lower than 2 cm).

From datasheet specifications, there are three ranges of maximum dI/dt:

●

dI/dt = 20 A/µs: for low current rating of TRIACs (0.8 A and 1 A).

●

dI/dt = 50 A/µs: for the other TRIACs (4 A up to 40 A).

●

dI/dt = 100 A/µs: for some ACSTs (6 A up to 12 A).

resistance must be typically higher than 47

Ω (refer to Figure 6). For a 20 A/µs maximum

dI/dt, the minimum resistance value is about 620

Ω. Therefore, depending on the component

used, some tests should be performed to define accurately the minimum resistance value.