Snubber circuit functions and drawback

AN437

4/18

Figure 4.

Z0103 TRIAC turn-off on inductive load without and with snubber circuit

(C = 10 nF and R = 2.7 k

Ω)

The snubber circuit design, detailed in Section 2: How to design snubber circuit for turn-off

improvement, is a trade-off between the maximum peak off-state voltage under pulse

stress (dI/dt). When low load inductances are controlled or, low damping factor or low slope

of reapplied voltage are considered, the snubber circuit design can lead to choose a low

snubber resistance value. To reduce the snubber capacitance discharge at turn-on, the

resistance value is limited to a minimum value (refer to Section 1.4).

1.2

Overvoltage limitation at turn-off

When a TRIAC controls low root-mean-square currents inductive loads, an overvoltage

may be even damaged. The protections against overvoltage at turn-off are:

●

A clamping strategy - use a varistor or an ACSTM / ACST (refer to AN1172 about

protected AC Switch™).

●

A damping strategy - a snubber circuit. An RC snubber circuit limits the slope of the

voltage rise and could maintain the overvoltage at a lower value than the maximum

allowed value.

Figure 5.

Overvoltage at TRIAC turn-off with and without snubber circuit

(C = 10 nF and R = 2.7 k

Ω)

V

I

Without

With

snubber

snubber

:dV/dt

OFF

V

I

Without snubber

With snubber

V

V

I

H