Why overvoltage protection is required

AN1966

2/14

1

Why overvoltage protection is required

In a typical circuit (see Figure 1.), a surge voltage superimposed on the network voltage can

turn the TRIAC on by exceeding its breakdown voltage.

Under these conditions, because of its internal structure, only a part of the TRIAC is

effectively turned on and the TRIAC can thus withstand only a very low di/dt. This explains

the high risk of component damage when TRIACs are used to drive purely resistive loads.

The di/dt can, in this case, reach very high values (> 100 A/µs) since only the inductance of

the connections limits the current rate of increase.

Figure 1.

Typical TRIAC application circuit without overvoltage protection (risk of

damage)

+

accidental

overvoltage

R

A2

A1

G

Triggering

at zero

voltage

Control