Conclusion

AN1966

12/14

5

Conclusion

A procedure has been given to choose the right Transil for TRIAC protection. The minimum

clamping voltage, experimental results are recommended to check the maximum current

which will flows through the Transil. This current is linked to the TRIAC turn-on delay, the line

surge voltage shape and the load impedance. So the current value is difficult to estimate

theoretically and it’s better to perform tests to measure it. It should be noted that the value is

then a typical one and a margin has to be added.

With the proposed protection circuit, the TRIAC voltage can be kept lower than the specified

is turned on as soon as the voltage exceeds the clamping level of the Transil. But this is the

price to pay to avoid any TRIAC damage in case of turn-on in breakover mode. The turn-on

is safe here as it is ensured by a gate current.

It should be noted that the spurious TRIAC turn-on only lasts a half-cycle (10 ms for 50 Hz

operation). It could be accepted for most applications, especially for heating applications

where a 10 ms switch on has no impact on the system behavior, as it could not be detected

by the end user and have no impact on excessive heating or power consumption.

This circuit enables a considerable increase in systems reliability and is mandatory to

implement with TRIACs driving high power resistive loads, where a varistor is not efficient

enough. ST has also introduced ACS and ACST devices, which feature such overvoltage

protection by crowbar. The protection is then directly integrated in the silicon die, and no

other external device, such as Transil and resistor, is required.