AN1966 Datasheet, PDF (1/14 Pages) STMicroelectronics – In most of their applications

AN1966
Application note
TRIAC overvoltage protection using a Transil™
Introduction
In most of their applications, TRIACs are directly exposed to overvoltages coming from the
mains, as described in IEC 61000-4-5 or IEC 61000-4-4 standards. When TRIACs are used
to drive resistive loads (ex: temperature regulation), it is essential to provide them with
efficient overvoltage protection to prevent any turn-on in breakover mode that could lead to
device damage.
A traditional method to clamp the voltage is to use a varistor in parallel across the TRIAC.
But with high power loads (a few kW), the current through the varistor is very high in case of
surge voltages (a few hundred amperes). The varistor is then not efficient enough, due to its
dynamic resistor, to limit the TRIAC voltage to a low value.
We present here a solution that can be used for these kinds of applications and also for all
applications where TRIAC voltage protection is required.
It should be noted that the overvoltages could also come from the overvoltages that appear
at device turn-off due to the TRIAC holding current. This phenomenon occurs mainly with
TRIACs controlling low rms current (15-50 mA), high inductive loads like valves. For more
information about such behavior, please refer to AN1172.
Contents
1
Why overvoltage protection is required . . . . . . . . . . . . . . . . . . . . . . . . . 2
2
Overvoltage protection solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3
Transil choice for efficient TRIAC voltage protection . . . . . . . . . . . . . . 6
3.1 Normal operation: check V_RM voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Surge voltage clamping: check max V_CL voltage . . . . . . . . . . . . . . . . . . . . 6
4
Experimental validation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
October 2007
Rev 3
1/14
www.st.com