AN4030 Datasheet, PDF (1/15 Pages) STMicroelectronics – Triacs and SCRs are power semiconductors

AN4030
Application note
Gate to cathode capacitor, impact on Triac immunity and reliability
Introduction
Triacs and SCRs are power semiconductors which are usually directly connected to the grid
line. As is well known, the AC grid voltage can be highly perturbed by important voltage
variation during very short times, for example, mechanical relay contact bounce, universal
motor disturbances. Several electromagnetic standards describe how appliances have to be
tested to check their immunity to such events. For example, the IEC 61000-4-4 standard
gives the test procedures and immunity requirements respectively for fast transient voltages.
To increase power-semiconductor device immunity, most designers add a capacitor across
the power device control terminal (the gate for SCRs or Triacs) and its reference terminal (K
or A1 respectively for SCRs and Triacs). This capacitor helps stabilize the control terminal
potential and so is believed to help increase resistance to fast voltage rises (dV/dt). As this
capacitor is also placed between the power device and the control circuit, which could be a
logic gate or a microcontroller unit (MCU), it acts as a filter on the path from the line to the
sensitive control circuit, and helps filter the noise coming from line disturbances.
This paper demonstrates why a gate to cathode capacitor is not efficient to improve Triac
immunity to fast voltage transients, especially for non-sensitive devices. This Application
note demonstrates that such capacitors can increase the risk of failure for repetitive or
accidentally high dI/dt.
The results presented in this Application note have been produced over a considerable
period of time. Some products, which are used as examples to present these results, may
no longer be available. However, the results presented apply to classes and types of
product, and thus are equally applicable to similar products available in the market.
March 2012
Doc ID 022635 Rev 1
1/15
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