March 2012

Doc ID 022635 Rev 1

1/15

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.

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