AN3169

IEC 61000-4-4 Tests

Doc ID 17194 Rev 1

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Figure 5.

IEC 61000-4-4 test schematic

2.2

Impact of a varistor on mains input

The IEC 61000-4-4 standard as been developed mainly to protect equipment from fast

voltage transients coming from bad mains connection, electrostatic discharge or inductive

load disconnection. Designers usually think that the impact of the burst is mainly due to the

high dV/dt, and so due to the capacitive currents circulating through all parasitic capacitors.

This effect is one of the major ones. Applying bursts can also cause some spurious device

turn on due to excessive voltage across the switches. For example, Figure 6 shows the

voltage and current waveforms of an ACST2 used in the previously described board but

without any varistor for a 1 kV burst. We see that when the voltage spikes are applied at

peak mains voltage, the device turns on, and turns off for the next ZCS (zero current

switching) point. If we zoom in on this oscillogram with a 0.4 µs/division time scale, we

clearly see that the triggering comes from an overvoltage. Indeed, Figure 7 shows that the

voltage across the ACST2 reaches more than 800 V, and then the device turns on in

breakover mode.

Such a turn on can be allowed with ACST2 devices, contrary to other technologies and even

contrary to other planar devices from other companies. For ACST2 there will not be any risk

of device failure, as long as the applied current remains below the guaranteed limit. But,

applying such bursts without any voltage protection on mains input could lead to devices

failure with top or mesa glass technologies. This is the reason why we put a varistor on the

board we used for IEC 61000-4-4 tests.

Figure 6.

ACST2 behavior during 1 kV burst (without input varistor)

V

Rg

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N

V

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OUT-COM

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OUT

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OUT-COM

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OUT