AN1172

Electromagnetic compatibility standards

13/23

The applied surge is fixed at +/- 2,4 kV in order to be equivalent to a 2 kV over-voltage

applied at the peak mains voltage, with the same bias.

4.2

IEC 61000-4-4 standard

4.2.1

2.1: Standard requirements

For IEC 61000-4-4 tests, two different stressing ways are demanded. One is to apply the

bursts to the Line, Neutral or Ground through 33 nF capacitors. In this case, the bursts are

entirely absorbed by the mains filter, which is always present at the input of electronic

systems. The second IEC 61000-4-4 stressing mode is to apply the bursts through a typical

100 pF capacitor (realized by an aluminum sheet), directly to the I/O ports of the system.

The I/O port test is in fact required for systems where there are control wires, as for

computers (wires between keyboard and central unit). But appliance manufacturers apply

similar test to check if their products can withstand fast voltage transients.

The standard requires that for burst voltages up to 2 kV, the system must operate without

problem. However, triacs can then turn-on due to high dV/dt rates. In this case, a snubber

must be added to smooth these rates. Designers must then manage with the following

trade-off:

1.

Reduce dV/dt rates: the snubber capacitance must be high and the snubber

resistance must be low;

2.

Reduce the dI/dt rate at turn-on: the snubber capacitance must be low and the

snubber resistance must be high.

4.2.2

Snubber removal thanks to ACSs

I/O tests have been carried out on an electronic board including an ACS402, where the

gates are short-circuited to the COM in order to avoid parasitic turn-on due to MCU bad

operations. The system under test is embodied by this board. The I/O wires are then the

OUT pins of each ACS cell (which are connected to the loads), plus the Line and Neutral

wires. The trial diagram is shown in Figure 14.

Figure 12.

2 kV surge on the mains

(IEC 61000-4-5 test)

Figure 13.

ACS breakdown zoom

(IEC 61000-4-5 test)