Fast transient immunity tests

AN1379

4/10

Doc ID 8317 Rev 5

2

Fast transient immunity tests

2.1

Standard requirements and mains filter utility

IEC 61000-4-4 tests propose two different kinds of stress. One, called the “supply test”,

consists of applying the bursts through 33 nF capacitors to the line, neutral, ground or

combinations of these terminals. The second EN61000-4-4 stressing mode is to apply the

bursts through a typical 100 pF capacitor (achieved with 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 the keyboard and the central unit). But appliance manufacturers

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

For both cases, the system under test is placed 10 cm above the bursts generator reference

plane. As the voltage waveforms increase and decrease respectively in 5 and 50 ns, the

impedance of the parasitic capacitance between the system board and the plane is very low

for such high dynamic waveforms. This results in the application of the major part of the

burst directly across the mains supply plug of the system under test. The following figures

show the line to neutral voltage measured during a 2 kV supply test. We see that the voltage

reaches up to 1.41 kV when no filter is used at the mains input of the system. If a filter, such

as that described in Figure 7, is added, the overvoltage, caused by the 2 kV burst falls to

584 V. This is below the breakdown voltage of most Triacs used in 230 V appliances and of

ACSs. Then, there is no risk of a spurious firing due to turn on in breakover mode.

But Figure 6 also shows that, despite overvoltages being limited, high dV/dt rates are still

present. Indeed, the 500 V overvoltage is reached in less than 10 ns. Silicon devices must

therefore feature a very high dV/dt capability in order to avoid parasitic turn on. The next

paragraph summarizes some comparative analysis made on Triacs and ACSs.

Figure 5.

Line to neutral voltage during 2 kV burst without mains filter

1.41 kV