Electromagnetic compatibility standards

AN1172

12/23

4

Electromagnetic compatibility standards

4.1

IEC 61000-4-5 standard

4.1.1

Standard requirements

The IEC 61000-4-5 standard has been established to check if systems can always work

after there has been a voltage surge super-imposed to the mains. A standard voltage

waveform has been chosen which embodies typical over-voltages due to thunder or

disconnection of running inductive loads from the line.

Two kinds of surges must be applied:

1.

Line to Ground surge: in this case the maximum voltage surge is 4 kV (for aerial

power network), but the energy is absorbed by the Y2 capacitors (connected between

lines and ground) of the mains filter.

2.

Line to Neutral surge: in this case the maximum voltage surge is 2 kV (for aerial

power network, N.B: 1 kV is required for public power network) and is applied across

the power device and the load controlled by this one.

A Line to Neutral over-voltage is then:

1.

entirely absorbed by the load if the power switch is ON;

2.

entirely held by the semiconductor device if it appears while the switch is at off-state.

As the Line to Neutral surge can appear at peak mains voltage, the overall amount of

voltage can reach 2.4 kV. This will be higher than the break-down level of the silicon devices

used in appliances. Then, in order to prevent components destruction, designers use a

varistor connected across silicon devices. The overvoltage is limited below the breakdown

level of the power semiconductor and the surge energy is absorbed by the metal-oxyde

component.

4.1.2

ACS behavior during IEC 61000-4-5 test

When a surge appears when an ACS is OFF, the mains over-voltage is first clamped by the

device. But an excessive energy surge can raise the ACS current above its breakover level.

Then, the switch turns on in break over mode. Such an event is particularly stressful on the

semiconductor especially so if the current and its rate of increase are both high. The worst

case occurs for ACS driving low resistance, non inductive loads.

For example, Figure 12. and Figure 13. have been recorded with a thermal active door lock

system at low temperature. The 2 kV surge is super-imposed to the 230 V - 50 Hz mains

and synchronized with its peak value, as shown on Figure 12. Figure 13. highlights the

device turn-on in this mode. As the load was previously off, its resistance is cold and equals

150

Ω. In this case, the current rises at a rate of 100 A/µs and reaches 15 A. Such transient

surges would damage triacs, but not ACSs which are designed to turn-on in breakover

mode. No more varistor is then needed in parallel across ACSs unlike triacs. The difference

between ACS and Triac + Varistor is that, with the ACS, the load is switched on during a half

or one mains cycle. This can be accepted as such events happen a few times in the

system's life.

Reliability tests are carried out on production batches to check the ACS robustness towards

IEC 61000-4-5. A standard surge generator is used directly across a load and an ACS. The

load is a 150

Ω resistor, including a 3 µH parasitic inductance, to simulate a cold door-lock.