AN1172

Inductive loads on/off control

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2

Inductive loads on/off control

2.1

Valves and relays

2.1.1

Turn-off overvoltages are clamped by ACSs

Valves and relays are both electromagnetic systems. In the case of AC high voltage

operation, their windings present a high series resistance (a few k

Ω) and a high series

inductance (tens of Henry). Hence, they absorb a low RMS current (typically, 10 to 50 mA).

In this case, the current rate of decrease is low and an automatic switch turn-off may result,

when its current becomes lower than the holding level (see References, 2.). There may be

an over-voltage due to the fact that there is still some current through the inductive load. The

inductive energy thus creates a back electromotive voltage which tends to force the switch to

conduct. If this over-voltage is not clamped, it can exceed the device breakdown level and

damage it.

ACSs are over-voltage self-protected. They can sustain their holding current in such an

operating mode, as shown in Figure 4.

Figure 4.

ACS voltage and current waveforms at turn-off (230 V 35 mA RMS valve)

During clamping periods, the inductive energy is dissipated both in the silicon die and the

series resistance of the load. The worst case appears when the load inductance is the

highest, i.e. for electromagnet loads.

In annex C, a theoretical analysis is performed with a 0.1 power factor load and an RMS

current lower than 40 mA (value which never appears in practice where, for such RMS

currents, the power factor is always higher than 0.7). Then, it is demonstrated that, even in

this worst case scenario, the transient junction temperature remains below 160°C. And the

ACSs dies thanks to their reliable planar technology.

Iout (10 mA/div)

Vout (200 V/div)