AN1172

Resistive loads on/off control

9/23

3

Resistive loads on/off control

3.1

Inrush current

In most systems, resistive loads are thermal effective. For example, light bulbs emit light

when their filament is hot enough. New types of door-lock actuators have emerged in which

the bolt move is due to a thermal expansion of a metallic part or a wax.

All these loads can be characterized by a very low resistance value in cold state.

Consequently, when the switch is turned on, there is a high inrush current. For low power

light bulbs, the inrush current lasts on average 10 ms. The worst scenario is in the case of

thermal door-locks. Figure 9. shows a typical inrush current in such loads.

Figure 9.

Inrush current in a 230 V thermal effective door-lock

3.2

Transient junction temperature

With such current shapes at turn-on, thermal calculation must be carried out in order to

choose the right package or heat-sink so as to avoid exceeding the maximal junction

temperature (110° C).

To perform the calculus, the current shape must be simplified. Let us work on the hypothesis

that the current average waveform of Figure 9. is similar to a 2 A peak 0.18 s time long sinus

shape.

Vout (200 V/div)

Iout (1 A/div)