AN441 Datasheet, PDF (3/16 Pages) STMicroelectronics – Inductive load control with AC switches

AN441
Reasons for inrush current in inductive loads
These two equations show that, when the inductive load is switched on in full-wave, the
transient current depends on the Triac first turn-on delay with respect to the mains voltage
zero point. Figure 2 shows the load current curve for triggering at zero voltage and triggering
at the peak mains voltage. This figure comes from PSPICE simulation for a 150 ohm and
5 H load switched on at a voltage of 230 V rms at 50 Hz.
Triggering at zero voltage brings the highest inrush current which can be up to twice the
peak current reached in case of triggering at peak voltage.
Figure 2. Inrush current difference according to Triac first turn-on delay
400
400
300
300
200
200
100
100
0
0
-100
0
20
40
60
80
100
-100
-200
-200
-300
-300
-400
Turn-on at zero crossing
Time (ms)
Turn-on at peak mains voltage
Mains voltage
-400
Due to this high peak current, two problems may occur at AC switch level.
● High peak current may be higher than I_TSM value (maximum surge peak current). In this
case the component can be damaged.
● AC switch temperature may exceed the maximum allowed junction temperature (this
will not lead necessarily to device failure but electrical parameters are not anymore
guaranteed if working temperature is above max allowed value).
Inrush currents have also to be checked to fit electromagnetic compatibility standards.
Actually, IEC 61000-3-3 standard make it mandatory to limit inrush currents of appliances
connected to the power network to reduce the flickering effect on lighting.
It should also be noted that reducing inrush current helps to increase the reliability of the
load and other switches or breakers used in series with the load.
1.2
Magnetic core saturation due to remanent induction
In transient operation, the induction can follow a different path and reach the saturation
value BS for which the magnetic field H increases very rapidly even for a low induction
variation (see Figure 3). At saturation level the magnetic material permeability decreases
drastically, down to air permeability. This leads to a lower inductance value. The load current
is then mainly limited by the load resistance, and can increase substantially. Saturation then
leads to a high increase of the coil current.
Doc ID 3579 Rev 3
3/16