DocID025296 Rev 1

7/20

AN4363

Current rating selection

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the application are:

•

Universal motors: Due to the brush commutations, the rate of decrease of the current is

higher than the one calculated from the rms current from the equations.Typically the

rate can be three times higher.

•

Loads (such as universal motor or valve) connected in a diode bridge (refer to

STMicroelectronics Application note AN439): In this case, the rate of decrease of the

Triac current is limited only by the mains inductance. This is the reason why an inductor

has to be put in series with the Triac to reduce this rate of decrease.

1.3

Inrush current and stalled rotor operation

AC switches thanks to their N-P-N-P layer silicon structure feature the best overcurrent

capability among semiconductor switches. For example, the maximum peak current an AC

switch can withstand during a line cycle (10 ms for a 50 Hz frequency) is usually 6 or 10

For a 1 ms pulse length, the ratio can even reach 20 to 40.

and two curves giving the variation of this parameter according to the pulse duration. One

for pulse durations shorter than 10 ms.

These two curves allow users to know if the selected device fits with their application

requirements, particularly if overcurrent due to inrush currents or stalled rotor operation can

occur.

These curves demonstrate that is not necessarily mandatory to over-rate an AC switch

current rating for a motor control application.