AN441

AC switch control with an inductive load

Doc ID 3579 Rev 3

7/16

If a power supply with reduced output current capability is used (for example, the case with

capacitive or resistive power supplies), a solution to reduce the gate current average value

is to apply a high-frequency (several kilohertz) burst to the gate. This solution allows

reduction of Triac off time that occurs after current has reached zero and before a new gate

current pulse (coming with the high-frequency burst) is applied. This maximum off time will

then approximately equal (at worst case) to one gate-burst period (see Application note

AN308).

2.3

Inrush current limitation

To limit the inductive load inrush current, triggering at peak mains voltage could reduce the

peak current as previously shown. Also, especially for transformers, it is better to ensure that

the device is switched-on at a different polarity compared to last half-cycle conduction (refer

to Section 1.2).

But this triggering method will only be efficient in applications where the inrush current

comes from inductive part of the load or from core saturation. This is mainly the case with

transformers or valves. The inrush current period usually lasts less than 100 ms.

For other applications, the inrush current could also come from other phenomenon.

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Low-resistance value at start-up (cold filament of incandescent lamp)

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High starting torque (induction motors used in compressors)

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Low back EMF at zero speed, i.e. at start-up (universal motor or permanent-magnets

induction motors used in pump applications)

For these applications, the inrush current can last up to 500 ms.

As the inrush current is not only due to the behavior of the inductive part of the load, the

problem cannot be resolved just by triggering the Triac at peak voltage for the 1st half-cycle

and then control it in full wave-mode. The solution consists of implementing a soft start by

progressively increasing the conduction angle of the AC switch. This solution allows the

maximum current at turn on to be limited to a value near to the steady state. This brings the

following benefits:

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Lower rate of rise of temperature for the device and so better device reliability

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Compliance with the IEC 61000-3-3

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Lower peak current and so better load reliability