AN441

Typical examples of inrush current reduction

Doc ID 3579 Rev 3

9/16

Triac turn on at peak mains voltage allows the reduction of the peak inrush current by 75%

compared to a zero voltage switch on as shown in Figure 8.

The reduction of first peak current allows:

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3.2

Transformer for lighting

A Triac could be used to dim SELV (safety extra low voltage) halogen lamps. In this case the

lamps are connected at the secondary winding of a 220 V to 12 V step-down transformer.

The Triac is then connected on this transformer in series with the primary winding (Figure 9).

The power applied to the lamp is controlled by the variation of the conducting angle of the

Triac and allows dimming the brightness of the lamp.

Figure 9.

Typical schematic of a light dimmer for SELV halogen lamps

Like microwave oven transformers, transformers for lighting application present an

overcurrent if they are switched on at zero voltage due to magnetizing current.

This inrush current is increased if the lamp filament is cold. Filament resistance decreases

when the temperature decreases.

The high current due to the cold filament effect could cause transformer magnetic core

saturation, which then drastically increases the primary winding inrush current.

To reduce the inrush current, an appropriate triggering needs to be used. Table 1 shows

different halogen lamp test results used with a transformer. The table shows that the

maximum peak current is lower when the load is triggered at zero current.

Indeed the ratio between peak current at zero voltage and at peak voltage triggering is in the

range of 2 to 4. It should also be noted that, in case of triggering at peak voltage, some

transformers (transformer 1 and 2) have a peak inrush current close to 13 times the steady

state peak current whereas others feature a ratio lower than 7 (transformer 3). This is linked

to the low-saturation level of some low-quality transformers.

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