AN4130 datasheet(29/41 Pages) STMICROELECTRONICS

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AN4130

Design guidance

Doc ID 023315 Rev 1

29/41

4.13

EMI filter and dimming

4.13.1

Damping the input filter

When the unit is dimmed a large transient voltage appears on the filter input capacitor:

Figure 35.

Undamped input filter waveforms with Triac dimmer

To maintain Triac holding current, the ringing must be damped. The best way to do this is to

add an R-C network across the filter output, where the network impedance is highest.

Ideally, the input current waveform should look like this:

Figure 36.

Properly damped waveforms

Actually, some high-frequency ringing can be tolerated, if the current reversal time is less

than the turn-off time of the Triac, about 20 microseconds. The input filter, however, would

ring at about:

Equation 8

so current reversal time would be in the 70 microsecond range. Damping is required.

The filter’s output impedance is high, the impedance at the line end is low. In fact, the line

end of the network is much less than the 100

Ω of the LISN at the ringing frequency, almost

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