LNK408EG Dimming Ref Design to 277V

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I modified the LNK408 reference dimming circuit to work at 277 volts.  (Scaled dampening components and increased voltage ratings.)  It runs fine (months continuously) in full output but when I connect a 277V dimmer it fails.  It dims beautifully at 120 volts.  Also, the current waveform at 277 is pretty ugly, is this normal?  Is dimming at 277 volts even possible with this IC?

Hi,

Yes, LNKPH can be used at high line, refer to some reference design.

It is much easier to dim for low line than high line application due to the following reasons:

1.  For leading edge dimming at high line application the peak input current is sgnificantly higher than low line application.  As such, the input current ringing is bigger therefore a proper damping circuit is needed.

2. Input capacitance is critical for leading edge dimming at high line, make sure proper damping circuit is used.

3. Make sure latching current is at least 500us period.

4. high line average input current is lower thatn low line, as such monitor the proper holding current.  Check the characteristic of the dimmer for minimum holding current.

5. Make sure that the trailing edge dimmers internal timing is met.  Determine the minimum power requirement of the dimmer.

You need to smoothen the input current in order to make the triac happy. Direct way in checking is series a resistance at least 150 ohms in the input.  Input capacitance before and after the bridge is 10nF maximum to avoid unwanted spikes.

Regards,

PI-Jono