Please see end comment first
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Thanks,
The idea was supposed to be that M3 clamps the voltage across itself to 413V, thus protecting the LED driver. As you say, the NFET M1 acts like a current clamp. Any voltage above 413V gets taken up across M1, which is 900V rated.
for a worst case ZR431F the xtor Q2 will be always on at high ambient ...
Thanks, though worst case cathode current for the ZR431 is about 40uA…..that would drop about 0.2V across the BE of the PNP….surely not enough to turn it ON?
One of the reasons that we put the M1/M3 based circuit in there, is because the blessed TVS voltage climbs above 450V as it trys to clamp a transient…so we put the M3 Based shunt in there in the hope that it would prevent the voltage across the TVS (and hence across the LED driver) from going above 450V.
If the shunt FET M3 needs to pass more than 0.6A in order to clamp its voltage to 413V, then we assumed that M3 would just simply short out the high voltage DC Bus and that voltage would then appear across the M1 NFET?
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There is, by the way, a littelfuse LSP05 MOV module at the mains input, but it "lets through" up to 900V.....therefore we needed the extra mains transient protection as shown here.
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1.5SMC400A datasheet
https://www.littelfuse.com/~/media/...littelfuse_tvs_diode_1_5smc_datasheet.pdf.pdf
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Damned me, ive just realised i forgot to put the 3n3 capacitor across the 10V zener on the schem...now as attached...sorry.