170V does not have high enough voltage

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3 years 4 months ago #7480 by nullpoexc
I have replaced my capacitor, and from the multimeter diode D2 seems good. It still can't work now, so the problem should be on the MOSFET. However, I can't get IRF840 from my local shop. The only one with 500V voltage is IRFP450 ( www.vishay.com/docs/91233/91233.pdf ). Is this a good replacement for IRF840?

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3 years 4 months ago #7481 by Ty_Eeberfest
I just compared datasheets for the IRF840 and IRFP450. They look like they have very similar specs. Should be okay to substitute.

Also - I just took a closer look at the photos in your OP. Something looks funny on the bottom side under capacitor C1. Are you positive there is not a solder bridge shorting across C1? It sure looks like there could be one.

Look into it later when the dust is clearing off the crater.

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3 years 4 months ago #7483 by Ian
Before we go swapping the MOSFET out, can you post a trace of the voltage at the Drain of the MOSFET?

The IRFP450 is really similar, the only real difference is the packaging, and the Rds, which is lower (= superior). You can go ahead with that one, and I have to say, I like that part after going through the data sheets. I've been looking for a better version of the IRF840 for a while, and this one might be worth some tests.

The only real question I have is that I would like a nice clean turn on, meaning that at Vgs = 5V, we have a low Rds. I don't see this in the data sheets. I think it needs a test.

One thing: it is physically a bigger part and it won't fit right on the board without some trickery.

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3 years 3 months ago #7489 by nullpoexc
I have substituted the broken IRF840 with IRFP450 using flying wire, and it worked! The voltage is around 250V at the startup, and will stay at ~185V when stabilized (with load). I can hear the crackling sound and nothing is hot. I think this one could be a good substitute for IRF840.

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3 years 3 months ago #7490 by Ian
Great news! Could you post a little picture of it?

Does it get hot at all? (Even after many hours of usage, especially during the day, when the brightness is at 100%).

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