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I have designed a Boost PFC circuit, But upon switch on transistor is getting blown up and fuse is burnt. So I removed transistor and just connected source to ground. the circuit worked fine. Then I kept gate open and connected drain to boost inductor. The transistor again blown up.

[Edit : Schematic is added]

sch from datasheet but components are same. I am now confused because even I kept gate open and transistor being technically off. Why it is keep blowing away?

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    \$\begingroup\$ Where is your analysis, test results? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 12 '19 at 6:52
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    \$\begingroup\$ In your second try you say "I kept the gate open" does this mean that you kept the gate floating? Please refer to your reference designators Q1, R6, D4, R5 and say which were mounted in this try. \$\endgroup\$ – Bonnevie Dec 12 '19 at 7:00
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    \$\begingroup\$ Did you apply the 12Vdc on J2? \$\endgroup\$ – Huisman Dec 12 '19 at 8:23
  • \$\begingroup\$ @Bonnevie I have kept gate floating. And R6, D4, R5 will be open. \$\endgroup\$ – Voldemort Dec 12 '19 at 12:24
  • \$\begingroup\$ @Huisman I have not applied 12V. that is what made me confuse even more. Because without IC getting enabled and transistor cannot be switched. So I have removed pins one by one (means drain and gate were floating first, later drain is connected and gate is floating. Transistor blown when gate is still floating) to check which pin is making fault \$\endgroup\$ – Voldemort Dec 12 '19 at 12:27
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To be absolutely sure that the FET is not supposed to conduct, you should've shorted the FET's gate to the "source". If you leave it floating, god knows what can happen - it's a high-impedance input.

If your FET is indeed an SPP20N60, it is supposed to be an N-FET, in which case it should work in your circuit. If you mistakenly inserted a P-FET, you know what would happen.

Is there any chance that your FET's are possibly fake?

Have you checked the function of the FET somehow in "safe conditions"? I.e., small Vds and using a resistor or a lightbulb to limit the current (or a lab PSU with a constant current limit) and apply some voltage to the gate... You could even try a breakdown test, if you short the gate to "source" and use a high-value series resistor in the "drain" against a full rail voltage...

With the PWM controller attached to the gate: is your D3 diode really present in the circuit? (Have you checked for its forward voltage? for cold joints?) Is C3 in the circuit? If you leave the PWM controller's output detached from the gate (and FET gate shorted, or FET removed from the circuit), does the PWM produce output pulses? You don't want the FET to conduct all the time :-)

Is the L1 really the value you think it is?

The SPP20N60 appears to have 600V nominal voltage. What's the rail voltage in your PFC circuit? Around 400V ? As a very general rule of thumb, in power electronics for 230V mains, I'm used to seeing 800V triacs or other silicon switches - and that's where the 400V PFC topology is NOT involved. If you're working with a 400V PFC application, try using an even higher-rated FET, say 1000-1200V, to stay firmly on the safe side - at least as an experiment while you're debugging your issue. Changing the MOSFET model may help you avoid a fake or a bad batch, and the higher-voltage silicon switch models (in a family of similar siblings) tend to have a slightly higher "sustainable thermal energy before destructive melt-down" - which may also be a factor in your prototype, for one reason or another :-)

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I have found the answer. Unfortunately transistor body is source and diode body is cathode which means technically I have shorted VCC and GND by giving common heatsink. Now that I have made them seperate, It is completely working fine.

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    \$\begingroup\$ heheh congrats :-) Meanwhile I was wondering why I don't see any kind of snubber attached to the node connecting FET drain to L1 and D3 - but I cannot see a snubber in any example PFC schematic that I've found anywhere, so it's probably okay. I have also noticed that your schematic is possibly a verbatim copy of the catalog schematic for the UCC28019. No problem with that, thanks for reporting success = for being a witness that it actually works :-) \$\endgroup\$ – frr Dec 12 '19 at 14:22
  • \$\begingroup\$ BTW I believe the FET body tends to be Drain, rather than the Source? But it probably doesn't matter much, you had a short nonetheless... \$\endgroup\$ – frr Dec 12 '19 at 14:29
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  1. In order not to blow a large number of transistors, it is advisable to connect some active load in series to the transistor's drain (back in the old times it was an incandescent bulb or two). Sure, it will get the circuit out of spec, but nothing will blow and you can attach a meter or an osciloscope and look what happens.

  2. The FET in your case (assuming it is not fake, not miswired and not killed by static before being mounted) blows because it stays in "switched on" state for too long and the current in L1 gets too high. Using the method above you can check why. It may be a wrong resistor or capacitor somewhere around the IC. Keep in mind that the IC may be blown from the first experiment, too.

  3. Never, ever power on a circuit where some MOSFET has it's gate unconnected. It is perfectly capable because of the parasitic capacitance between the drain and the gate, to saturate itself just because of the drain voltage change. Bipolars are (mostly) OK with floating base.

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Your MOSFET is likely blowing up because it it connected incorrectly. Although it is possible that static transients exited your MOSFET gate when you replaced it unconnected, this behavior seems more like an incorrect routing on the PCB, but I wouldn't know, as you have not posted your board layout, or even a photo of the burned MOSFET on the board.

MOSFETS have an internal parasitic body diode that renders them unable to block in the reverse direction. Connect them in reverse, and you are asking for an instant short. This is probably what gave your fuse a run for its money too.

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