Electronic – Why is this MOSFET burning


I have a system that powers a transformer via a line to line power supply of 115Vac(L-N) for a total of 200Vac(L-L)

In order to control the ON/OFF of the transformer, the NEUTRAL line is used to control a set of MOSFETS triggered off by the Line to Neutral connection. The circuit is as following:

Power Supply
The circuit works well as intended during normal operation for ON/OFF purposes. But if I induce a diode (D7) to one of the phase (L1), the MOSFET attached to it will burn up (M1 in this case). I am using MOSFET IRFBF20S

I have simulated the circuit as I can't run measurements during the induced fault. But all I can think of is that somehow the body diode is conducting or the MOSFET is turning ON but not completly causing the RdsON to be too high.


Best Answer

If you put a diode in a circuit that is in series with a transformer then you are applying a DC voltage to that transformer primary and it will saturate its magnetic core and take a phenomenal current (theoretically) and blow fuses and burn the MOSFET and probably burn the diode.

If the load isn't a transformer (but say a heater element) then the addition of the diode prevents the gate-source capacitor (C3) being charged up and, as a result, the MOSFET (M1) doesn't turn on properly and will likely burn due to not enough gate-source voltage. If you don't apply enough gate-source voltage to a MOSFET it will enter its so-called linear region and suffer from thermal runaway

The whole point about this circuit is that C3 is charged up to D2's zener voltage when Phase_A is negative relative to neutral - because of D7, you are preventing M1's source going negative hence you are only partially turning on M1 and it gets hot and burns.