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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.

Ideas?

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    \$\begingroup\$ What is used to turn the the MOSFETs ON? 310V DC on ON_OFF? \$\endgroup\$ – τεκ Mar 1 '18 at 17:41
  • \$\begingroup\$ It is a 115Vac 2 phase system. So Phase A-B= 200Vac \$\endgroup\$ – DIODEX Mar 1 '18 at 19:40
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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.

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