I designed a circuit around LTC4366

I designed a circuit around LTC4366 as per the picture below. However numerous MOSFETs have been damaged and become useless.

Mosfets used : STH12N120K5-2; IXFT50N85XVH; STW13NK100Z - All are rated to above 800V and 12A

The Load is operated with 380VDC and can handle 450VDC MAX (the threshold in the circuit is set to 420VDC). The power supply can deliver 800VDC if it fails (and this is what I want to protect the load from)

1) If the Gate Cap (Cg) is 3.3nF the Vout never goes up. The circuit behaves as if it was in the protected mode.

2) If the Gate Cap (Cg) is 10pF the circuit works and turns off the MOSFET when the input voltage reaches approx. 420V. However if I set the power supply to 550VDC, the LTC4366 resets the failure every 9 seconds and after a few cycles the MOSFET simply dies and shows a short between Drain and Source

title edit
[edited by: lallison at 6:20 PM (GMT 0) on 19 Dec 2018]
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