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



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[edited by: lallison at 6:20 PM (GMT 0) on 19 Dec 2018]
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  • Hi John,

    In the first case with 3.3nF gate cap, the inrush current is very small  and it takes roughly 62 msec to charge the output cap . The timer timesout much before this and shuts the gate off. the part will still retry after 9 sec but fail to start as timer is set for almost  10.5 msec.  You might want to take a look at scope shot of the timer to confirm if it reaches the timeout threshold during startup .

    In the second situation , since gate cap is small , the inrush is larger . Hence the part turns on. Note however due to resistive load , the FET carries load + inrush current .  Since it retries with several 100V Vds the temperature rises and eventually the MOSFET fails.

    I will recommend  using the LTC4366-1 instead of LTC4366-2 so that the part remains latched-off after such a fault . With such high Vds and current latching-off would be better instead of auto-retry functionality.

    Best Regards,

    Ritesh

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