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LT3757 High Voltage flyback MOSFET choice

On page 30 of the LT3757 boost/flyback/sepic/inverting controller; there's an application for a 300V flyback design.

The switching MOSFET selected for this design is a VISHAY SILICONIX Si7850DP which has a max drain-source voltage of 60V.

When simulating the circuit with LTSpice I see voltages in excess of hundreds of volts at the drain.

Am I missing something here? It seems like this MOSFET selection is not adequate for the application.

[edited by: tpttr at 2:43 AM (GMT -4) on 14 Sep 2018]
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  • Hello W.G. Sorry I never received a notification for the reply but I just checked your answer and I do appreciate the answer!

    You are correct this is due to the coupling factor. I have set it to 0.98 and not 1 to account for leakage in the transformer. In this scenario there are huge voltage spikes at the drain that would trigger the 60V MOSFET into avalanche.

    So a 60V MOSFET probably works but I'm sure we are greatly shortening its lifespan.