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MOSFET to switch AC load that is a transformer

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Member level 4
Sep 23, 2008
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A transformer is a big inductive load. When you switch it OFF, you will see a very high voltage pulse across the switch, this is killing your FET.

Connect a snubber circuit across your switch or tvs, or a mov or similar overvoltage protection circuits.

Energy across the primary side of the transformer appears when the MOS is off and this energy is converted into voltage and this can be very high depending on inductor value and ON time pulse width.
As KlausST said, a snubber circuit must be used to prevent this energy discharging over the transistor.

Well I guess the snubber circuit doesnt work. In theory it should. Not really sure what the issue it. There was no load on the secondary side.

The Resistor was 62ohms and the cap was 680uF. I put an MOV across the L and N of the transformer as extra protection. Snubber circuit was something like this

Not sure why its not working. The MOSFET works great when its not connected. The switch is placed on the neutral side of the incoming AC source. Once turned on its permanently shorted.

The Resistor was 62ohms and the cap was 680uF
Really 680 µF? That's bad. Hopefully just a typo.

I would rather try with bipolar TVS diodes or varistors than a snubber, giving better defined peak voltage.

Recent MOSFET have a considerable avalanche energy rating, it's no completely clear to me why the MOSFET was damaged. Are you sure that the connected transformer is healthy?
Thanks for the reply. I just checked the connection and realized I connected the Drain to Drain and using the source on connectors.

I think I will use your advise and place a MOV instead of a SNUBBER.

Yes its works perfectly. Thanks.

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