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I am using high voltage MOSFET in TO-220 package with a heatsink like this one:

heatsink

Under normal conditions, the temperature of heatsink does not exceed 90 degrees.

My MOSFET can run up to 120 degress according to the datasheet.

What kind of protection can I use so that the MOSFET temperature does not go above 120 degrees in an extraordinary situation? (For example, 150-200 degrees).

Could I connect a thermal fuse in series with the MOSFET? I suppose I'd have to attach the thermal fuse to the cooler.

Any ideas?

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    \$\begingroup\$ Any information on the attached circuit? If you have a reference and a spare comparator, using a thermistor may be the best option \$\endgroup\$ Commented Oct 16, 2021 at 13:52

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There are too many variables to calculate the temp based on on-state resistance, switching speed, air circulation, etc.

Directly measuring the case temperature will be your only option. Ideally, measure as close to the MOSFET tab as possible with a very small thermal fuse or, yes, mount on the back side of your heat sink.

Use heat-sink grease to thermally bond the MOSFET to your heat sink. If not a thermal fuse, you can use a thermal sensor and back off on the power via PWM in over temperature situations (the strategy really depends on your application.)

One-time use thermal fuses can be a pain when the trip. Polyfuses that reset are a bit more user friendly.

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