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Temperature of TO220 MOSFET tab measured through layers of kapton tape

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treez

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Hello
We were measuring the temperature of the tab of a TO220 FET (dissipating 3W) in an 60W offline flyback SMPS.
We glued the the k type thermocouple end to a small piece of kapton tape which was covering a small bit of the tab. However, the high dv/dt switching voltage coupled through to the thermocouple and afflicted the reading. Therefore we solved the problem by using 2 layers of kapton tape instead of one.

We would like to know how far off our temperature measurement of the tab is? …. given that there are 2 layers of kapton tape between tab and thermocouple
We know the formula for heat transfer….

Q/t = k.A.[delta(T)] /d
Where:-
A = 6 e-6
K = 0.12
Q/t is a variable which we cant find
Delta(T) is what we want.
 

betwixt

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dv/dt doesn't cause a temperature error but it might be producing voltage into the thermocouple by induction or capacitive coupling. Did you try simply adding a capacitor across the thermocouple wires or alternatively, physically reversing (flipping over) the thermocouple probe and taking the average of the two readings so you cancel all but temperature generated voltages?

Brian.
 

FvM

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There's no easy calculation. Q/t (heat flow through the kapton film is caused by convection from the surface plus conduction through the thermocouple wires. It depends on many factors, e.g. ambient temperature, wire cross section, air flow.

A reasonable approach is to keep the heat dissipation as low as possible, by attaching a piece of styrofoam as thermal insulation on top of the temperature probe and using smallest available thermocouple gauge.

As for switching noise affecting the thermocouple measurement, that's just a matter of appropriate filtering.
 

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