+ Post New Thread
Results 1 to 3 of 3
  1. #1
    Advanced Member level 5
    Points: 32,316, Level: 43
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,734
    Helped
    488 / 488
    Points
    32,316
    Level
    43

    Temperature of TO220 MOSFET tab measured through layers of kapton tape

    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.

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 80,462, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,188
    Helped
    4404 / 4404
    Points
    80,462
    Level
    69

    Re: Temperature of TO220 MOSFET tab measured through layers of kapton tape

    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.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



    •   AltAdvertisement

        
       

  3. #3
    Super Moderator
    Points: 258,450, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,123
    Helped
    13718 / 13718
    Points
    258,450
    Level
    100

    Re: Temperature of TO220 MOSFET tab measured through layers of kapton tape

    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.



--[[ ]]--