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heat sink

Discussion in 'Electronic Basics' started by Kelvin, May 27, 2005.

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  1. Kelvin

    Kelvin Guest

    Hi:
    this is a question of 2 transistors mounted on the same heat sink...

    Two 2N3055 transistors are mounted close together on a heasink
    consisting of a square piece of bright aluminum 1/8 thick. the
    transistors are electrically insulated from the heatsink by using MICA
    washers coated with silicone grease between each transistor and the
    heatsink.
    for a 2N3055. Qjc = 1.5 C/W and Q of a greased MICA washer = 0.5 C/W
    Tj Maximum of a 2N3055 is given as 200C.
    a) If each transistor is producing 20W of heat at its collector junction,
    calculate the maximum allowable value of Qsink if the ambient temp cam be
    as high as 45C
    b) if Qsink = 2.5 C/W, and one transistor is dissipating 30 Watts and the
    other
    is in cutoff, calculate the case temp of each transistor if T ambient = 50C.
    State
    any assumption made.

    after filtering out the decorations... i figure the following are the useful
    part of the question:
    2 transistor with Qjc = 1.5 C/W each is mounted on a Qsink = unknown
    heatsink
    between the heatsink and two transistors, there is 2 MICA Q = 0.5 C/W
    Tj(max) = 200C
    Tambient = 45C
    a) im looking for Qsink
    b) i have no clue :p

    I did Tj(max) - Tambient = 200C - 45C = 155C (which is the heat allowed on
    my components)
    Total Power dissipation = 2 x 20W = 40 W
    155C/40W = 3.875 C/W --> total thermal resistance I can have in my circuit
    then im stucked... lol...
    two transistor mounted on the same heatsink... so the total thermal
    resistance they have
    should be the sum of parrallel of the two?? the same for two MICAs...

    and for (b)... i have totally no clue about it...

    thanks for any help...
     
  2. It may help you visualize this problem, by representing the thermal
    resistances as resistors, temperatures as voltages and power flows as
    currents. If you assume that the thermal resistance of the aluminum
    between the two transistors has an insignificant thermal resistance
    (one of those assumptions you have to state) then the problem can be
    analogized by the following schematic (viewed in fixed width font):

    Qjc1 Qmica1
    TJ1--/\/\/--/\/\/--+
    Pj1-> Pj1-> |Tsink Qsink
    +-------/\/\/--Ta
    Qjc2 Qmica2| Pj1+Pj2->
    Tj2--/\/\/--/\/\/--+
    Pj1-> Pj1->
     
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