Thermal resistance of an identical but larger heatsink

Discussion in 'General Electronics Chat' started by Norfindel, Jun 23, 2010.

  1. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
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    Hi,

    I have the specifications of a heatsink, including the thermal resistance for a certain lenght, but it can be buyed in longer sizes (it's likely an extruded heatsink).

    My question is: knowing the thermal resistance of the 75mm long heatsink, how to calculate the thermal resistance of a longer one?

    This is the page of the heatsink manufacturer, by the way: http://www.disipadores.com/ingles/media_potencia.htm

    Thanks.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    inversely proportional to the square root of the length
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  4. bertus

    Administrator

    Apr 5, 2008
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  5. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
    235
    9
    Thanks. That table is very useful.
    They say that it's not linear, because the heat source is in the center, and it's colder the farthest away from it, and it makes a lot of sense.

    Probably that means that if using several, correctly distributed devices, the factor will get closer to 1 again, right? If i have a big heatsink with two transistors, it would work the same that if the two transistors where on separate heatsinks of half the length?

    I had already readed it. Very good document. Thanks.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Should be pretty darn close assuming your larger heatsink was selected so that heat from one device is not effecting the other significantly.
     
  7. timrobbins

    Active Member

    Aug 29, 2009
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    I don't have empirical data to hand, or a good book reference, but you have to significantly bias the position of the devices to a point 'lower' in the heatsink if only relying on natural convection. This is easy to comprehend, as the air flowing over the upper section of the sink has increasing temperature, and the rate of change of temperature with distance in the sink body is less as you move 'higher'. This equates to an effective length of heatsink that is shorter.

    For diodes and thyristors, the nominal design case temperature will be much higher than for more exotic devices such as Fets and igbts, in which case the heatsink body is significantly higher in temp and so the effective thermal resistance of the sink is lower. One needs to always check on the test conditions for the rated sink thermal resistance.

    Ciao Tim
     
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