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Determining correct heatsink size to use.

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New Member
:?: Is there a formula used to determine the size of heatsink to be used on power transistors or IC? I have seen some on datasheets, but I dont know how to use them. I have been installing heatsink on my projects but am not sure of their sizes. I have not encountered thermal runaways in them because I have been using big sizes of heatsinks, but I know I am overdoing it and it is causing me problems on how to fit them in a case. Your help would be highly appreciated. Thanks. :lol:


Active Member
There is a pretty good explanation in the Radio Amateur's Handbook too - as I recall, with an good example problem worked out for a power supply transistor. I think the methods were described as crude but yeilded conservative results. I don't know enough about the subject to comment one way or the other.


New Member
Well, in general you calculate a heatsink this way:

You got:
Ptot = max. power produced by the semiconductor in Watt
Tambient = ambient temperature in Kelvin

From the datasheet:
Tmax = max. temperature of the semicond.
Rjc = junction - case thermal resistance in K/W

We calculate:
Rja = junction - ambient thermal resistance
Rca = case - ambient thermal resistance (-> the heatsink)

Rja = (Tmax - Tambient) / Ptot ... (Tmax, Tambient in Kelvin; Ptot in watt)

Rca = Rja - Rjc

Rca is the max. thermal resistance your heatsink should be. i.e. if you got 3.5K/W I would use a heatsink of 3K/W - just to be sure

We assume that the heatsink is mounted with thermal conducting matter (you know, the white stuff between the heatsinks and the semiconductors),
so we can neglect this additional thermal resistance. If you want to get sure, assume a max. thermal resistance between case and heatsink Rch with 0.2K/W:
Rca = Rja - Rjc - Rch

I hope this is all correct!

Best regards,
Hannes / Austria

PS: if your circuit is in a box, Tambient can easy be about 10-15K higher than the ambient temperature. Make sure air can easily get into and out of the box.


New Member
thank you note

:lol: To all the persons who answered my heatsink calculation query, thank you very much. More power to you people. Again thanks
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