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If I have a TO-220, with max power dissipation of 10w and the worst case ambient temp is 50°c, with a \$R_{jc}\$ of 4°c/w and a max allowed junction temp of 150°c (ignore any thermal resistance between case and heatsink).

Would the heatsink's required thermal resistance \$R_{sa}\$ be 6°c/w?

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  • \$\begingroup\$ Your calculations are correct, so yes, the maximum required heatsink thermal resistance would be 6ºC/W. \$\endgroup\$ – stefandz Sep 5 '15 at 11:59
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How is this not just a straight forward math problem? Work it out:

You say the thermal resistance from junction to case is 4°C/W, and that the junction is producing 10 W of heat. It will therefore be 40° hotter than the case. The max allowed junction temperature is 150°C, so the max allowed case temperature is 110°C.

You say the worst case ambient temperature is 50°C, so that means 10 W of heat flow from the case to ambient can't drop more than 60°C. Overall the case to ambient path must have (60°C)/(10 W) = 6°C/W thermal resistance or less.

Of course in practise you'd want to make sure the case to ambient resistance is some margin below the absolute maximum of 6°C/W.

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