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?

  • \$\begingroup\$ Your calculations are correct, so yes, the maximum required heatsink thermal resistance would be 6ºC/W. \$\endgroup\$
    – stefandz
    Commented Sep 5, 2015 at 11:59

1 Answer 1


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.