Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
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?
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.
