When design power electronics, to prevent devices from overheating and damage, heatsinks are commonly used. How to calculate heatsinks is thus an issue.

The basic theory is a thermal equivalent of Ohm’s Law. For a power transistor:

T(j-a) = Pd x Rth(j-a)

T(j-a) (in degrees Celsius) is the temperature rise of the transistor above ‘ambient ’ temperature.

Pd (in watts)is the power disspipated.

Rth(j-a) (in degress Celsius per watt)is the total thermal resistance between the junction and the ambient.

A design example:

Heatsink Calculation

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