Heat Sink Calculator

This heat sink calculator works out the maximum thermal resistance a heat sink can have while still keeping an electronic component below its safe operating temperature, which matters because overheating a transistor, MOSFET or processor shortens its life or causes it to fail outright. You enter four figures: the maximum junction temperature the device is rated for, the ambient air temperature around it, the power in watts it dissipates, and the combined junction-to-case and case-to-sink thermal resistance from the datasheet. From these the calculator works out the total allowed thermal resistance, the temperature rise budget divided by the power, then subtracts the junction-to-case and case-to-sink figure to give the maximum heat sink rating you can use, in degrees Celsius per watt. Alongside the main result it also shows the total allowed thermal resistance and the temperature rise budget, so you can see how the numbers add up. A lower degrees-per-watt figure means a more effective, usually larger or finned, heat sink; if the result is negative, no passive heat sink will do the job and you need to lower the power, allow a bigger margin, or add active cooling such as a fan. Try the worked example figures of 125, 25, 10 and 1.5 to see how the maths flows through. Treat the result as an indicative engineering guide and confirm against the datasheet before relying on it.

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