Heat Sink Calculator
This heat sink calculator finds the thermal resistance a heat sink must have to keep a power device below its maximum junction temperature. Enter the maximum allowed junction temperature, the ambient temperature, the power the device dissipates, and the combined junction-to-case and case-to-sink thermal resistances, and the calculator returns the largest heat sink thermal resistance you can use. Heat flows like current through a chain of thermal resistances, and the total temperature rise must fit within the budget set by the junction and ambient temperatures. A smaller degrees-per-watt figure means a bigger, better heat sink. The formula, a worked example and the assumptions are below.
The formula
The total allowed thermal resistance is (Tj minus Ta) / P. The heat sink resistance must be at most that total minus the junction-to-case and case-to-sink resistances: Rsa = (Tj minus Ta)/P minus (Rjc + Rcs). Lower is better.
Worked example
For a device at 10 W, max junction 125 degrees, ambient 25 degrees, with 1.5 degrees per watt from junction to sink: total allowed is (125 minus 25)/10 = 10, so the heat sink must be 10 minus 1.5 = 8.5 degrees per watt or less. Enter 125, 25, 10, 1.5 to confirm.
Frequently asked questions
How do I size a heat sink?
Find the total allowed thermal resistance (temperature rise over power), then subtract the junction-to-case and case-to-sink resistances. The remainder is the maximum heat sink rating.
What does degrees per watt mean?
How many degrees the temperature rises per watt of heat. A lower figure is a more effective heat sink.
What if the required value is negative?
No passive heat sink can do it; you need lower power, a larger temperature budget, or active cooling.
Who this calculator is for
This calculator is for electronics students, hobbyists and engineers.
What this calculator assumes
- You enter values in the units shown.
- Ideal components are assumed.
- Results are rounded for display.
Formula and sources
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