How to use the heatsink thermal calculator
- Choose what to find: the junction temperature, the heatsink needed or the largest power the part can dissipate.
- Enter the ambient temperature and the power dissipated if you are not solving for it.
- Enter the junction to case resistance from the data sheet, the case to heatsink resistance of the pad or paste, and the heatsink to ambient resistance.
- Read the junction temperature, the total resistance, and the temperatures at the case and the heatsink.
Formula
Total thermal resistance R = R(junction to case) + R(case to heatsink) + R(heatsink to ambient)
Junction temperature Tj = ambient temperature + power x R
Heatsink needed: R(heatsink to ambient) = (Tj max - ambient) / power - R(junction to case) - R(case to heatsink)
Largest power = (Tj max - ambient) / R
Thermal resistance in C/W is like electrical resistance. Temperature difference is like voltage, power is like current, and the resistances in series add.
10 W through a 6 C/W path at 25 C
With 1.5 C/W from junction to case, 0.5 C/W through the pad and 4 C/W from the heatsink to the air, the total is 6 C/W. At 10 W the temperature rise is 60 C, so the junction is at 25 + 60 = 85 C.
The case is at 25 + 10 x 4.5 = 70 C and the heatsink is at 25 + 10 x 4 = 65 C. If the largest junction temperature is 125 C, the largest power through the same path is (125 - 25) / 6 = 16.67 W.
Choosing a margin
A part may survive at its largest rated junction temperature, but life and reliability improve when it runs cooler. The page lets you enter a lower largest temperature to build in a margin, and the value is yours to choose. Also allow for the rise in ambient temperature inside an enclosure, which is often well above room temperature.
To find the power that flows through the part, use the Ohm's law calculator. For a resistor that sets an LED current, use the LED resistor calculator.
What the numbers depend on
The heatsink to ambient resistance on a data sheet assumes particular airflow and orientation, so use the figure for your conditions. Thermal paste or a pad adds a case to heatsink resistance that depends on the pressure and the material. Measure the heatsink or the case temperature if you can, and compare it with the page to check the model.
A temperature sensor can help. See the PT100 temperature calculator for converting a sensor reading to a temperature.
Assumptions and limits
- Heat flows in one series path from the junction to the air, and the resistances add.
- Thermal resistances come from data sheets and are valid only at the conditions they were measured in.
- Power is steady. Short pulses have a lower effective resistance, which this page does not model.
- The ambient temperature is the air near the heatsink, not the room.
Frequently asked questions
What is thermal resistance?
It is how many degrees the temperature rises for each watt of heat that flows. It is given in C/W, and a lower number means better cooling.
How do I pick a heatsink?
Choose the mode that finds the heatsink needed. It gives the largest heatsink to ambient resistance that keeps the junction at or below your limit.
Why include a case to heatsink resistance?
The joint between the part and the heatsink is never perfect, and the pad or paste adds a small resistance that you should count.
Does this apply to LEDs and power supplies too?
Yes, for any part with a junction temperature and thermal resistances. Use the power that turns into heat, not the total power.