Paritian

Electricity

Junction Temperature and Heatsink Calculator

Temperature of a semiconductor's junction, case and heatsink from the power it dissipates and the chain of thermal resistances, plus the largest heatsink resistance you can get away with.

Results

Junction temperature 80.0 °C
Case temperature 72.5 °C
Heatsink temperature 70.0 °C
Margin to the maximum 70.0 K
Largest heatsink resistance you can use 20.00 K/W
Power that takes the junction to its maximum 13.75 W
How it changes
Junction temperaturePower dissipated in the component 0.0 – 11.0. Junction temperature: 40 → 128Junction temperature4060801001201400.02.04.06.08.010.0Power dissipated in the component (W)

The dashed line marks the value you entered.

What this tool does

Heat leaving a transistor, regulator or LED crosses three thermal resistances in series, from junction to case, case to heatsink and heatsink to air, and they add up exactly like resistors, with watts in place of amps and kelvin in place of volts. θJC is in the datasheet; a thermal pad or paste adds roughly 0.1 to 1 K/W; and the heatsink's θSA is in its own datasheet, usually for still air. Do not design right up to the maximum junction temperature: a common rule of thumb is that every 10 °C less roughly doubles the life of electronics, so many designers aim for 20 to 30 °C of margin. In a closed box, use the air temperature inside the box, not the room.

Formula

Tj = Ta + P × (θJC + θCS + θSA)

Variables

SymbolMeaningUnit
taSurrounding temperature°C
pPower dissipated in the componentW
jcJunction-to-case resistance θJCK/W
csCase-to-heatsink resistance θCS (pad or paste)K/W
saHeatsink-to-air resistance θSAK/W
tmMaximum junction temperature (datasheet)°C
TJJunction temperature°C
TCCase temperature°C
TSHeatsink temperature°C
MGMargin to the maximumK
SMLargest heatsink resistance you can useK/W
PMPower that takes the junction to its maximumW

Worked example

  • Surrounding temperature40 °C
  • Power dissipated in the component5 W
  • Junction-to-case resistance θJC1.5 K/W
  • Case-to-heatsink resistance θCS (pad or paste)0.5 K/W
  • Heatsink-to-air resistance θSA6 K/W
  • Maximum junction temperature (datasheet)150 °C
  • Junction temperature80.0 °C
  • Case temperature72.5 °C
  • Heatsink temperature70.0 °C
  • Margin to the maximum70.0 K
  • Largest heatsink resistance you can use20.00 K/W
  • Power that takes the junction to its maximum13.75 W

Limitations

  • Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
  • The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.