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Thermal Resistance Junction To Case Calculator

Work out thermal resistance junction to case instantly with clear inputs, formula shown and shareable results.

About Thermal Resistance Junction To Case Calculator

Thermal resistance is temperature difference divided by power. Junction-to-case is fixed by the package and is the part the designer cannot change; case-to-ambient is the heatsink and airflow, which is where all the design freedom lies.

Formula

Thermal resistance

Rjc = (Tj - Tc) / P; Rja = Rjc + Rcs + Rsa

Worked example

With the default values below, Thermal Resistance Junction To Case Calculator returns Junction-to-case thermal resistance of 0.3177. Change any input above and every figure here is recalculated in your browser.

Inputs used

Power dissipated
85 W
Junction temperature
95 °C
Case temperature
68 °C
Ambient temperature
35 °C

Results

Junction-to-case thermal resistance
0.3177 °C/W
Junction-to-ambient resistance
0.7059 °C/W
Case-to-ambient resistance
0.3882 °C/W
Headroom to a 105 C limit
10.00 °C

Step by step

  1. 1Values used
  2. 2Thermal resistance
  3. 3Junction-to-case thermal resistance= 0.3177 °C/W
  4. 4Junction-to-ambient resistance= 0.7059 °C/W
  5. 5Case-to-ambient resistance= 0.3882 °C/W
  6. 6Headroom to a 105 C limit= 10.00 °C

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Which resistance can I actually improve?

Case-to-ambient. Rjc is a package property, so once it and the power are fixed, only better thermal interface material, heatsink and airflow help.

Why is Rjc so important at high power?

At 85 W even 0.3 C/W of Rjc costs 25 degrees of junction temperature that no external cooling can recover.

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