Junction Temperature Calculator – Tj = Ta + P × RθJA
Steady-state semiconductor thermal estimate
Required Parameters
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Quick Answer
Tj = Ta + P × RθJA. Example: 25°C, 0.85 W, 50°C/W → rise 42.5°C and Tj = 67.5°C. The number is an estimate, not a measured die temperature.
Junction Temperature Calculator
Tj = Ta + P × RθJA
- Temperature rise ΔT = P × RθJA
- Remaining margin = Tj,max − Tj when you enter a maximum
What this model does not include
The calculation is a simplified steady-state thermal resistance chain. Real junction temperature also depends on:
- PCB copper area and layer count
- Airflow and nearby heat sources
- Package and exposed-pad soldering
- The board used when the vendor measured θJA
Do not treat the result as a thermocouple reading on the die.
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Design Notes
This is a simplified steady-state model: Tj = Ta + P × RθJA. Accuracy depends on PCB copper, airflow, heatsinking, package, and the vendor’s test conditions. The result is an estimate, not a measured die temperature.
Common Mistakes
- 1
Using θJA from a different board size or copper area than the real layout.
- 2
Ignoring airflow, nearby heat sources, or pulsed power.
- 3
Treating the estimate as a thermocouple reading.
Knowledge Base
What is RθJA?
Junction-to-ambient thermal resistance in °C/W. It depends on package, PCB copper, and the vendor’s test board — not a universal constant for a given part number.
Is this the actual chip temperature?
No. It is a steady-state estimate. Airflow, nearby heat, copper area, and pulsed power all move the real Tj.
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