Junction Temp & Thermal Resistance Calculator
Estimate semiconductor junction temperature and the max power for a temperature limit.
Input
ℹ θJA is from the datasheet (package + board). Lower θ = better cooling.
Results
Junction Temp (Tj)—
Margin to Tj max—
Max Power for Tj max—
💡 Usage & Formula
Tj = Ta + P × θJA. Max power for a limit: Pmax = (Tjmax − Ta) / θJA.
Usage: Use the datasheet θJA. If margin is negative, you need a heatsink (lower θ), less power, or lower ambient.
When you need it: Computing a part's junction temperature from its power dissipation and the thermal-resistance chain, to keep it safely below Tj(max).
Worked example: P = 1 W, θJA = 50 °C/W, Ta = 25 °C → Tj = 25 + 1 × 50 = 75 °C. With a heatsink the path becomes Tj = Ta + P × (θJC + θCS + θSA).
Tips & gotchas:
- Datasheet θJA assumes a specific board copper area and airflow — treat it as optimistic.
- For SMD parts the copper pour is the heatsink; more area and thermal vias lower θJA.
- Derate to about 80% of Tj(max) for lifetime; electrolytics and plastics age fast when hot.
- The interface (thermal pad/grease, θCS) often dominates — a dry or air-gapped joint wrecks the budget.
📖 References:
Thermal resistance (Wikipedia)