Junction-to-Case Temperature Rise

Compute ΔTj-c from power dissipation and RθJC. Optionally estimate junction temperature using case temperature.

How to Use

  1. Enter Power Dissipation (P).
  2. Enter RθJC from the datasheet (junction-to-case thermal resistance).
  3. Optional: enter Case Temperature (Tcase) to estimate junction temperature.
  4. Open Show Work for formulas and base-unit steps.
Thermal Path View
ΔTj-c scales with P and RθJC. Enter values to update instantly.
P
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RθJC
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ΔTj-c
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Tj (opt)
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Status: —
Junction (Tj) Heat source Power: P RθJC (°C/W) Case (Tc) Optional input Estimate: Tj = Tc + ΔT Model: ΔTj-c = P × RθJC (steady-state, 1-D approximation). Real results depend on mounting, TIM, airflow, and transient conditions.
Inputs & Settings
Enter power and RθJC to compute ΔTj-c. Add case temperature to estimate junction temperature.
Use actual dissipation (not nameplate). Example: MOSFET loss, regulator heat, LED driver loss.
From datasheet: often “RθJC” or “θJC”. Lower is better.
If provided, tool estimates Tj = Tc + ΔTj-c (after unit conversion).
Used for status highlighting only when Tj is being estimated.
Quick Adjust
5 W
2.5 °C/W

Tip: Sliders are for fast “what-if” testing. Use inputs for precise values and units.

Show Work (step-by-step)
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Work is shown in base units (W, °C/W, °C) for consistency. If Tc/Tmax are in °F or K, they are converted internally.

Reference

Core model: ΔTj-c = P × RθJC

If case temperature is provided, estimated junction temperature is: Tj ≈ Tc + ΔTj-c

  • P = power dissipated in the device (W)
  • RθJC = junction-to-case thermal resistance (°C/W)
  • ΔTj-c = junction-to-case rise (°C)
  • Tc = case temperature (°C)
  • Tj = junction temperature (°C)
Note: This is a steady-state approximation. Real results depend on mounting, TIM, heatsink, airflow, and transient thermal impedance.

FAQ

Where do I find RθJC?

In the datasheet thermal section. It may be listed as RθJC, θJC, or “junction-to-case thermal resistance.”

Is °C/W the same as K/W?

Yes—temperature rise uses differences, so 1°C rise equals 1K rise.

Why is my real temperature higher than the estimate?

Because this model only covers junction-to-case. Additional rises exist from case-to-heatsink and heatsink-to-ambient (plus interface materials and airflow limits).

Can I use ambient temperature instead of case temperature?

Not directly with RθJC. For ambient-based estimates you need the appropriate resistance such as RθJA (junction-to-ambient) or a full stack model.

Tool Info

Last updated:

Updates may include unit support, UI improvements, and calculation edge-case handling.