Heatsink Thermal Resistance Calculator | Junction Temp (Tj)
Thermal Management

Heatsink Thermal Resistance Calculator

Managing the thermal path from the Junction to the Ambient air is critical for power electronics reliability.

This calculator sums the resistances of the Component Case ($R_{\theta JC}$), Thermal Interface Material ($R_{\theta CS}$), and Heatsink ($R_{\theta SA}$) to predict the Junction Temperature ($T_J$) or size a required heatsink.

Power & Environment

Conditions
W
°C
°C
°C
Safe Limit: 110 °C

Thermal Stack

Resistance
°C/W
°C/W
°C/W

Analysis Result

Output
Chip Die Temp (TJ)
— °C
Enter Power
Total RθJA — °C/W
Temp Rise (ΔT) — °C
Thermal Resistance Stackup Visualization: Junction to Case to Heatsink to Ambient Sink TIM Device Heat Source (Tj)

Typical Thermal Resistances

Use these reference values if you don’t have the specific datasheet.

Package / MaterialParameterTypical Value
TO-220 PackageRθJC~0.5 – 2.0 °C/W
TO-247 PackageRθJC~0.3 – 1.0 °C/W
Thermal GreaseRθCS~0.1 – 0.5 °C/W
Sil-Pad (Insulator)RθCS~1.0 – 4.0 °C/W

Formulas Explained

Analyze Mode (Find TJ)

TJ = Ta + Pd × (RJC + RCS + RSA)

Calculates final temp based on selected heatsink.

Size Mode (Find RSA)

RSA = ((Tsafe – Ta) / Pd) – RJC – RCS

Calculates max allowed heatsink resistance.

Safe Limit

Tsafe = Tj,max – Margin

Derates the component max temp for reliability.

Thermal Resistance (Rθ)

Unit: °C/W

Measures temperature rise per Watt of power.

How to Use

Optimize your thermal design in 3 steps.

1

Choose Mode

Select Analyze Chip Die Temp to check an existing heatsink, or Size Heatsink to find out what heatsink you need.

2

Define Constraints

Enter the Power, Ambient Temp, and your desired Safety Margin (e.g., 40°C below max).

3

Build Stackup

Fill in the resistances for the Device (RJC) and Paste (RCS). The tool will calculate the result instantly.