Required Thermal Resistance Calculator

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A calculator provides an initial engineering target, but the final cooling solution also depends on geometry, available space, heat source distribution, airflow or coolant conditions, mounting and manufacturing requirements.

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Thermal Design Calculator

Required Thermal Resistance Calculator

Calculate the allowable system resistance, required heat sink resistance, and estimated junction temperature for a selected cooling solution.

T_j = T_a + P_totalR_θSA + P_device(R_θJC + R_θCS) Lower thermal resistance means better heat transfer.

Thermal path inputs

Device Heat Load Required

Power dissipated by the device whose junction temperature is being checked.

Maximum Junction Temperature Required

Use the design limit from the component datasheet, not an absolute theoretical value.

Maximum Ambient Temperature Required

Air temperature entering the heat sink or the local environment around it.

Junction-to-Case R_θJC Required

Obtain this value from the component or power-module datasheet.

Case-to-Sink R_θCS Required

Includes the thermal interface material, contact quality and mounting pressure.

Selected Heat Sink R_θSA For Tj estimate

Enter a supplier, test or simulation value at the intended airflow and orientation.

Multiple devices share the same heat sink Enable this when other components also heat the same sink. Their power raises sink temperature even if their RθJC paths are separate.

Design margin

Junction Temperature Margin

Subtracted from maximum junction temperature when sizing the heat sink.

Calculation Results

Steady-state preliminary estimate

Target met

Required Heat Sink R_θSA

≤ 0.417 °C/W

The selected heat sink should have this resistance or lower.

Allowable System R_θJA

0.667 °C/W

Estimated Junction T_j

137.5 °C

Estimated Sink T_s

100.0 °C

Junction Margin to Limit

12.5 °C

Heat sink resistance comparison Lower is better

Required

0.417

Selected

0.400

Estimated temperature path Single device

Ambient40.0°C

Sink100.0°C

Case107.5°C

Junction137.5°C

Preliminary target met

The selected heat sink resistance is below the calculated requirement and the estimated junction temperature remains below the design target.

Thermal resistance depends on airflow, orientation, heat spreading, contact area, TIM, mounting, radiation and test conditions. Verify the final design by simulation or prototype testing.