Heatsink Thermal Resistance Calculator
Calculates the required heatsink thermal resistance (Rθsa) from a device's maximum junction temperature, ambient temperature, power dissipation, and junction-to-case/case-to-sink resistances, with heatsink recommendations.
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How to Use
- Enter the device's maximum junction temperature Tj_max (°C; typically 125–150°C for silicon devices).
- Enter the ambient temperature Ta (°C; default 25°C).
- Enter the device power dissipation P (W).
- Enter the junction-to-case thermal resistance Rθjc (°C/W, from the device datasheet).
- Enter the case-to-sink thermal resistance Rθcs (°C/W; approximately 0.2–0.5°C/W with thermal grease, up to 1–2°C/W without).
- The required heatsink thermal resistance Rθsa and the resulting actual junction temperature Tj are calculated in real time, along with a text representation of the full thermal chain (Tj → Rθjc → Tc → Rθcs → Ts → Rθsa → Ta).
- Below, a four-tier heatsink recommendation is given based on the Rθsa value: ≥60°C/W means no heatsink needed; 20–60°C/W suggests a small natural-convection heatsink; 5–20°C/W a medium heatsink; 0–5°C/W a large heatsink or forced-air cooling; negative values indicate a heatsink alone is insufficient — power must be reduced or active cooling (liquid/heat pipe) used.
- Click "Load Sample Data" to restore the default example: Tj_max=125°C, Ta=25°C, P=5W, Rθjc=2°C/W, Rθcs=0.5°C/W.
Features
- Calculates the required heatsink (sink-to-ambient) thermal resistance for a power device using the classic thermal resistance chain model: Rθsa = (Tj_max − Ta) / P − Rθjc − Rθcs.
- Also computes the actual junction temperature Tj = Ta + P×(Rθjc + Rθcs + max(Rθsa,0)) under the given cooling conditions, plus the case temperature Tc and heatsink surface temperature Ts.
- Displays the complete thermal resistance chain path as text (Tj→Rθjc→Tc→Rθcs→Ts→Rθsa→Ta) with temperature values labeled at each node.
- Automatically tiers heatsink recommendations based on the computed Rθsa: no heatsink needed / small natural-convection heatsink / medium natural-convection heatsink / large heatsink or forced-air cooling / heatsink alone insufficient, active cooling required.
- Validates inputs (power ≤0, junction temp ≤ ambient, negative thermal resistances) and shows a clear error message rather than silently producing incorrect output.
Use Cases
Heatsink sizing for power devices (MOSFETs, regulators, amplifiers)
Determine the thermal resistance a heatsink needs for heat-generating components like linear regulators, power MOSFETs, or audio power amplifiers, to keep junction temperature within the device's rated limit.
Assessing whether an additional heatsink is needed
For low-power designs, calculate whether the bare device package's natural convection alone can meet temperature requirements, avoiding over-designed cooling.
Deciding between passive and active cooling upgrades/downgrades
When a product needs to move from natural convection to forced-air cooling, or conversely to evaluate whether a fan can be removed, compare Rθsa requirements across different power and ambient temperature scenarios.
Diagnosing overheating or junction over-temperature failures
When a device trips thermal protection or fails from overheating, back-calculate the actual junction temperature under the current heatsink's thermal resistance to determine if undersized cooling caused Tj to exceed Tj_max.
FAQ
What does a negative Rθsa result mean?
It means that even with zero heatsink thermal resistance (i.e., no heatsink at all), the device's own junction-to-case and case-to-sink resistances alone cannot keep the junction temperature below Tj_max. The tool flags this as "heatsink alone insufficient" — power dissipation must be reduced first, or stronger active cooling (liquid cooling, heat pipes) must be used; simply choosing a bigger heatsink won't solve it.
What value should I use for Rθcs (case-to-sink resistance)?
It depends on the thermal interface material used: with thermal grease, typical values are about 0.2–0.5°C/W; with bare metal-to-metal contact and no interface material, it can be as high as 1–2°C/W; thermal pads or phase-change materials fall in between. The tool defaults to 0.5°C/W as a common estimate.
Where do I find Rθjc (junction-to-case resistance)?
This is an intrinsic thermal characteristic of the device package, found in the "Thermal Characteristics" or "Thermal Resistance" section of the device datasheet. It varies significantly by package type (TO-220, TO-247, SOT-23, etc.). The tool has no built-in device database — you must look it up and enter it manually.
Why does the junction temperature calculation use max(Rθsa, 0)?
When the computed Rθsa is negative (implying a theoretical "negative thermal resistance" that doesn't physically exist), the tool substitutes 0 for display purposes when calculating the shown junction temperature, to avoid a distorted result. The status message still accurately flags the "heatsink alone insufficient" warning, and the negative value is still used to select the appropriate recommendation tier.