Calculator
Formula used
This tool uses a steady-state thermal resistance model and an optional geometry-based estimate.
- Tj = Ta + P · (RθJC + RθCS + RθSA)
- RθSA,required = (Tj,max − Ta)/P − (RθJC + RθCS)
- Convection estimate: Rθ ≈ 1/(h·A) with A in m2.
- Radiation estimate: hr ≈ 4·ε·σ·T^3, then heq = h + hr.
- Optional transient (lumped): Ts(t) = Ta + P·Rθ·(1 − e^(−t/(Rθ·C))), where C = m·Cp.
How to use this calculator
- Pick a mode: required RθSA, predict temperatures, or estimate from area.
- Enter power, ambient temperature, and your junction temperature limit.
- Add RθJC and RθCS from datasheets and interface materials.
- For estimation, enter surface area, emissivity, airflow, and orientation.
- Press Calculate to show results above the form.
- Use CSV or PDF export for reports and documentation.
Example data table
| Scenario | P (W) | Ta (°C) | Tj,max (°C) | RθJC (°C/W) | RθCS (°C/W) | Required RθSA (°C/W) | Notes |
|---|---|---|---|---|---|---|---|
| Compact controller | 8 | 35 | 110 | 2.0 | 0.5 | 6.875 | Often feasible with natural convection. |
| Motor driver | 20 | 30 | 125 | 1.1 | 0.3 | 3.350 | Consider forced airflow inside enclosures. |
| High-power module | 60 | 40 | 150 | 0.6 | 0.2 | 1.033 | Heatsink + fan or cold plate likely. |
Values are illustrative. Always verify with measured airflow and vendor thermal data.
FAQs
1) What is RθSA?
RθSA is sink-to-ambient thermal resistance. Lower values mean better cooling. Vendors rate it for specific airflow and mounting conditions.
2) Why does ambient temperature matter so much?
Thermal rise adds on top of ambient. If the enclosure air is already hot, the same heat sink produces a higher junction temperature.
3) Should I use typical or worst-case RθJC?
For reliability, start with worst-case or datasheet maximum. Then compare with typical values to understand real-world headroom.
4) What is a realistic RθCS value?
It depends on material and pressure. Thermal grease is often lower than thick pads. Poor flatness or low clamp force increases RθCS sharply.
5) Does black anodizing always help?
Higher emissivity improves radiation, especially in still air. In strong forced airflow, convection dominates and coating impact is smaller.
6) How accurate is the area-and-airflow estimate?
It is a first-pass estimate using simplified heat transfer. Fin spacing, ducting, turbulence, and enclosure restrictions can change performance significantly.
7) When should I add a fan?
If required RθSA is very low or the enclosure runs hot, forced airflow often gives the biggest improvement per cost and size.
8) Can I use this for liquid cooling plates?
You can budget junction temperatures with Rθ values, but liquid systems need different coefficients and flow-based models. Use vendor data for plate-to-fluid resistance.