Calculator Inputs
Example Data Table
| Device | Heat Load | Ambient | θjc | θcs | Suggested θsa Target |
|---|---|---|---|---|---|
| Power LED array | 28 W | 30 °C | 1.2 °C/W | 0.3 °C/W | Near 1.5 °C/W |
| MOSFET module | 55 W | 40 °C | 0.5 °C/W | 0.2 °C/W | Below 0.8 °C/W |
| DC regulator | 18 W | 45 °C | 2.0 °C/W | 0.4 °C/W | Near 1.0 °C/W |
Formula Used
Design heat: Qd = Q × (1 + safety factor / 100)
Allowed rise: ΔT = Tj max − Ta
Required sink resistance: θsa required = ΔT / Qd − θjc − θcs
Final junction temperature: Tj = Ta + Qd × (θjc + θcs + θsa)
Fin efficiency: η = tanh(mLc) / (mLc)
Fin constant: m = √(2h / kt)
Convection heat: Qconv = hAeff(Ts − Ta)
Radiation heat: Qrad = εσArad(Ts⁴ − Ta⁴)
How to Use This Calculator
- Enter the heat load produced by the device.
- Enter ambient temperature and maximum junction temperature.
- Add package and interface thermal resistances.
- Enter known sink resistance if the data sheet provides it.
- Leave known resistance blank to estimate it from fins and area.
- Use a realistic convection coefficient for natural or forced airflow.
- Set emissivity from the surface finish.
- Press the calculate button and read the margin.
Heat Sink Heat Transfer Guide
Why Thermal Resistance Matters
A heat sink moves heat from a device into surrounding air. The device first sends heat through its package. Heat then crosses any pad, paste, or insulator. Finally, the sink spreads heat through its base and fins. Each step adds thermal resistance. Lower resistance gives a lower junction temperature.
Start With Heat Load
Good design starts with the heat load. This is the power that becomes heat. It may come from a transistor, LED, processor, regulator, or power module. The allowed temperature rise is also important. It equals maximum junction temperature minus ambient temperature. That rise must cover every resistance path in the stack.
Two Calculation Paths
This calculator checks that path in two ways. You can enter a known sink to air resistance. That is useful when a data sheet gives a rated value. You can also estimate performance from surface area, convection coefficient, emissivity, and fin details. The estimate includes natural or forced convection input. It also adds radiation from exposed surfaces. Radiation is small at low temperatures. It becomes more helpful at high temperature rise.
Fin Efficiency
Fin efficiency matters in real sinks. Long thin fins do not stay at the base temperature. Their tips run cooler than the base. A fin efficiency factor reduces useful fin area. Better conductivity, thicker fins, and shorter fins improve efficiency. Strong airflow also changes the result because it raises convection heat transfer.
Design Margin
Use conservative inputs for early design. Airflow inside an enclosure is often weaker than expected. Dust, blocked vents, mounting angle, and nearby hot parts can raise temperature. Thermal pads can add more resistance than paste. Contact pressure also changes the result.
Reading the Output
The result shows required sink resistance, estimated sink resistance, sink temperature, junction temperature, and thermal margin. A positive margin means the design meets the chosen limit. A negative margin means heat must be reduced, airflow improved, or the sink enlarged.
Testing the Design
This tool is still a design aid. Real parts should be tested with thermocouples or built in sensors. Measure under full load. Test at the highest ambient temperature. Then compare the measured values with the predicted values. Small adjustments can prevent thermal shutdown, early aging, or unsafe surface temperatures. Repeated testing also catches fan failures, uneven paste spread, and enclosure recirculation before production or field use.
FAQs
What is heat sink thermal resistance?
It is the temperature rise per watt between the sink and air. A lower value means better cooling. It is usually written as θsa in °C/W.
What is junction temperature?
Junction temperature is the internal device temperature. It is often the most important thermal limit for chips, LEDs, regulators, and power transistors.
When should I enter known θsa?
Enter it when a manufacturer data sheet gives sink-to-air resistance. It is usually more reliable than a rough area estimate.
Does airflow improve heat transfer?
Yes. Forced airflow increases the convection coefficient. That lowers sink temperature and improves thermal margin, if the air path is not blocked.
Why does emissivity matter?
Emissivity controls radiation heat loss. Dark or anodized surfaces usually radiate better than shiny bare metal surfaces.
What does negative margin mean?
A negative margin means the calculated junction temperature is too high. Use a larger sink, better airflow, lower heat load, or better interface material.
Are fin calculations exact?
No. They are engineering estimates. Real fin spacing, airflow direction, turbulence, and mounting conditions can change heat transfer.
Should I test the final design?
Yes. Always test under full load and worst ambient temperature. Measurements confirm assumptions and expose enclosure or airflow problems.