Calculator Inputs
Formula Used
Heat from COB: Heat = Input Power × (1 - LED Efficiency) + Extra Driver Heat.
Design heat: Design Heat = Heat × Safety Factor.
Allowable total resistance: RθJA = (Tj Max - Ambient) ÷ Design Heat.
Maximum heat sink resistance: RθSA = RθJA - RθJC - RθCS.
Estimated sink resistance: RθSA Estimate = 1 ÷ (h × Effective Area).
Effective area: Physical Area × Fin Efficiency × Orientation Factor.
How to Use This Calculator
Enter the COB electrical power and expected optical efficiency. Add any driver heat that warms the same housing. Set ambient temperature and the allowed junction limit from the LED data sheet.
Enter RθJC from the COB data sheet. Enter RθCS for paste, pad, or mounting layer. Use the heat sink size fields to estimate surface area. Press submit to compare the required sink rating with the estimated rating.
Example Data Table
| COB Power | Efficiency | Ambient | RθJC | RθCS | Safety | Suggested RθSA Max |
|---|---|---|---|---|---|---|
| 50 W | 35% | 25 °C | 0.9 | 0.3 | 1.20 | 1.65 °C/W |
| 100 W | 35% | 30 °C | 0.7 | 0.25 | 1.25 | 0.62 °C/W |
| 200 W | 40% | 35 °C | 0.5 | 0.2 | 1.30 | 0.15 °C/W |
COB LED Heat Sink Design Guide
Why COB Thermal Design Matters
A COB LED puts many dies on one small board. That small source can make intense light. It can also make intense heat. A heat sink must move that heat from the junction to the surrounding air. Poor cooling raises junction temperature. Output drops. Color can shift. Lifetime becomes shorter.
This calculator builds a practical thermal path. It starts with electrical input power. It subtracts useful optical output by using LED efficiency. The remaining power becomes heat. Then it checks junction to case resistance, interface resistance, and heat sink to air resistance. The final result shows the maximum sink resistance allowed.
Using the Results
A lower thermal resistance means stronger cooling. If the required sink value is 1.20 °C/W, a 0.80 °C/W sink is better. A 1.80 °C/W sink is risky. The tool also estimates area from base size, fin count, fin height, and convection strength. This helps compare passive and fan cooled designs.
Airflow changes the answer a lot. Natural convection may use a low heat transfer coefficient. A fan can raise it several times. Orientation matters too. Vertical fins usually work better than trapped horizontal channels. Dust, enclosures, and hot ceilings reduce real performance.
Design Margin
The safety factor makes the check tougher. A factor of 1.25 treats the LED as if it makes twenty five percent more heat. This helps cover driver tolerance, ambient spikes, paste aging, and blocked airflow. For sealed fixtures, use a higher margin. For lab tests, use real measured temperatures.
Material and Mounting
Aluminum is common because it conducts heat well and costs less. Copper spreads heat faster, but it is heavier and costly. The interface layer is still important. Use flat surfaces. Apply thin paste. Tighten screws evenly. A thick pad can add unwanted resistance.
Always test the final fixture. Measure case temperature after thermal stability. Compare it with the predicted case and junction values. A calculator is a design guide. Real fixtures need measurement, because airflow and mounting details control final temperature. Record each test condition carefully. Note ambient temperature, driver current, enclosure state, and fan speed. Good notes make later upgrades easier and safer for future fixture changes.
FAQs
What is a COB LED heat sink?
It is a metal part that carries heat away from a chip on board LED. It lowers junction temperature and protects light output, color, and service life.
What does RθSA mean?
RθSA means sink to ambient thermal resistance. Lower values move more heat for each degree of temperature rise. High power COB modules need lower values.
How much safety factor should I use?
Use 1.15 to 1.30 for open fixtures. Use a higher value for sealed housings, dusty areas, hot ceilings, or uncertain airflow.
Does LED efficiency affect heat?
Yes. Useful light is not treated as heat in the main estimate. Lower efficiency leaves more input power as heat, so the sink must work harder.
Can I use natural convection?
Yes, but natural convection needs more area. Use lower convection coefficients for passive designs. Add a fan when size or temperature limits are tight.
Why include case to sink resistance?
The mounting layer blocks some heat flow. Paste, pads, flatness, screw pressure, and dirt can change this resistance. A thin, even interface performs best.
Is surface area the only heat sink factor?
No. Fin spacing, airflow path, base thickness, material, enclosure shape, and mounting direction also matter. The calculator gives a practical estimate.
Should I still test the finished lamp?
Yes. Measure the case temperature after it stabilizes. Real airflow and installation details can differ from assumptions used in any calculator.