Calculator
Formula Used
LED electrical power: P = N × I × Vf
LED heat: Qled = P × (1 - optical efficiency)
Driver heat: Qdriver = (P / driver efficiency) - P
Total heat: Qtotal = Qled + Qdriver
Usable temperature rise: ΔTusable = (Tj max - Ta) × (1 - safety margin)
Allowed total thermal resistance: Rtotal = ΔTusable / Qtotal
Required heat sink resistance: Rsa = Rtotal - Rjc - Rcs
Predicted junction temperature: Tj = Ta + Qtotal × (Rjc + Rcs + Rsa)
How to Use This Calculator
- Enter the LED count, current, and forward voltage.
- Select calculated heat or enter a manual heat value.
- Add ambient temperature and maximum junction temperature.
- Enter package and interface thermal resistance values.
- Add your available heat sink rating for comparison.
- Choose airflow, mounting, and safety margin.
- Press Calculate and review the result above the form.
- Use CSV or PDF export for records.
Example Data Table
| Example | LED Setup | Heat Load | Ambient | Thermal Path | Suggested Sink |
|---|---|---|---|---|---|
| Small lamp | 3 LEDs, 700 mA, 3.2 V | 4.70 W | 40 °C | Rjc 4, Rcs 0.5 | About 6.3 °C/W or lower |
| COB module | 1 module, manual heat | 18 W | 35 °C | Rjc 1.8, Rcs 0.3 | About 2.2 °C/W or lower |
| Strip fixture | 24 LEDs on aluminum bar | 12 W | 45 °C | Rjc 3, Rcs 0.8 | About 2.5 °C/W or lower |
LED Heat Sink Design Guide
Thermal Path Basics
An LED heat sink is selected by checking the complete thermal path. Power enters the diode, but only part becomes visible light. The remaining power becomes heat. That heat must move from the junction, through the package, across the interface, and into the sink. It then leaves the sink by convection and radiation.
Why This Calculator Helps
This calculator helps you size that path before buying parts. It combines LED count, current, forward voltage, optical efficiency, ambient temperature, junction limit, package resistance, interface resistance, airflow, mounting position, and safety margin. The result is a required sink rating in Celsius per watt. Lower values mean stronger cooling.
Safety Margin
A design with no margin can pass on a bench and fail later. Dust, high room temperature, tight housings, aging paste, and poor screw pressure can all raise junction temperature. For this reason, the margin field reduces the usable temperature rise. A ten to twenty percent margin is common for lighting work.
Comparing Real Parts
The available sink field lets you compare a real part. Enter the catalog value for the sink, then choose airflow and mounting. Natural vertical mounting is usually easier to cool than horizontal or enclosed mounting. Fan airflow can improve cooling, but fans add noise, power use, and failure points. Do not rely on a fan unless the product can detect fan failure.
Assembly Quality
Good thermal design also needs good assembly. Use flat surfaces. Apply a thin interface layer. Tighten fasteners evenly. Keep pads within their rated temperature. Place the sink away from hot drivers and sealed pockets. Test the final fixture at the highest expected ambient temperature.
Testing the Fixture
The calculated value is an estimate, not a certification. Real sinks depend on shape, surface finish, spacing, nearby walls, and air movement. Always compare the result with manufacturer curves. Then test the actual fixture with a thermocouple. Measure case temperature after the lamp reaches steady state. If junction temperature is still too high, use a lower resistance sink, reduce current, improve airflow, or increase margin.
Best Uses
This method is useful for COB modules, high power stars, strips on aluminum bars, and compact lighting fixtures. It also supports early cost planning. You can compare several operating currents and select the smallest heat sink that keeps the LED junction within a safe limit.
FAQs
1. What is heat sink thermal resistance?
It is the temperature rise per watt of heat. A 5 °C/W sink rises about 5 °C for each watt under its rated test condition.
2. Is a lower °C/W value better?
Yes. A lower value means the sink moves heat more effectively. It usually has more area, better airflow, or a more efficient shape.
3. Why does optical efficiency matter?
Not all electrical power becomes heat. Some becomes light. Optical efficiency estimates the useful light portion, so the remaining power becomes the heat load.
4. Should I include driver heat?
Include it when the driver shares the same sealed housing or warms the sink area. Exclude it when the driver is mounted far away.
5. What safety margin should I use?
Ten to twenty percent is common for many fixtures. Use more margin for sealed boxes, dusty areas, high ambient heat, or long service life.
6. Can this replace real testing?
No. Use it for planning and part selection. Final fixtures should be tested at steady temperature under the worst expected ambient condition.
7. Why is mounting condition included?
Orientation and enclosure affect air movement. A vertical open sink often cools better than a horizontal sink or one inside a tight cover.
8. What if the required sink value is negative?
The thermal path is too restrictive. Reduce heat, improve package resistance, reduce interface resistance, lower ambient temperature, or use a different LED module.