Luxeon Star LEDs

All high power LEDs produce heat that needs to be dissipated in order to ensure that the LED junction (the part of the LED that actually generates light) is kept well below it's maximum operating temperature.

Significant damage to the LED can be caused by even a small amount of overheating, including:

• Significantly shortened life.
• Reduced light output.
• Change in color output. (Especially with white LEDs)
• Complete LED failure.

Heat is typically removed and dissipated from the LED using some sort of heat sink or heat sink/cooling fan combination; the size and shape of which will depend on:

• The LED wattage.
• The number of LEDs you are powering.
• The temperature of the environment the LEDs are operated in.
• Whether the LED is mounted in an enclosed or open space.

Ideally you would perform a detailed thermal analysis using thermal modeling software to design an effective cooling system, but since this type of software is usually quite expensive, most designers take a more empirical approach using trial and error to determine just how to effectively cool the LEDs they are powering.

As a starting point, you can get an idea as to what size of heat sink you will need using a fairly simple thermal model that calculates the thermal conductivity (°C/Watt, or just °C/W) value of the heat sink you will need by considering:

• The sum of all the thermal resistances between the LED junction and the ambient air.
• The amount of heat to be dissipated in watts.
• The ambient air temperature that the LED will be operated in.

With all of this information you can use the calculator below to determine what the minimum °C/W rating you will need for your heat sink.

Note that the calculator we provide can only determine the °C/W rating of the heat sink needed for a single LED. You could use the results as guidance in selecting a heat sink for multiple LEDs, but that will not consider factors such as the distance between each LED, total wattage of multiple LEDs, etc. So you would need to experiment further to ensure that the heat sink you select will be adequate.

Enter the forward voltage (Vf) rating of the LED and the current in mA that you will be operating the LED at into the Vf and I boxes. This will determine the amount of power in watts that needs to be dissipated. As this calculator can only provide results for a single LED, you will not be able to enter a value in the Number of LEDs box

Enter the maximum junction temperature that you want to operate the LED at.

The technical documentation for the LED will provide the recommended maximum LED Junction Temperature, but you will want to stay well below that value as the cooler you can keep the junction temperature of the LED, the longer the LED will last. The color output of the LED will also be more accurate and stable if the junction temperature is kept low.

For example, if you check the technical document for cool white Rebel LEDs, you will find that the maximum LED Junction Temperature is listed as 150°C. However ideally you would want to operate the LED well below that limit. Say, 100°C for example. The trade off of course, is that you will need a much larger heat sink to keep the junction temperature of the LED at a lower temperature.

If you are having trouble coming up with a high enough °C/W value using the calculator (higher °C/W values means you can use smaller heat sinks), then try making small increases in the junction temperature until you find a °C/W value that will work for you.

If at all possible you should avoid operating Rebel LEDs at junction temperatures much above 100°C. This is because there is a significant drop off in LED projected lifetime as you get closer to the maximum operating temperature of the LED. You will find a number of graphs that illustrate this drop off on pages 7 to 12 of Lumileds Rebel LED Reliability document.

Enter the air temperature that the LED and heat sink will be operated in.

Enter the thermal conductivity of the interface between the LED junction and the base (the thermal pad) of the LED.

This is usually called Typical Thermal Resistance Junction to Thermal Pad (°C/W) in the LED technical documentation.

Enter the thermal conductivity of the LED mounting base. Usually the base is an aluminum or fiberglass PC Board. If you are purchasing a Rebel LED from us that is pre-mounted to an aluminum star or square base, then use 6 °C/W in this box. If you will be mounting a Rebel LED to your own PC Board, then enter the thermal resistance value of the PC Board material you will be using.

Enter the thermal resistance of the material that you will be using to attach the LED base to the heat sink. If you are using our pre-cut Berquist thermally conductive tape, then enter 4.5 °C/W in this box. If you are using a different method of attaching the LED to the heat sink, then enter the °C/W value in this box.

Once you have entered all of the values into the boxes in the calculator, click the Calculate button to determine the °C/W value of the heat sink that you will need to operate the LED at the junction temperature you specified in step 2. Once you have this value, you can then decide which heat sink to purchase.

You will find the °C/W rating for all of the heat sinks that we offer on each products detailed information page on this website. Any heat sink with a value that is equal to or lower than the °C/W value generated by this calculator should be suitable. However you will still need to confirm that the heat sink is providing adequate cooling by measuring the LED junction temperature. Lumileds has produced a document that details exactly how to measure LED junction temperature here.

Total Wattage
Vf		I (mA)		No LEDs		Watts
	x		x		=

Maximum LED
Junction Temp °C

LED Junction To
Thermal Pad °C/W

LED Mounting
Base °C/W

Heatsink
Attachment °C/W

Typical Ambient
Temp °C

Maximum Heat Sink
Thermal Resistance °C/W