Thermal calculation

Calculation of the required LED Cooler Thermal Resistance Rth

In this document we show how to calculate the required thermal resistance of the LED cooler.

As an example we take a LED COB model, which has a nominal forward current If of 450mA and a maximal forward current of 900mA.

We will drive the module at a forward current of 500mA with a forward voltage Vf of 35.5V.

The maximal case temperature Tc is 105°C but in our design we aim at a life time case temperature of 75°C.

The ambient temperature for our application is 35°C.

A LED luminaire can be seen as an electrical scheme with the thermal resistances of the LED internally, the interface material and the LED cooler is series.

The electrical power Pe = Vf x If or 35.5V x 0.5A = 17.75W.

The dissipated power Pd = Pe x efficiency where the efficiency of the COB is around 32% or 17.75W x 0.68 = 12.07W.

This is the amount of energy which need to be cooled down.

dT is the temperature difference between the case temperature Tc we want to acquire and the ambient temperature Ta

dT = Tc = Ta or 75°C – 35°C = 40°C

The required maximal thermal resistance Rth of the LED cooler + the thermal interface material Rth = dT / Pd = 40°C / 12.07W = 3.31°C/W

The thermal interface you use has a major impact on the performance.

We recommend to use either arctic silver, a good thermal grease or a thin 0.1 to 0.15mm phase change or graphite thermal pad.

In this case the thermal resistance from the interface material will be between 0.1 and 0.2°C/W.

This interface resistance you substract from the calculated interface resistance to determine the cooling performance your heat sink needs to be.

So Rth heatsink = 3.31°C/W – 0.2°C/W = 3.11°C/W maximal.

Any led cooler which does better than a thermal resistance (lower value) of 3.11°C/W in free air conditions would make that our LED case temperature Tc will remain below the required 75°C.

Keep in mind that an enclosure around the LED cooler, tilting and other variations will affect the performance of the LED cooler – contact us in case assistance is needed.

Lab measurement of the required LED cooler

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MechaTronix has performed over 1000 hours of lab measurements on the combination of world leading LED manufactures LED COB’s and modules, with various LED coolers and alternative driving currents.

These lab tests are done under free air convection environment, vertical positioning with the LED on the bottom. For the tests we use a phase change thermal interface Laird TPCM 585.

To conduct thermal measurement we use good digital meter (2 channels or more) and thin wire thermocouples which can easily measure the Tc point under operation. Besides a thermocouple on the Tc point, also attach a thermocouple on the base of the LED cooler near the LED module. When we see a temperature difference of more than 5°C between the two points, we will verify if it’s caused by interface material or wetting surface (the contact between the LED module and the LED cooler).

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