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.
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.
This is the amount of energy which need to be cooled down.
dT = Tc = Ta or 75°C – 35°C = 40°C
So Rth heatsink = 3.31°C/W – 0.2°C/W = 3.11°C/W maximal.
Lab measurement of the required LED cooler
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).