Junction temperature of the LED chips

When we`re talking about the lifespan of a lighting product , we mainly talking about the lifespan of LED driver and LED chips, as these 2 parts decide the lifespan of the whole fixtures, especially the LED chips, when the Lumen Maintenance is 70%, lower than the original lumen, it will be regarded as a failed light. Temperature control is the most important thing if you want to make sure your light will last long enough.

For the driver part we can test the case temperature easily, but for the chips ,do you know how to get the temperature in the middle of the chips which is supposed to be the hottest point? This temperature is called junction temperature (Tj).

Now we randomly take  NF2L757GRT-V1 chips from Nichia for example, refer to the specification (page 24), we know Tj=Ts+RθJS･W, so we need to know the Ts and RθJS.

The soldering point(Ts) temperature can be tested by yourself as below (point 2) . Here is the temperature test from 200W HiCool high bay at Ta 50℃。

Thermal resistance from junction to Ts  (soldering temperature )measurement point can be found at the chips spec Page 2, the max thermal resistance is 17, this resistance is for one single chip, we need to know the driven power for each single chip first.

Let`s still take the 200W HiCool high bay (252pcs chips) as an example, we can calculate the driven power for one single chip.  200W*0.93(power efficiency)/252pcs=0.74W/PC (per chip).

So here is the Junction temperature Tj=Ts+RθJS･W=92.5+17*0.74=105 ℃

From the chips specification page 2, we know the maximum junction temperature is 120°C, so our 200W high bay only has a junction temperature of 105 ℃ at Ta=50℃. Ta( ambient temperature)

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