My CULPM1.TG 25W COBs have finally arrived. Unfortunately, I have already managed to fry one, I guess due to insufficient cooling. I would like to avoid that fate for my other LEDs.
How do you guys measure the junction temperature to make sure it doesn’t get too hot?
I was thinking of getting a pyrometer. Is it possible to point it at the die directly, and if so, what would the correct emissivity factor be? I could also point it at the copper die instead, what is a sensible de-rating in this case, assuming a Tmax of 150°?
I only want to measure it once to determine which voltage and current is appropriate for the heat sink that I have, and then leave it be.
COBs often have a temperature management point.
You solder a thermal probe there.
Then then multiply thermal resistance (taken from the datasheet) by the power usage to learn how much hotter is the junction than the measurement point. Please note that some manufacturers specify thermal resistance for electrical power and some use thermal power.
I remembered that my multimeter has a thermocouple and used that to measure the temperature. The results were disappointing. Using a 20mm copper PCB and an aluminum heat sink with some thermal paste, the die reached 150° with just 3-5W of electrical input power, which is far from the design maximum. Maybe thermal conductivity is bad, I’m not sure.
Actual junction temp can only be inferred indirectly, not measured. Got all the piecemeal thermal resistances, junction-to-case (θJC) is about as good as you’ll get. Everything else is case to board, to shelf, to shell, then all the way to ambient.
True, I couldn’t find any precise resistance values in the data sheet. I was hoping the difference between junction temp and case temp shouldn’t be that high, probably below 10°, so I’ll just add a bit of safety buffer.