Help me calculate LED Junction temp

Hi I have all the figures but I don't know quite how to put it together to calculate a realistic junction temperature.

My stabilised aluminium heatsink (old CPU heatsink) temp: 45deg C (with a big fan pointed roughly at it)
LED board 7up cutter XP-G R2 at 1000mA (23W)
stuck directly to heatsink with Artic silver 5: 0.0045°C-in2/Watt (0.001 inch layer)

XP-G thermal resistance: 6degC/W

Help on calcs please!!

Marco.

Calcs? GIGO. The heatsink temperature is not so useful.

Mount a thermistor on the PCB, right up against one of the central XPG's, and measure the temperature there. Much closer to junction.

-Jeff

Hey marcopolo, you said LED board 7-up, is that an aluminum board, or FR4 type PC board stuff with lots of through-hole vias? Also, a heatsink sized fan, bolted directly to the unit, will be more efficient at heat removal than one generally pointed at it. What's you application for this setup? Cheers, Jeff O.

marcopolo said:

Hi I have all the figures but I don't know quite how to put it together to calculate a realistic junction temperature.

My stabilised aluminium heatsink (old CPU heatsink) temp: 45deg C (with a big fan pointed roughly at it)
LED board 7up cutter XP-G R2 at 1000mA (23W)
stuck directly to heatsink with Artic silver 5: 0.0045°C-in2/Watt (0.001 inch layer)

XP-G thermal resistance: 6degC/W

Help on calcs please!!

Marco.

Click to expand...

For a reasonable estimate, you can guess on somethings and frame it within a range.

First, within the LED itself = 3.5 Vf x 1 amp = call it 4 watts

4 watts x 6 C / watt = 24 C within each LED above its connection.

From this, the absolute lowest it is is 24 + 45 = 70 C

Adding in a few more factors
- Another 10 for the MCPCB + interfaces
- Another 10 for measuring in non ideal place

Rough estimate = 95C

You only use the wattage per LED for the temp rise within one led, not the total power, unless - it is all in one package.