thermal comparison between 2 LED Setups

[mds82]

I have been thinking about this for a bit and i'm not quite sure which one would win. I'm thinking of modifying a LED setup that i am currently using, but want to make sure that it will not get too Hot.

Current setup - 2 Cree XR-E, driven in series, 1000ma each die
Proposed setup - 2 Cree MC-E, driven in series, 1400ma to each Led, so 8 dies running at 350ma each

Obviously all 8 dies running at 350ma will be much more efficient = less heat, but in total there will be an extra 800ma of power being used.

so questions is, with the 2 setups, which will produce more heat?? the MC-E's will obviously produce much more light

 

[tonycollinet]

First mA is not power - so you can't have 800mA of power.

Power (Heat) is produced by volts x amps (Watts). So to find out which will create the most heat, you need to know what voltage each setup will be running at.

You can do this approximately (from the data sheet) or accurately by measuring the actual setup.

Find out the nominal voltage at your specified current (1000mA for the XRE, and 1400mA for the MCE). Multiply by 2 to get the voltage of 2 leds in series (if you are measuring, then you can just measure accross the 2 leds to get this directly), then multiply by the current to get the power.

So the power of the two setups will be

2 x XRE_VOLTS_AT_1000mA x 1.0
2 x MCE_VOLTS_AT_1400mA x 1.4

Some of this power will be light, the rest will be heat. For the purposes of the comparison, it is reasonable to ignore the differences in light efficiency between the two setups, since probably only 10 to 15% of the power is light.


Incidentally the 8 die setup will also have a larger surface area to transfer heat through to your heatsink (should result in lower thermal resistance) - so even at the same power, the dies are likely to run a little cooler, if the LED construction is similar.

 

[LukeA]

Some of this power will be light, the rest will be heat. For the purposes of the comparison, it is reasonable to ignore the differences in light efficiency between the two setups, since probably only 10 to 15% of the power is light.


Incidentally the 8 die setup will also have a larger surface area to transfer heat through to your heatsink (should result in lower thermal resistance) - so even at the same power, the dies are likely to run a little cooler, if the LED construction is similar.

I don't know if it is appropriate to ignore differences in efficiency between the two setups. EZ1000 dies at 350mA, are IIRC, around 20-25% more efficient than dies at 1A. At 350mA, the EZ1000 die (Q5) can be as much as 40% efficient.

With a Vf of 3.5V@1A, the two XR-Es will draw 7W and output around 2.24W of light.

Assuming the same efficiency per die in the MC-E, the MC-E setup will draw 9.8W and will put out 3.9W of light.

More heat, but also more light per unit of heat.