Avago "3W Mini-Emitter" Power LED temperature

I tried playing with the 3W Avago power LEDs.

I soldered two of them onto a very thin piece of PCB material (0.5mm / 20 mil) and connected them in series. Also added a resistor to limit the current to about 700mA when connected to a 12V source. The manufacturer datasheet states that the maximum continuous current is 700mA, but when I did that, the LEDs got extremely hot after only a few seconds of operation. The flux residue from soldering started smoldering. Seemed quite strange that they didn't die right there and then...but anyway.....

The second experiment I tried was with a power MOSFET and a precise variable resistor that allows me to limit the current through the LEDs quite precisely. The odd thing about them is that with only 200mA going through them (less than 1/3 of the rated maximum current) they were already getting too hot to hold on the back of the PCB after about 30-50 seconds of operation.

My question is: Am I doing something wrong with them? The manufacturer documentation contains very little information about cooling, but I would have thought any additional heatsink wouldn't be required for cases other than prolonged operation at it's maximum rating.

The problem is, I chose these LEDs for their rather small size (4mm x 4mm x 2mm) and mounting them onto a large heatsink entirely defies the purpose of small LEDs

Didn't you know that high power LEDs need a pretty good heatsink?
If you use Name-Brand LEDs then their datasheet tells you exactly how much heatsinking they need.

Yes, I expected them to need a heatsink if I intended to keep them powered at 700mA, for hours at a time, not at 1/3 of the power for seconds at a time.

They are very small so need a heatsink to keep them cool.

I bought a simple aluminum heatsink intended for a TO-220 device, and though the LED should be easily solderable to the heatsink. I was going by the impression that the solder material easily sticks to aluminum, which did not appear to be the case.

What about a copper heatsink? Like a 1mm thick sheet of copper, now I take it the ideal heat transfer method would be to somehow solder the actual LED onto it (keeping the power leads isolated), or continue with the 0.5mm PCB I made, and just glue it onto the copper sheet with a proper thermal adhesive...


And advice on that front?

It is very difficult to solder to aluminum. A special flux is needed and maybe a special enclosure that seals out air.

High power LED manufacturers like Philips Luxeon have tutorials about how to properly heatsink their LEDs.

yes for power leds, you are lucky if they have an isolated slug that you can "white paste" to a heat sink, some have screw placements on the led so it can be screwed to a heatsink, otherwise , if the slug is not isolated, you just have to glue it to a piece of metal with thermal adhesive as you say.

Hi Jumper,

I had a similar experience in the past for a hobby project. I found out this applciation note from Avago, which helped me a lot. I did at that time a PCB heat sink. I hope it helps to you too. Give a try!

Thermal Management of Moonstone LEDs:


Cheers

Yeah, I found that eventually.
I made a 0.5mm thin PCB like the one on Figure 6f from that document, and intend to glue it to a piece of Copper plate with thermal adhesive hoping that would do the trick.