Avago "3W Mini-Emitter" Power LED temperature

[jumper2high]

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

 

[Audioguru]

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.

 

[jumper2high]

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.

 

[Audioguru]

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

 

[jumper2high]

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?

 

[Audioguru]

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.

 

[eem2am]

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.

 

[hakeen]

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

 

[jumper2high]

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.