Cree XML high output LEDs

[Zac Hall]

Hi all,
I'm attempting to make my own bicycle headlight, via a cree xml t5 LED. I purchased a ready-made buck driver circuit to power the LED from Li Ion batt. pack.

the issue is, there doesn't appear to be terminals coming off the LED. Is there some kind of "star" or base PCB I need for this LED? if so, how do I attach it to the board? data sheet mentioned something called "reflow soldering". is this what I'd use?

 

[Harald Kapp]

The xml t5 s a SMD LEd which means it is mean to be soldered directly to a PCB. See datasheet page 12.
You can solder wires to anode and cathode (long bars at the side). The tricky part ist that this LED needs cooling. This is where the solder point in the center come into play. The LED is meant to be soldered onto a thermally conducting substrate here. This usually requires reflow-soldering as you have no access to these center pads with a soldering iron.
As a workaround you can drill holes in your substrate (see the 8 squares in the recommended stencil pattern) and carefully solder the LED through these holes to e.g. a copper plane on the back of the substrate.
Be careful not to overheat the LED during this procedure, let it cool down between soldering through holes. Solder isn't the best of thermal conductors, but better than no cooling at all.

You'd have an easier life using this LED already mounted on a substrate.

 

[Zac Hall]

Thank you for this information! By substrate, did you mean the PCB that the LED gets soldered to? Also, the long bars at the side; were these those two long strips underneath the LED (I assume these are the anode and cathode)?

 

[Harald Kapp]

Yes and yes.

For improved cooling this type of LED sometimes is mounted directly on aluminum heat sinks (with appropriate insulation for the anode and cthode, of course).

 

[Akrylamid]

Personally I'd go for copper MCPCBs, like Noctigon. Copper MCPCBs with direct thermal paths can suck up the heat a little better than those aluminum ones. If you have no experience with hot air soldering or other reflow processes, do yourself a favor and get a LED already reflowed into the MCPCB.

When getting a LED pre-reflowed onto a copper MCPCB you have multiple choices. What is more important? Output strength, color temperature, color rendering (CRI)? beam profile (flood bs throw)? Personally I do not like cool white LEDs. They maybe brightest XM-L2s, but colors appear too flat if you ask me.

Have a look at XM-L2s here: http://www.mtnelectronics.com/index.php?route=product/category&path=60_61
If you want higher CRI, go for Nichia LEDs, the 219Cs have a good mixture of output vs CRI: http://www.mtnelectronics.com/index.php?route=product/category&path=60_94

MTN electronics is US based. I use them a lot even though I live in Sweden. They know LEDs and flashlights, and have good customer service.

As you have a buck driver, I'm assuming the battery pack has the cells in series? If you want to go crazy you could opt for a MT-G2 LED, or XHP-50. You would however need a different driver as they are not 3V LEDs.