Production CREE XR-E Testing (SLOW lots of pictures!)
First off, CREE made a vast improvement in the Vf of their LEDs, and it holds quite well with increasing current. In fact, I know of no other power LED company producing LEDs with Vf's this low. Why does it matter? Power consumed is Current * Voltage. So, here is the first chart, compared with a Philips LumiLEDs Luxeon K2, their latest device:
Next, I measured the lumen output (I figure accuracy is +/-10%), vs. current:
This is also a vast improvement vs. any other LED I've tested to date, and pretty much, producing roughly 2x more light than other similar LEDs at the same current. Or looking at it another way, at the same light output, this part consumes about half the current, so your batteries will last 2x as long, and the LED will produce half the heat.
Due to the extra low Vf, and the outstanding amount of lumens comming out of the device at a given current, when you multiply these two together, the total power is often less than half, compared to most devices.
The extra high lumen output of this device, vs. power consumed, means it is more efficient, and more of you power leaves the device as light, instead of being converted to heat. Heat is the bane of LEDs and reduces both their expected lifetime and lumen output.
Yes, you can find the rare unobtainium bin of some whizbang Philips Luxeon, but they are rare, and still won't come close to matching the lumens vs. current of the CREE XR-E. CREE also makes some premium bins, this one tested here is just a run of the mill part.
For testing, the device was directly soldered to a 2" by 3" plate of 0.162" copper slab, and set on a table with no cooling air. Your results may be better, if you are more agressive in your heatsinking.
I can't recall, but, I'm wondering if the datasheet has any information of output vs heat? Just curious to know of the thermal degradation versus temperature is relative to the competition. I wonder if the Cree die is better or worse in this.
My question is this? Assuming you could drive it at say 2A and die temperature rises to 120C as an example. Would it be better to drive it at say 1.5A and have a lower temperature die rise. It's possible that the same light configuration might produce more lumens at the lower current since it doesn't get as hot. There might be a sweet spot that increasing current to LED might generate less lumens output when the light gets to thermal equilibrium.
Nice graphs Newbie! I was told the die is rated at 2 amps but the lumen maintenance and packaging are not up to this current.
Some might falsely assume that twice the light means 1/2 the heat. Instead of lumens, it would probably make sense to measure the light in radiometric power and get a feel for the light output in watts and compare that to the input power of watts to get an idea of the actual power in heat generated. I suspect that in some cases, a light with the XR-E and a good MCPCB may deliver more heat to the flashlight host than a Luxeon driven at the same power level. The low thermal resistance of the Cree XR-E and MCPCB may be low enough to bring this about, relative to the Luxeon. Anyone soldering one of these LED's will appreciate how well the heat transfers throughout the package!
Any idea of Vf shift as a function of temp? I think that flat curve shows the difficulty for a DD set up. There is one DD composition though that I bet would be real viable and that would be on a single CR123.
Theta Jc for XR-E is 8 C/W and the K2 is 9 C/W. I think if everything else allows, these can be driven to 1.5A like the K2. I prefer moderation where the incremental increase in current results in a somewhat linear fashion with the output which is 700mA or less which incidentally is how Cree specs their part.
Wow, this is more than an incremental improvement - more like a leap. Looks like Cree has a real winner here. The conformal phosphor coating is icing on the cake (no pun intended).
It's VI curve looks like it would resemble a UxxH LuxIII (but with a greater light output). Luxeon did not do themselves much of a favour in releasing the K2 with such high Vfs. It was a more of one step forward and two steps back.
Correction: Oops - I was looking at the wrong point (500mA instead of 700mA) binning for the Cree. Looks like a good VxxH instead of a good UxxH bin!!! Outstanding.
Cree XR-E, bin WC-P4, right out-of box, epoxied to heatsink. Measured on the emitter. Current source: nFlex.
20 mA - 2,71 V
58 mA - 2,84 V
100 mA - 2,94 V
175 mA - 3,05 V
350 mA - 3,24 V
500 mA - 3,34 V
700 mA - 3,49 V
1000 mA - 3,59 V
Last one is a little suspicous, I will check it later, maybe I was blind and missed a segment on my multimeter.
Vf's will typically vary from part to part, but your Vfs are much closer to what I measured.
The reason why they are closer, is that it may be because you measured at the emitter for the voltage, typically, when you have current going down a test lead, it causes a voltage drop, which means voltages measured at the power supply will be higher than normal.
But remember, Vf's will vary from part to part, just like the Luxeons do.
Luxeon III binning current is 700mA
Your sample looks like it would be a Philips Luxeon III bin J.
My sample looks like it would be a rare Philips Luxeon III bin H.
@ 701.5 mA and half the voltage of a Lux V (3.2716
Volts), I'm getting ~134.075 lumens and 2.2950274 Watts.
This would make it a VXOH bin, and a very upper end of the flux bin at that.
At 50 mA you have more than 100 Lm per watt, 5 mm LED don't do as well.
At .4 A you have about 70 Lm/W
At .6 A about 60 Lm/W
At 1 A it degrades to 50 Lm/W
You would need pretty agressive cooling to use the Cree above this level.
For a bicycle light it could still be useable since the air displacement is important but for a flashlight I would certainly not go above this level.
Thanks Newbie, great work .
Last edited by Calina; 10-29-2006 at 07:42 PM.
Health is merely the slowest possible rate at which one can die.
As my understanding, CREE doesn't allow user to choose Vf value. They said there has no bin code for forwading voltage.
So, if they can keep low working voltage is very good for user.
But, if they can't control well. It may be a challenge to engineer.
Did you measure those Vf's Kelvin connected at the emitter, or did you measure them at your power supply?
It makes a big difference.
Measured with a voltmeter right at the LED. I know there's a huge IR drop thru the leads from the power supply especially at 1A. I realize my data shows higher Vf but I'm sticking to it. I see some incosistency in Vf since I have two other parts that I went ahead and measured and they're closer to what you got though still a bit on the higher side. I re-measured the data I posted again and got the same results. Maybe a single LED fluke?
EDITED TO ADD:
Two other leds measure 3.50V and 3.55V at 700mA. Still very happy with the output of these. I'm now looking at my S bin and U bins with contempt
Nice work. I'm starting to get excited A little not so excited about the lack of hosts though I won't be buying an aleph or a TI McCree so i'm kinda out in the cold on that one. Maybe a minimaglite and just put up with the beam pattern.
Okay, I poked around a bit and found that I did find a loose reflector from McGizmo handy, it is 45mm in diameter.
I know that this isn't a fair comparison, but what I happened to have that was portable is a circuit that delivered 450mA to the CREE XR-E. The exact wattage drawn by this CREE at this current is 1.423 Watts.
I do not know how many mA the Fenix P1 delivers to the emitter, but I had one with a fresh battery I put in earlier today, so I used that as a reference. I just checked the current draw from the cell, and it is measuring 625mA, with the cell voltage dropping to 3.05V. This is 1.91 Watts. I suspect the converter is ~80% efficient, so the power delivered to the Luxeon III in the P1 would be roughly 1.525 Watts- but this is not fact.
I held the reflector in my hand, over the CREE XR-E and thus it isn't centered, and the photo is at an angle, as the truck was in the way. So, we have the larger reflector I used (so please keep this in mind!), and this is what I saw:
Anyhow, I need to run off to bed, maybe I can put together something tomorrow that is a much more fair comparison.