Technical evaluation - Cree XR-E Q4 bin
The Cree XR-E has been out in the market for almost a year now. It was a bit of a revolution when it came out, even at the 70-ish lm/W level (which is considered "low" these days), it was far above and beyond any offering prior in terms of efficiency; and easily became the brightest single-junction LED and essentially single handedly doubled LED efficiency.
I figured I'd produce a report just as I've done with the 100 lumen rebel and improved K2 (yet unreleased). While I may not be presenting new or amazing information, it'd still be nice to have this all in one place in the LED forum.
The partcular XR-E I'm testing is a Q4 bin, which is supposed to produce 100-107 lumens at 350mA. It's not the absolute brightest XR-E available (the Q5 bin gets that honor), but seems to be easily widely available, and I didn't particularly feel like going out of my way or spending tons of extra for the Q5 bin. Perhaps in the not to distant future, I'll obtain one and present test data on it as well.
Just in case you haven't been around here for the last year, here is what a Cree XR-E looks like:
This is a 4 bond wire version of the XR-E. The P4 I've tested prior only had 3 bond wires. The extra bond wire should help reduce the Vf at higher currents. While extra bond wires do help, there's only so much that packaging can do for the Vf of a device. Ultimately, the dominant factor in the Vf is the LED chip its self, and there is still variation among individual samples. So having extra bond wires still doesn't ensure a low Vf.
The optical system of the XR-E is interesting - there's a thick silver ring, with the optical dome (made of glass). The LED chip sits quite deep down relative to the glass dome, indicating that the beam pattern will be relatively narrow and forward projecting, compared to other power LEDs that we've become accustom to that have nice, wide, lambertian distributions. This could pose a problem for certian reflector configurations.
Here's the backside of the XR-E:
The XR-E is intended to be reflow soldered to a PCB. Because of this, there are power contacts on the bottom of the package, connected to the power contacts on top. This can present a problem if you want to directly mount the XR-E to bare metal for a heatsink, as the heat sink could short the power pads together. The top and bottom contacts are connected with vias in the corners of the package substrate. There are quite a few solutions that are possible to address the shorting issue: clipping the corners of the package, grinding down the bottom contacts, using a thin insulating layer of thermal epoxy, or raising the central portion of the heat sink to isolate the power contacts. I chose to use my dremel and a sanding disk to sand down the contacts in the corners where the vias are present. To me, this is the least destructive solution that allows the simplest mounting, as I can use as thin of a layer of thermal compound/epoxy as possible, and not worry if part of the package makes physical contact with the heat sink.
The central pad is the thermal pad, indended for heat transfer from the package to the heat sink. The thermal pad is fully isolated!!! YAY!! When using multiple XR-E's in various electrical configurations, there is no need to electrically isolate the thermal pads on XR-Es, simplifying mouting and usage.
Here's the XR-E amongst similar power LEDs: the updated K2, and Rebel-100:
The XR-E is about the same size as the K2 (and original Luxeon, Luxeon III, and Luxeon V). The Rebel is substantially smaller (and much more difficult to deal with).
The beam profile of the XR-E is substantially different than other power LEDs. It is significantly more narrow and forward projecting than other power LEDs, and projects much less out the sides.
Here is the XR-E's beam profile, compared to the Rebel-100:
The relative intensity is a good way to compare different beam profiles, but it doesn't tell the whole story. Another way is to compare the absolute beam intensity at a given current, for two different LEDs with the same brightness. This will show which part of the beam each LED delivers its lumens to, and allows you to estimate how a given optical system will behave with the given beam distribution.
Here's the beam profile for the rebel and XR-E, at 310mA. This is the actual beam brightess in lux, not scaled relative to 0 degrees, like in the previous graph:
In the central 35 degrees, the XR-E delivers more lumens, while in the part of the beam outside 35 degrees, the rebel delivers more lumens. Thus, the rebel could create a brighter focused beam with a reflector, but the XR-E would produce a brighter beam with optics (such as an aspherical lens). The lack of side-emitted light makes the XR-E less than ideal for reflectors. An especially deep reflector would be needed to be optimal with the XR-E.
One must wonder how the beam profile and total output would change if the thick silver ring were absent, and a shorter silicone dome were in the place of the ring/glass dome combination. The output could be significantly higher if the side emitted light were permitted to escape directly. This might be one reason why the Seoul P4 was hitting near 100 lumens last year when the XR-E was at around 80ish, when they both used the same blue die underneath. Granted, there are phosphor differences between the two, but I have a feeling the optics of the packages also have an effect on that.
Here's the raw performance data for this Q4 XR-E.
Current (mA) 0.1 30 130 310 670 980 1260 1570 1930
Vf 2.53 2.82 3.15 3.37 3.62 3.75 3.86 3.96 4.06
Watts 0.41 1.04 2.43 3.68 4.86 6.22 7.84
Lumens 42 94 178 234 278 312 341
Lumens/W 103 90 73 64 57 50 44
The extra bond wire in the Q4 XR-E's construction should help lower the Vf. This isn't a guarantee, since the Vf is mostly dependent on the individual LED die. Let's see how the Vf of this particular XR-E stacks up:
This particular XR-E actually has a rather high Vf. This will hurt it in the efficiency department. This doesn't mean that all XR-Es will have such a high Vf, but this particular one does. Even though it is the 4 bond wire version, it still has a higher Vf across all current levels than even my 3 bond wire P4 XR-E. So, when it comes to Vf, your milage may (and will) vary.
Here is how the Q4 stacks up to other power LEDs with overall brightness:
The Q4 XR-E (100-107 lumens at 350mA) performs almost identically to the Rebel-100 (100-120 lumens at 350mA). In fact, it starts to peform a bit better than the Rebel at very high current levels. The better packaging of the XR-E allows for a better thermal path, and makes heat sinking much easier. The Q5 XR-E should perform even better.
The high Vf of this XR-E hurts it efficiency wise. With a lower Vf, the XR-E will be much more efficient. The efficiency of this XR-E could be about 10% higher if it had the same Vf of the P4 XR-E.
Finally, the infamous "droopyness" graph - an indication of how efficiency scales with drive current:
The Q4 XR-E scales about the same as all other current power LEDs.
So the Q4 XR-E has about the same performance specs as the Rebel-100, and performs pretty much identically as well. Ultimately the choice as to which one to use depends on a lot of factors. As far as ease of use and mouting, the XR-E wins hands down - it's much easier to deal with than the rebel, even with the bottom electrical contacts - it's also much easier to get good thermal transfer to the heat sink with the XR-E as well. The only gotcha would be with the beam profile, which is much different than other power LEDs that have a more lambertian beam distribution. This could affect performance with certian optical configurations.
So which one to use? Don't ask me! I'm still up in the air - with power LEDs coming in different packages, with different levels of performance, and varied aspects of beam profiles, electrical configurations, etc, there are more choices now than ever in the power LED market, but it's less and less clear what the best "go-to" power LED is for general applications.
Given that, the Q4 XR-E is a great performer, and should definitely be seriously considered for any application.