What really causes (White) LED's to fail?

What really causes (White) LED\'s to fail?

Especially the 5W Luxies...


500 hr is such a short life even if it's the half life.

Does the current start to ramp up and efficiacy drop?

Does this happen at the actual diode junction OR is it a result of the phospohor coating being damaged and loosing conversion efficiency?

Cheers!

Re: What really causes (White) LED\'s to fail?

We've theorized that it must be related to the phosphor, since the blue/green 5W devices are rated at a 50,000 hour life by Lumileds. Everything else is the same between the colored and white 5W devices, thus the phosphor must be what's causing the reduced light.

We can see the relative junction temperature difference between the various families of Luxeons to determine how the phosphors are being treated:

(assuming the emitter mounted directly to an infinite 25C heatsink, 1C/W S-A thermal resistance)

1W:
Rj-s = 15C/W
Tj = 25 + (16 * (3.3 * 0.35) ) = 44C

3W (700mA) :
Rj-s = 13C/W
Tj = 25 + (14 * (3.5 * 0.7) ) = 60C

3W (1A):
Rj-s = 13C/W
Tj = 25 + (14 * (3.7 * 1.0) ) = 77C

5W:
Rj-s = 8C/W
Tj = 25 + (9 * (6.8 * 0.7) ) = 68C



What is interesting is that the 3W at 1A has a higher Tj than a 5W, yet a 3W at 1A is rated at 20K hours to 50% brightness.

However, we are just given the j-s thermal resistance. The phosphor must be at a higher temperature, since it is farther from the heat sink slug than the junction is. It could be that there is so much heat produced at the phosphor of a 5W (compared to any of the others) that the phosphor is being baked alive, even though the junction temperature is at reasonable levels.

If anyone is willing to sacrifice the above 3 LEDs, and have a very sensitive thermocouple probe, they could drill into the dome of the emitters, then measure the temperature at the top of the phosphor and report back what they found. It might be an interesting experiment.

Re: What really causes (White) LED\'s to fail?

LED crystal emitters, phosphors, and conductor junctions all have operating temperature limits. If sufficient heat is not removed from an LED, then degradation leading to complete failure will occur. Excess heat can destroy the chemical integrity of solid state devices. Therefore, adequacy of heat sinking becomes a major consideration.

Heat is a major factor in defining reliability in most electronic devices.

Re: What really causes (White) LED\'s to fail?

As I figured...

I have some WWII radium buttons that don't glow at all but read 80+ mr/hr! Plenty of "juice" but the scintillant is completely shot.

I guess way past its effective point of expiration, a white LS would act more like a blue bulb with a cap on it letting out some light.

Heat wouldn't really affect the phosphor but certainly can damage the junction. Most well designed products do take this into consideration.

Cheers!

Re: What really causes (White) LED\'s to fail?

[ QUOTE ]
Especially the 5W Luxies...


500 hr is such a short life even if it's the half life.

So are you saying the effective life of most 5 watt Luxeons is only 500 hours?

[/ QUOTE ]


Please explain. (I'm a newbie and know little)

I have 3 watt luxeons, are they also short life span?

I thought the 1 watt luxeons are usually rated at least 10,000 hours?

If 5 watt luxeons are indeed that fragile, I won't be buying any.

Re: What really causes (White) LED\'s to fail?

gremlins!

Re: What really causes (White) LED\'s to fail?

500 hours is really not that bad for a flashlight. Incandescent flashlight bulbs normally last less than 20-30 hours. A 5 watt flashlight like a Surefire L4 costs $120 and uses two $1.25 CR123 cells that last about an hour. So after 500 hours, you've used $1250 worth of batteries, ten times what the light itself cost. Even if you have to throw away the light after 500 hours, it's not so bad. Normally though, you can get the light serviced, maybe even under warranty.

The 500 hour issue matters more for fixed lighting than flashlights, I'd say. Also, it's possible that it only applied to the early 5 watt leds, and that newer ones last longer, as reflected in the spec of the new 3 watt units.

Re: What really causes (White) LED\'s to fail?

Just a thought, but could the phosphor be "insulating" the surface of the die at all, increasing its temperature slightly - but significantly? Or could the phosphor be creating any heat at all as a result of the absorbtion/emission process?

I'm no expert in this field, but it's a thought.

Re: What really causes (White) LED\'s to fail?

I get the impression from this thread that no one here has driven the 5W unit past its half-life, else we'd know by now what happens.

I find that rather hard to believe, given what else has died in the hands of CPF's best. /ubbthreads/images/graemlins/huh.gif

Re: What really causes (White) LED\'s to fail?

Yeah, it would seem that no one has achieved 500hrs yet. Probably too many toys. /ubbthreads/images/graemlins/buttrock.gif

But let's see... I'm planning to replace my KL4 emitter with a side-emitting version sometime in the near future (to get better focus from the reflector), and I have a good bench top power supply... 500hrs is just under 21 days or 3 weeks of continuous running. Could be done.

Anyone care to sponsor say half the cost of a new SE LuxV and loan me some monitoring equipment? /ubbthreads/images/graemlins/naughty.gif Or I could extract the LED and send it on to someone who is already set up.

What would we be measuring anyway? Initial brightness level and subsequent fall off, power consumption through out its life, chromatic shift, spectrum distribution?

Run it sitting on a big, fan-cooled heatsink and monitor the junction temperature while measuring all the above.

Re: What really causes (White) LED\'s to fail?

357: The 5W are no more fragile than any of the other Luxeon LEDs. We suspect that it is because of the heat generated by the 5W of power being dissipated that is causing the reduced lifespan. The heat probably damages the phosphor and the diode junction. Just like a Pentium 4 chip would cook itself to death if not for the fan-cooled heatsinks.

5W LuxV are rated for 500hrs
Lux III are rated for 20,000hrs
1W LS are rated for 100,000hrs

Seems like a clear case of heat damage?

Anyway, try looking at the LumiLEDs website for the data sheets to get a clearer picture of their lifespans and the fair bit of literature about designing for the heat.

Re: What really causes (White) LED\'s to fail?

fwiw i have heard of luxeons shifting towards blue as they age.i still have some arc rejects from the auction so i will try running one hot and see what happens.

Re: What really causes (White) LED\'s to fail?

[ QUOTE ]
Especially the 5W Luxies...


500 hr is such a short life even if it's the half life.
...
Cheers!

[/ QUOTE ]

Don't forget - it doesn't "burn out" at 500hr's. This is the point at which they predict light output will be down by 50% when operated at 105 deg C (from an early data sheet). Reality is that if you had a 5w in a well heat sinked design, only ran it when it was cool enough to hold the light in your hand, and didn't overdrive it - you'd probably get a lot more than 500hrs till half brightness.

Looks like they learned from the 5W and designed the 3W to avoid some of the degradation issues. The market they cater to is lighting, so you can bet they are working on some powerful white LED emitters to beat flourescent technology in brightness AND life (>20K hours).

Re: What really causes (White) LED\'s to fail?

The white LEDs from all suppliers are either blue or UV activated, with Phos to emit secondary colors. In the case of Luxeon 5 watt monochromatic LEDs, the life is pretty long, so the main degredation is primarily a combination of Phos and encapsulant degredation. More Lumens = more photon flux = faster chemical reactions = shorter life.

In the case of the white LEDs from the secondary market and most (not all) of the far east suppliers, there is an even more serious limit - The LEDs themselves have fairly short lifetimes, even monochromatic versions. This is just a basic difference between an LED from premium suppliers (like Cree and Lumileds) and the suppliers with limited engineering staff.

You will be able to easily see a factor of 5 x difference in lifetime between parts from a premium LED supplier and a commodity LED supplier.

I agree in general with the comment about the effect of battery cost - other than real commercial users and CPF testers, most people have never put even 10 sets of batteries in a flashlight before either it broke or they lost it.