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hotbeam said:
jtr... Good points. However, you are coming from the school of thought that specs should be observed and that is that. Sure, if we were making lights for the masses, I'd keep it at its max ratings. Definately. Here at CPF, we have no constraints. We push the envelope MUCH harder and faster. No sooner does something new come out, it is pushed and pushed. Sometimes at the detriment of the parts we use. /ubbthreads/images/graemlins/icon3.gif That's just part of the experience. Would you considering running a 5w LS (max rating 700mA) at 1.5A? I do.
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There's nothing wrong with pushing the envelope and a little destructive testing. I hope I didn't give the impression otherwise. While I personally can't afford to destructively test $10 parts on a regular basis, I'm certainly interested in reading about it here, and seeing exactly how far these parts can be pushed.
With regard to absolute maximum ratings, yes, I would observe them also on anything sold commercially to a general audience. I'm quite aware that you exceed those ratings on items you sell, but those who purchase your products are well aware of this, and you do seem to take every reasonable precaution. Again, for my own personal use or a small audience, I might exceed absolute maximums provided those who bought my products knew the ramifications of this.
I personally have a couple of thoughts on exactly how much an LED like the Luxeon star can be overdriven. There are two failure modes-the die can cook from overheating, or you can exceed the limits of the wire bonds even without overheating the die. For the 1-watter I would probably feel fine going to 500 or 600 mA, provided the heatsinking is adequate to keep the LS heatsink at 40°C or less. At 600 mA and with a die to heatsink impedance of 20°C/W this would mean a die temperature of ~85°C. This is certainly within limits and you have some safety margin. The bond wires can withstand 350mA for 50,000 hours(at least). They can certainly take maybe 600 mA for the much shorter lifetime the LED would have at that current level. In something like a flashlight the much shorter lifetime at 600 mA is irrelevant. I'm not so sure how I would feel going much past about 600 mA(maybe 700 mA tops). At that point I think we might run into issues with the bond wires.
The 3-watter is a different animal, and here I think bond wires aren't the issue for any current level because the thermal limits would be reached long before the bond wires were unduly stressed. Drawing an analogy similar to what I did for the 1-watter, I would say we're safe keeping the current level for the 3-watter under 1.5A at least with regard to the bond wire issue. However, we run into heat-related problems long before that. Once again, say we manage to keep the Luxeon III heatsink at 40°C with good external heatsinking. At a die to heatsink impedance of 17°C/W the die is already at 106°C running at the absolute maximum rated current of 1A. If you go to 1.5A(and say 4.2V), the die temp is now 147°C. At 2A and 4.5V(i.e. 9W) you're up to 193°C. The absolute maximum die temperature is 120°C. Beyond that you're literally cooking the die to death in a very short time. This is why I felt so strongly about overdriving a L3 at 2A. I would feel fine driving one at maybe 1.2 or 1.3 A with a really good heat sink, or going to 1.5A(or even a bit more) with active cooling. However, at 2A you need to keep the star's heatsink at -33°C or less not to exceed thermal limits, and I feel the bond wires are being unduly stressed at that current level regardless. Of course, for a second or two just to get a Lux reading I might do it out of curiosity. /ubbthreads/images/graemlins/wink.gif
Regarding the 5-watter, 1.5A is probably a bit much for the bond wires, and certainly well beyond the thermal limits of the package. Even driven to specs, the 5-watter has a very short lifetime(presumably due to heat-related issues). Of course, for a second or two I have no problems driving at 5-watter at that level. It does appear Lumileds solved the thermal problems with the LS3, which can be driven at 1A and ~3.9W yet still have a 20,000 hour lifetime(or 50,000 hours at 700 mA).
One reason I've raised eyebrows at overdriving LEDs is because past a point there just isn't much gain in lumen output. If the lumen gain were nearly linear, I might at least see some reason for it. Nothing wrong with having an LS3 driven at 700 mA normally, and maybe a 1.1 or 1.2A "turbo" mode. Beyond that I don't think there's much more light to be found. Overdriving bulbs, on the other hand, makes more sense provided you're aware of the greatly shortened lifetime. Bulbs actually get much more efficient(and whiter) when they're driven harder, and these are both desireable characteristics.
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Oh, by the way, welcome to the CPF /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif
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Thank you very much. /ubbthreads/images/graemlins/smile.gif I appreciate it. I've been lurking here the past few weeks on and off, and finally a thread came up that I just had to post in(that other thread regarding Moore's Law and LEDs). The topics of most interest to me generally are similar to that thread and this one. While I don't see myself ending up with a huge flashlight collection, I do like to tinker with LEDs from time to time. I already modified two bike lights with some LEDs from ChiWing on eBay. Good LEDs for the price-maybe a bit less efficient than Nichia, but nice even beams and good quality. The one I did with 20 8,000 mcd LEDs gives a really nice light on a dark street, although here in NYC you barely notice the beam on most streets due to the streetlights. I have yet to do anything productive with my 5 1-watt LS's, but those were something I wanted to play with ever since I learned about them. /ubbthreads/images/graemlins/grin.gif