As much as the term "realistic simulation" is an oxymoron /ubbthreads/images/graemlins/tongue.gif, I've attempted to model the output of various luxeons.
Lumileds provides output ratings for luxeons, but based on the quite unrealistic situation with a junction temperature of 25C. Unless you plan on immersing your luxeon in ice water, or running it at a 1% duty cycle, the junction temperature will be higher than 25C. Thus, the output you can expect will be much different than the data sheet specifies, depending on your application.
Lumileds also provides output degridation data for various junction temperatures. This is one point that has prevented me from posting this sooner is a conflict in information that I've come across, between lumileds datasheets, and data given by lumileds representatives in technical presentations.
The degridation curves from the datasheets show a linear decrease for white/blue/green luxeons (1, 3, 5 W) starting at 0% at 25C, and decreasing to 70% output at 100C. The curves for red/orange/amber show 0% at 25C, and decreasing to 30% at 100C (exponential).
Now, a presentation I've run across, dated November 2003, (available at http://www.netl.doe.gov/ssl/PDFs/Krames.pdf ) that shows the white/blue/green output at 90%+ at 100C, and 70% for red at 100C, 38% for amber at 100C. This could reflect an improvment in the high temperature performance of luxeons, or could reflect best, in-lab results.
As a result, and the extremely limiting nature of the datasheet numbers (modeling based on the data sheet numbers gives performance that has easily been exceeded by CPF members - numbers that the model says can never be achieved have been by several modders), I've decided to bump up the numbers from the datasheet degridation curves slightly, but not upto the numbers in the presentation.
So, with the output degridation model, samples of Vf curves, and a curve of theoretical output vs. input current, I've constructed some curves that show the relative expected output at different current levels, based on different Junction-Ambient thermal paths. All curves modeled with an ambient temperature of 25C.
Remember, the thermal resistances each curve represents is the junction-ambient thermal resistance. To find the thermal resistance of the heat sinking source (head sink, light body, hand, etc.) you must subtract the junction-slug thermal resistance of the emitter. For example, the 1W has a 25C/W curve modeled. To find the thermal resistance of the heat sink, subtract the 15C/W junction-slug resistance of the emitter, to get 10C/W. Thus, if you wanted your emitter to behave like the 25C/W curve, your heatsink must have a maximum thermal resistance to ambient of 10C/W. The lowest resistance curves are provided as an example of the best-possible performance - that is, if the emitter were bonded directly to an "infinite" heat sink.
The "relative to rated output" scale means that the output has been normalized to the number of lumens. Thus, if a luxeon is rated to output 35 lumens at rated current, then 1.0 on the scale = 35 lumens...1.2=42 lumens, etc. This lets you see relative gains from various levels of overdrive.
Rated current used:
1W White - 0.35A
1W red - 0.35A
3W white - 0.7A
5W whote - 0.7A
First up, the venerable 1W white:
The 15C/W curve represents the best possible thermal path from the emitter to ambient. The vertical red lines represent the current at which the junction temperature is exceeded for the curve that it stops at.
This shows the junction temperature. The thick red line is the maximum junction temperature allowed. The vertical line traces down to the current axis, so you can see the current at which the junction temperature for that thermal path is exceeded.
Next, a 1W Red:
The 18C/W curve represents the best possible thermal path from the emitter to ambient. Note that I only modeled to 1A for the red, since I don't have any Vf data beyond 1A (translation - I haven't intentioanlly destroyed one yet /ubbthreads/images/graemlins/tongue.gif )
Next, 3W white:
The 13C/W curve represents the best possible thermal path from the emitter to ambient.
You'll notice that the 3W essentially behaves the same as the 1W, except for the best-case curve, because of the L3's improved junction-slug thermal path. Most of the 1/3W curves are modeled with the same junct-ambient thermal resistance.
Finally, the 5W:
Best thermal path is 8C/W.
The one interesting thing to note about the whites is that the better thermal path, the higher the output will be, the more current can be pumped in before exceeding the maximum junction temperature, and the peak output happens at higher currents (none of those observations should be surprising).
The red curve is interesting, mostly because of the exponential dropoff in output vs. temperature. The peak for red/amber/r-o luxeons is near 500-600mA for most conditions.
Okay, so that was a lot of information. I'm not sure if I presented this in the best way possible. I would really like to present what we can realistically expect as far as real-world performance of luxeons at various current levels. One thing that comes to mind right away is to model the thermal paths as having equal heat sink devices attached. So instead of all being modeled at 30C/W, instead being modeled at R(j-s)+15C/W. Sound good?
What do you think?
Lumileds provides output ratings for luxeons, but based on the quite unrealistic situation with a junction temperature of 25C. Unless you plan on immersing your luxeon in ice water, or running it at a 1% duty cycle, the junction temperature will be higher than 25C. Thus, the output you can expect will be much different than the data sheet specifies, depending on your application.
Lumileds also provides output degridation data for various junction temperatures. This is one point that has prevented me from posting this sooner is a conflict in information that I've come across, between lumileds datasheets, and data given by lumileds representatives in technical presentations.
The degridation curves from the datasheets show a linear decrease for white/blue/green luxeons (1, 3, 5 W) starting at 0% at 25C, and decreasing to 70% output at 100C. The curves for red/orange/amber show 0% at 25C, and decreasing to 30% at 100C (exponential).
Now, a presentation I've run across, dated November 2003, (available at http://www.netl.doe.gov/ssl/PDFs/Krames.pdf ) that shows the white/blue/green output at 90%+ at 100C, and 70% for red at 100C, 38% for amber at 100C. This could reflect an improvment in the high temperature performance of luxeons, or could reflect best, in-lab results.
As a result, and the extremely limiting nature of the datasheet numbers (modeling based on the data sheet numbers gives performance that has easily been exceeded by CPF members - numbers that the model says can never be achieved have been by several modders), I've decided to bump up the numbers from the datasheet degridation curves slightly, but not upto the numbers in the presentation.
So, with the output degridation model, samples of Vf curves, and a curve of theoretical output vs. input current, I've constructed some curves that show the relative expected output at different current levels, based on different Junction-Ambient thermal paths. All curves modeled with an ambient temperature of 25C.
Remember, the thermal resistances each curve represents is the junction-ambient thermal resistance. To find the thermal resistance of the heat sinking source (head sink, light body, hand, etc.) you must subtract the junction-slug thermal resistance of the emitter. For example, the 1W has a 25C/W curve modeled. To find the thermal resistance of the heat sink, subtract the 15C/W junction-slug resistance of the emitter, to get 10C/W. Thus, if you wanted your emitter to behave like the 25C/W curve, your heatsink must have a maximum thermal resistance to ambient of 10C/W. The lowest resistance curves are provided as an example of the best-possible performance - that is, if the emitter were bonded directly to an "infinite" heat sink.
The "relative to rated output" scale means that the output has been normalized to the number of lumens. Thus, if a luxeon is rated to output 35 lumens at rated current, then 1.0 on the scale = 35 lumens...1.2=42 lumens, etc. This lets you see relative gains from various levels of overdrive.
Rated current used:
1W White - 0.35A
1W red - 0.35A
3W white - 0.7A
5W whote - 0.7A
First up, the venerable 1W white:
The 15C/W curve represents the best possible thermal path from the emitter to ambient. The vertical red lines represent the current at which the junction temperature is exceeded for the curve that it stops at.
This shows the junction temperature. The thick red line is the maximum junction temperature allowed. The vertical line traces down to the current axis, so you can see the current at which the junction temperature for that thermal path is exceeded.
Next, a 1W Red:
The 18C/W curve represents the best possible thermal path from the emitter to ambient. Note that I only modeled to 1A for the red, since I don't have any Vf data beyond 1A (translation - I haven't intentioanlly destroyed one yet /ubbthreads/images/graemlins/tongue.gif )
Next, 3W white:
The 13C/W curve represents the best possible thermal path from the emitter to ambient.
You'll notice that the 3W essentially behaves the same as the 1W, except for the best-case curve, because of the L3's improved junction-slug thermal path. Most of the 1/3W curves are modeled with the same junct-ambient thermal resistance.
Finally, the 5W:
Best thermal path is 8C/W.
The one interesting thing to note about the whites is that the better thermal path, the higher the output will be, the more current can be pumped in before exceeding the maximum junction temperature, and the peak output happens at higher currents (none of those observations should be surprising).
The red curve is interesting, mostly because of the exponential dropoff in output vs. temperature. The peak for red/amber/r-o luxeons is near 500-600mA for most conditions.
Okay, so that was a lot of information. I'm not sure if I presented this in the best way possible. I would really like to present what we can realistically expect as far as real-world performance of luxeons at various current levels. One thing that comes to mind right away is to model the thermal paths as having equal heat sink devices attached. So instead of all being modeled at 30C/W, instead being modeled at R(j-s)+15C/W. Sound good?
What do you think?