I wonder what it is that limits how much current can flow through a power LED. If the heat removal is managed properly how large can the current get? Why 45×45 mil size LED chips are rated 3W (700mA) but smaller ones 350mA?
I've got an LED module, it has 9 45x45mil LED chips (It is RGB), I want the maximum light intensity possible, what is the maximum safe current? I don't care if the life is shortened to half or one third compared to if it were drived at 350ma (the manufacturer rated current).
What limits the maximum allowable current of a power LED
led
Related Solutions
There's a bunch of LED driver ICs, for instance TLC5951DAP. See it in use here: Mbled
Similar problems has been solved in a few open source projects, like Peggy2. The chip Peggy2 is using seems to be going out of production, MBI5026 might be a replacement.
If somewhat more than minimum cost is acceptable you could implement a buck converter per LED. These could be very crude and low component count and not a vast price. Each converter would be able to set a current that was proportional to a PWM or analog input. Probably under $1 per channel.
Your 3 x Power supplies plus resistors idea is acceptable BUT can be improved in performance (with increased cost) by making it 3 x power supplies plus constant current sources. The constant current sources are not much different from the buck converters above but with no inductor.
Example. Imagine that your lowest Vf LED (probably red) has a Vf range of 2.0 to 2.5V.
You will need at least 2.5V supply plus headroom. Say 2.7 V.
Minimum efficincy will be when Vf = 2V = 2/2.7 = 74% BUT this is worst case Vf and in many cases the Vf range is lower.
eg say VF = 2.2 - 2.4. Vsupply = 2.6. Zmin = 2.2/2.6 = 85%.
Boost supply to 2.8V for more headroom. Zmin = 2.2/2.8 = 78%
Again, this is worst case, so acceptable.
A buck converter will improve on the above.
One solution of zillions. Rsc not needed. Rather than sensing output voltage, LED current is sensed. This C had Vref = 1.25V = far too hight. It can be reduced to 0.6V effective using a diode but still too high. More modern ICs has a lower Vref OR a 1/4 IC section can be used per channel. PWM per channel then becomes 1 x SOIC MC34063 + 1/4 LM324 + a few resistors and a catch diode per channel. May be cheaper than more modern IC.
This circuit, from sound.westhost.com may be suitable.
I independently arrived at a similar version - works reasonably well.
This is voltage feedback. Can be made current feedback. Vsense here = Vbe of Q1. This can be reducued with care.
Best Answer
One of the things that determines the LED lifetime is the junction temperature (the temperature of the LED die inside the package). Then it's a matter of how fast you can remove heat from that, given the thermal resistance between the junction and the pad where the heatsink attaches (upon which you have no control, and which may be lower for larger LEDs).
Some manufacturers have published data on temperature and current vs. lifetime which shows that indeed if you accept some reduction in lifetime and have proper heatsinking you can drive their LEDs at 2x the rated current.
See for instance the OSRAM Dragon reliability information. Note how the lifetime drops smoothly from 100,000 to 20,000 hours with increasing current/temperature and then suddenly to 0 hours when you exceed the maximal temperature (fig. 5). Note also how this depends on the LED color (fig. 5 vs fig.7), with blue/green/white LEDs much more fragile than yellow/red LEDs.