Hey all,
My question is focused on the design of long series-parallel strings of LEDs driven by a constant current, and whether active current equalization with current sources like the NSVC2050 or current mirrors are necessary when all LEDs are thermally coupled, and the strings are very long such that the statistical variation of forward voltage between each string of LEDs is about the same.
I'm developing a round light engine PCB using 4 parallel strings of 28 LEDs as seen below for a total of 112 LEDs
figure 1: Four series-parallel strings of LEDs, each drawing 60mA of current which are each regulated by the AL3066
My first design had each of the parallel strings of LEDs being current-matched and regulated through an IC called the AL3066 https://www.diodes.com/assets/Datasheets/AL3066.pdf, an amazing PWM controllable constant current step-up LED driver. An outright beast of its kind.
figure 2: Application circuit for the AL3066 to show the full schematic
In my first version, I ordered a 2-sided FR4 PCB (about 5.5" in diameter) with LEDs on one side and the LED Driver circuit on the other, and hit temperatures around 70-80°C with LEDs facing downwards (measured using a non-contact thermometer 6" from the surface) and at full brightness (about 22 watts and nearly 3000 lumens). In either case I decided to run the lights at a lower duty cycle/power so the light engine hit a more reasonable temperature of around 65°C
In any case I decided to work on a V2 to improve the thermals. The LED Driver is now on a separate PCB and wires will be connected to the light engine, which will be a single-sided MCPCB instead of an FR4 PCB (aluminum instead of fiberglass). Rather than running 5 wires between the boards (Vout and 4 channels), I want to simplify the connections so that only 2 wires are required. This means I have to tie the 4 channels together and can't use the AL3066's current matching feature to balance current....But is it still necessary?
figure 3: Four series-parallel strings of LEDs, driven by a constant current of 240mA
I'm trying to determine if I need to design a current mirror or use CCRs on the light engine to balance currents. The LEDs are thermally coupled on the FR4 board from my V1 and will be even more coupled with the very thermally conductive MCPCB, and like mentioned above, 28 LEDs brings the Vf variation between LEDs to a fairly consistent level across the strings.
An example of a product that uses the same approach would be Forge's light engines https://www.forge.co.uk/products/light-engines/200mm-round-light-engine-select where in their datasheet (page 4) they use fewer series-parallel LEDs but probably much stricter binning.
I've been trying to think of the best way to do this for a while now and I've also tested running a smaller set of 21 LEDs/string in series-parallel without equalization, resulting in no thermal runaway, no nonuniform lighting, or any LED failures or color shifts. So I feel fairly confident that I have a good design here but could use a sanity check from someone.
All that being said, I appreciate all for reading this post and giving it some thought in an effort to help and/or learn.
My question is focused on the design of long series-parallel strings of LEDs driven by a constant current, and whether active current equalization with current sources like the NSVC2050 or current mirrors are necessary when all LEDs are thermally coupled, and the strings are very long such that the statistical variation of forward voltage between each string of LEDs is about the same.
I'm developing a round light engine PCB using 4 parallel strings of 28 LEDs as seen below for a total of 112 LEDs
figure 1: Four series-parallel strings of LEDs, each drawing 60mA of current which are each regulated by the AL3066
My first design had each of the parallel strings of LEDs being current-matched and regulated through an IC called the AL3066 https://www.diodes.com/assets/Datasheets/AL3066.pdf, an amazing PWM controllable constant current step-up LED driver. An outright beast of its kind.
figure 2: Application circuit for the AL3066 to show the full schematic
In my first version, I ordered a 2-sided FR4 PCB (about 5.5" in diameter) with LEDs on one side and the LED Driver circuit on the other, and hit temperatures around 70-80°C with LEDs facing downwards (measured using a non-contact thermometer 6" from the surface) and at full brightness (about 22 watts and nearly 3000 lumens). In either case I decided to run the lights at a lower duty cycle/power so the light engine hit a more reasonable temperature of around 65°C
In any case I decided to work on a V2 to improve the thermals. The LED Driver is now on a separate PCB and wires will be connected to the light engine, which will be a single-sided MCPCB instead of an FR4 PCB (aluminum instead of fiberglass). Rather than running 5 wires between the boards (Vout and 4 channels), I want to simplify the connections so that only 2 wires are required. This means I have to tie the 4 channels together and can't use the AL3066's current matching feature to balance current....But is it still necessary?
figure 3: Four series-parallel strings of LEDs, driven by a constant current of 240mA
I'm trying to determine if I need to design a current mirror or use CCRs on the light engine to balance currents. The LEDs are thermally coupled on the FR4 board from my V1 and will be even more coupled with the very thermally conductive MCPCB, and like mentioned above, 28 LEDs brings the Vf variation between LEDs to a fairly consistent level across the strings.
An example of a product that uses the same approach would be Forge's light engines https://www.forge.co.uk/products/light-engines/200mm-round-light-engine-select where in their datasheet (page 4) they use fewer series-parallel LEDs but probably much stricter binning.
I've been trying to think of the best way to do this for a while now and I've also tested running a smaller set of 21 LEDs/string in series-parallel without equalization, resulting in no thermal runaway, no nonuniform lighting, or any LED failures or color shifts. So I feel fairly confident that I have a good design here but could use a sanity check from someone.
All that being said, I appreciate all for reading this post and giving it some thought in an effort to help and/or learn.