Controlling high-power RGB LED COB? 2-part Question - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 17, 2021, 07:04:19 pm
content: Hey all, I bought a high-power RGB LED COB (90W rated) from (https://www.ebay.com/itm/10W-20W-50W-100W-RGB-High-Power-Full-Color-LED-Lamp-Spotlight-COB-LED-DIY/252710808301). I anticipated that the COB would have a common negative rail, allowing for easy control using 3x boost converter, but alas, it is common-positive. Nearly all boost converter's that I've ever used are common negative, so they won't work (I think).Instead, I've thought about using 1x boost converter to boost the voltage to ~36v, modyifing a cheap [12v RGB controller box](https://i.ebayimg.com/images/g/9pUAAOSwGH9gD-Tt/s-l1600.jpg) to support 36v in (which would involve swapping out the output transistors, current limiting resistor, and possibly zener diode), and a few large power resistors to drop the voltage appropriately for the LED's.The controller box is [super simplistic](https://ibb.co/3ND012j), so it shouldn't be hard to modify.Forgot to show in the image; the output transistors are N-channel mosfets (A2SHB). [Datasheet.](https://datasheetspdf.com/pdf-file/1088459/SLSSEMICONDUCTOR/A2SHB/1)How does this plan sound, to control my 90W RGB LED COB?

author: Manul
author_rank: Super Contributor
timestamp: March 17, 2021, 11:49:41 pm
content: That 12v RGB controller box looks like PWM, not a current driver. So it is not the best idea. Also it does not look like it has strong gate drive, so there could be some switching loss, especially if you add bigger mosfets with more gate capacitance.In my opinion, if you are serious about it, you should invest into constant current controller with good ouput filter. Personally I hate all this PWM flickering and high EMI emissions. Flickering light is also not good for health.36V boost converter at 90W output is not a small converter. Current limiting resistors at such power is not very elegant.That are my thoughts.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 18, 2021, 02:48:51 am
content: Quote from: Manul on March 17, 2021, 11:49:41 pmThat is correct, only PWM. The LED strip that it is meant to power has resistors to limit the voltage/power (? maybe both?). Anyway, yeah I remember learning about needing constant-current driving for high-power LED's; completely forgot that this module lacks that.Quote from: Manul on March 17, 2021, 11:49:41 pmWow I had no idea this was a thing! TIL!Quote from: Manul on March 17, 2021, 11:49:41 pmDo you have any recommendations from EBay? I haven't found any that have a common-positive instead of common-negative.

author: Manul
author_rank: Super Contributor
timestamp: March 18, 2021, 03:22:48 am
content: I do not know off head, but you can spot constant current drivers easy, they have inductors on PCB. Not sure about common anode configuration, how common it is.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 18, 2021, 06:15:21 pm
content: Quote from: Manul on March 18, 2021, 03:22:48 amYeah I tried finding common-positive (I have a collection of boost/buck converters) and they all appear to be common-negative. Heck, even tried finding fully-isolated boost/buck converters on EBay, and they don't seem to exist for cheap prices (understandable, because they'd require a transformer).However, what I came across is a fantastic YouTube video explaining how to use a buck converter to create an adjustable negative voltage rail, which *could* work.. I'm not sure if it's only possible on the LM2596, or if it can also be done on something like an XLSemi 4016 (buck IC), or even on a boost converter. The XL4016 has a higher current output, so it would work better for use in a high-power LED. Tried this on a boost converter that I have, and it sadly didn't work, even when feeding in like 8A.Edit: Just tested with a XL4016 module CC module that I have, and feeding in 12v does infact allow me to create a -20V rail. This may work...Edit2: So probing the Vin and Gnd of the IC, at -20v output, the voltage at IC is 32v, so this may/may not work. More researching to do...Edit 3: So this may be the potential final edit. Feeding in 24v will create -8v at the output, however the IC sees 32v across gnd and VIN. Increasing the output voltage to around -12v, the IC sees around 34V, which is nearing its max VIn. Therefore, even though I could use the XL4015 to generate a negative voltage, for my application, where I'll need ~32V to the LED, this won't work. HOWEVER, I stumbled upon the LM2596HV, which supports 60V across VIN and gnd, and some modules come with CC.I'll likely go the route of buying 3x LM2596HV modules (~10 Euro) with CV CC support, and then modifying the modules to create negative voltages (so the positive rails are commoned, but the gnd rails are isolated, allowing for controlling the high power LED).