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Hi there,

I'm currently working on a project where I'd like to slowly fade the brightness of a series of high brightness RGBW LEDs.

Basically I'd like to drive these 4W (1W per channel) current-driven LEDs:

Controlling high intensity RGBW LEDs with PWM Screenshot 2022-04-20 130420 - EletriciansForums.net

SmartArray Q4 LED Module, 4W, RGBW, 4000K, CRI 90 | the leading LED-shop by LUMITRONIX - https://www.lumitronix.com/en_gb/smartarray-q4-led-module-4w-rgbw-4000k-cri-90-32454.html

What surprised me was that they only require 3V, and then I learned that with these types of high intensity LEDs, the voltage is not important, but the (consistency of the) current is.

I looked a long time for a suitable LED driver, and ended up buying one of these, which can take in 5-36V and provide a stable current.

Controlling high intensity RGBW LEDs with PWM colibri-driver-for-power-led-rgbw-7305-colibri-500x500 - EletriciansForums.net

Colibrì - Driver for power led RGBW - https://store.open-electronics.org/colibri

Which also came with a long article explaining the usage of high intensity LEDs

COLIBRĂŚ: the driver for RGBW LEDs - Open Electronics - https://www.open-electronics.org/colibri-driver-for-rgbw-leds/

Normally people use LED drivers in combination with a whole series of them. Though in my case, I want to drive 54 of them, in pairs of 2. (ie, 27 "unique" LEDs with each 4 channels (RGBW)).

The test setup I have works well:


My main questions is, does anyone have experience with powering these types of LEDs? And are there alternatives to using the Colibri? I was surprised that there seems to be hardly anything out there. Especially something which can support more channels (there's a lot of single-channel LED drivers out there).

Controlling high intensity RGBW LEDs with PWM Screenshot 2022-04-20 130005 - EletriciansForums.net

Above you can see my hardware setup, it seems crazy to need an LED driver per LED.

If anyone wonders why its important to have constant current, its to prevent the LEDs from flickering. Using an LED strip for my project is also not an option, each LED needs to sit at the end of a 2 meter cable and light up an acrylic rod.

Any help or suggestions would be greatly appreciated!
 
If you want 27 unique controllable light then you need 27 controllers. However, you might find you can get boards that control more than one unit in an addressable manner, so fewer PCBs.

LEDs product light proportional to the current (although the eye is not linear in perception), and the resulting voltage changes a bit with temperature and LED construction. So if you want N x LEDs same brightness the simplest solution is to connect them all in series and then control the drive current. However, each LED takes 1.2-2.5V or so (depending on colour) so if you have a significant number then the overall voltage can become quite high.

If you want to adjust the brightness then you need to control the drive current. You can do that either by adjusting the constant current value, or by switching them on/off fast enough the eye does not see the flickering and then the mark:space ratio allows brightness control (the principle of Pulse Width Modulation = class D amplification, etc).
 
So you're saying I might be able to for instance (in the case of lets say per 10 LEDs or so) use a regular constant current source, and then control the brightness using a transistor on the GND side of each channel with PWM?
 
If anyone wonders why its important to have constant current, its to prevent the LEDs from flickering.

This is not really the reason.

Like any semiconductor junction, LED chips themselves are by nature easiest to drive with a constant current, because they have a very low dynamic resistance. If you attempt to apply a fixed voltage and let the LED determine the current, tiny variations in voltage or temperature will result in huge variations in current and therefore power. What has to be done instead is to externally regulate the current and let the LED determine the voltage that appears across it.

Assuming the power is coming from a fixed voltage PSU, there are two ways to regulate the current - passively using a resistor (using a PSU voltage higher than the LED forward voltage to allow for the drop in the resistor) or actively using a current source. Resistors dissipate the voltage overhead as heat. The greater the voltage overhead the nearer to a true current source it appears, but the more heat dissipation. Switched-mode current sources convert the source voltage to whatever voltage the LED is developing at the set current, achieving higher efficiency / lower heat dissipation at the expense of greater complexity.

You might find you could get perfectly good results using a 5V PSU, resistive ballasting to set the current, and tiny transistors to implement the PWM. Depending on the permissible peak chip current, you might be able to drive them as a matrix and use less transistors than LEDs. With matrix drive the duty-cycle is low so the peak current must be increased to achieve the same mean output. The advantage is mainly that you won't need one output pin per chip. E.g. with a matrix of 4x32 chip series-pairs you need only 4+32 = 36 outputs. 8x16 needs only 24 outputs but then you can't have a duty cycle >1/8 at full brightness and the peak currents might be a problem.

As you mention, this might also be a way to save channels of active constant-current drivers, if you don't want to go with resistors. However it is important to account for the dynamic behaviour of the driver when its load changes. E.g. it might be necessary to enable the incoming column select transistor before disabling the outgoing one, to prevent energy stored in the output inductor of the constant-current circuit being dissipated in the transistors as they switch.
 
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