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film grade controller for rgbww leds

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vns:
Hi,

I want to design/implement a controller for rgbww leds to use in film/photography illumination.

It has to be flicker and banding free, and also it cannot emit any sound in the audible range, which means that leds need to be controlled with pwm above 20khz, my plan is to use 25khz.

All leds will be emitting the same color.

rgbww leds integrate 5 different leds, red, green, blue, and two white leds with different color temperatures, to create different colors and intensities I need 5 pwm pins with different duty cycles running at 25khz, I need enough precision to have a good color spectrum coverage while I am able to dimm intensity, I am not sure how many bits I need, but probably no less than 12, maybe 16.

I will use wifi to control the color/intensity.

These are the leds I will be using: https://a.aliexpress.com/_BO6lwY

the question is, how do I get 5 pwm pins at high frequencies with enough resolution?

Mi initial idea was to use a eps12 or esp8266 module with a pca9685 or pca9635,  but the first one doesn’t provide enough frequency and the second one lacks the precision.

PWM dithering to increase resolution could be an option, but I don’t know what hardware I can use...

This is something I am doing as a hobbyist, so my resources are limited. There are out there illumination products providing what I want, but I really want to learn how to do this.

Thanks in advance.

TheUnnamedNewbie:
I would look at external PWM controller ICs. You either feed these with an analog reference voltage which you can generate with any of the many multi-channel DACs out there, or some you can control directly digitally. Those often go well into the hundreds of kHz switching frequency. If you need more current capacity, just add a good switch after it that can keep up. Some of the controllers might even have a FET driver output stage as many of the PWM controller ICs are aimed at DC/DC power supplies.

Siwastaja:
Why do you want to use PWM dimming?

For illumination, you would be typically using proper constant-current LED driver anyway. You would be then use PWM to toggle the enable pin of said converter; and they have requirements for the pulse width, limiting the maximum PWM frequency.

Instead, adjust the current limit; many have current limit adjustment pins, or a register if it's digitally controlled. The downside is, constant current drivers typically do not go all the way to zero. Additional PWM dimming is then required if you need very low intensities. The flicker is still much better because if you want to output say 5%, having a 50% PWM of 10% current is way less flickery than having a 5% PWM of 100% current.

I don't think PCA9685 or PCA9635 are suitable at all. For photography illumination, you would likely use high-power LEDs, and drive them with (current, not voltage)regulated switch mode supplies, not with linear supplies. Then adjusting the current limit becomes the obvious way to control light output.

TheUnnamedNewbie:

--- Quote from: Siwastaja on June 22, 2020, 12:31:16 pm ---Why do you want to use PWM dimming?


--- End quote ---

When you want very high CRI and consistent color, you usually need to go for PWM because of the non-linear relationship between input current and output spectrum. (note that there is also a difference with respect to temperature, but that should be similar for both constant-current and PWM if the average power is the same). You need to drive them at the same current to get the same light out.

I seem to recall that the differences can be quite significant.

Syntax Error:
So how many colors is your video light going to produce? Can your RGBWW leds produce a color gamut that will render colors correctly in a camera? RGBW leds are often quite crude in their chromacity, because they're not made for use in accurate color reproduction.

Most professional video lights are grids of color matched cold white and warm white leds, which give adjustable color temperatures from 6500K sunlight to 2400K tungsten.

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