LED driver and PWM dimmer selection

I'm building a remote phosphor light for videography. I want to make sure there won't be noticeable flicker at high frame rates and shutter speeds (up to 120fps at 180 shutter) if I need to dim. It'll consist of 120 cree xt-e royal blue leds and remote phosphor panels.

My knowledge of LEDs is somewhat novice. I want to go with PWM dimming to make sure the LED's wavelength doesn't change as much as it would using a resistor potentiometer. The chromaticity and CCT of the light being produced is a important factor to me. My main concern is flickering. Although I understand the fundamentals of how LED's flicker I don't completely understand which component is responsible, the driver or the PWM dimmer or both?

I understand a PWM dimmer at a lower frequency will cause visible flicker even up to 500hz in video at high shutter speeds but what I don't understand is, I would think any decent dc constant current driver by Meanwell would be flicker free and the PWM dimmer be the source of flicker yet I see drivers from Meanwell advertised as flicker free or flicker free design like their IDLC series while their HLG-C series which would be better suited for my application because of the power isn't advertised as flicker free.

I will probably need to power the 120 LED's by two or more drivers. Meanwell's HLG-C series look good for my power requirements, I just want to make sure they are flicker free with or without the PWM dimmer.

The driver causes the flicker. Even though many drivers have "PWM" dimming capability, most just convert it to an analog level and use that to control the output current. They do not actually PWM the output.

You can get video effects with true PWM on the output at frequencies much higher than 500Hz.

There is color shift w.r.t. LED current, but the shift may not be noticeable depending on the current range adjustment. Are you manually or automatically white balancing. If you are automatically balancing, that is going to have a much bigger impact. Note that temperature also shifts white point.

Have not been into professional video in ages, so not sure what the present gen-lock capabilities are, but if you locked the PWM to the frame rate, then flicker would be less of an issue.

I'm no expert, but as I understand it the problem with PWM and digital photography is that unlike a film camera, the digital camera does not read all the pixels at the same time, but rather scans them over a period of time. And unlike incandescent bulbs, LEDs really do turn on and off with PWM, even if it's really fast. If they don't, then what you have is analog dimming. So as any given pixel is being scanned, the LED is either on or off. If you scan some while it's on and some while it's off, you get lines, bars, or other effects. Locking the PWM frequency to the frame rate would stabilize the effect, not eliminate it.

The camera's detector does have a response time, and this varies from camera to camera, and possibly even with settings on the same camera. The only way to get effective PWM dimming in digital photography is to have the PWM period substantially shorter than the response time of the camera. That might require quite a high PWM frequency.

Thank you both for your answers. Btw, I will be manually white balancing

What part of a led driver makes it flicker free like Meanwell's IDLC series? I would choose one of those but their power output isn't high enough. Even if I were to use several.

Also, is it possible to use one PWM or resistor potentiometer dimmer to dim 3 or 4 led drivers?

Thanks

Well, there's flicker-free, and then there's flicker-free. Most people can't see flicker above 100 Hz or so, but some are affected by it up to 200 Hz, possibly even higher in rare cases. So if you run it at 500 Hz or higher, humans can't see it, except by sweeping vision across it. But digital devices certainly can still see it. It may well be possible to run fast enough to blind at least some digital devices to it.

If you filter the output to create DC, then even the digital devices won't see flicker, because there won't be any. But this is effectively the same as analog dimming, with its associated color changes.

Flicker is definitely the result of PWM, but flicker-free may mean analog, filtered PWM, or high frequency PWM. I suspect most (maybe all) of the drivers in the category you are talking about are filtered PWM. PWM signals on wires create electrical noise, and that makes it hard to get through certifications, and pure analog is too inefficient.

In most cases you can use one PWM dimmer for several drivers. Probably a potentiometer too, but I would want to check the specs before saying that.

Thanks DIWdiver. Much appreciated!

Steveggz said:

What part of a led driver makes it flicker free like Meanwell's IDLC series?

Click to expand...

- Part of it is marketing, part may be reality

- Almost all higher powered drivers, say 75W and up, have always been "flicker free". However, as flicker has only been recently raised as an issue, they are now taking advantage of it from a marketing standpoint.

- Most so called flicker free drivers use two power conversion stages. The first stage provides a somewhat stable voltage and the second stage provides a stable "flicker free" current. This is a opposed to a single power conversion stage to constant current. Some flicker free designs may use a single stage, but use lots of capacitance and/or an analog filter stage.

If you are manually balancing I would not get that hung up on PWM dimming for perfect color unless you are dimming over a very wide range. Not many high powered drivers PWM dim the output. I can't think of any off the top of my head but I am sure they exist. Most are not PWM and just change the current.

Thanks for your answer ssanasisredna.

I decided to switch from linear dimming to PWM using a power supply and Mean Well LDD-1500H buck drivers. I'll be using a Coralux PWM controller to control the buck drivers at frequencies in the kilohertz range which won't be visible and output constant color even at lower light levels. The LDD-1500H outputs 1500mA. Will that pose a problem since the LED's use 1500mA max? And if I wanted to reduce the current to around 1490mA (I need as much light output as possible without damaging the LED's) would using a current limiting resistor in series with each led string mess with the slew rate or pulse frequency of the buck drivers? I don't want the slew rate or pulse frequency to be effected in any way.

The LED life is just reduced at high current assuming it is well heat sinked but note that temperature affects color too.

Maximum PWM for this driver is 1KHz. You can probably go faster but who knows how much.