Dimming a constant-current LED power supply with a half-bridge?

Hello.

I have a high-power LED light armature, the light is dimmable from about 150W to 280W, but the 160W is still to bright.
I am using a constant-current LED power supply from Mean Well(which feature an adjustable output current, hence the dimmable light) and I was thinking of simply using a PWM controller and a half-bridge(or simply a high- or low-side switch) to PWM dim the output of that constant-current switching power supply.
But I don't know if it's a good idea to PWM the output of a constant-current switching power supply.

Is that a problem or should the LED power supply be able to manage such a load?

Regards

Hi,

It depends.

Usually a constant current switching supply for LEDs isn´t vary fast.

Now it depends how you switch ON/OFF the LEDs.
You might build it a sseries switch to the LEDs. When the sitch is high impedance...the LED is OFF. But the SMPS tries to maintian the current an thus increases the voltage... up to it´s maximum limit. Now what happens when the switch becomes low impedance again? Is it the SMPS fast enough not to overload the LEDs wuth the - now - high voltage?
A cpacitor is counter productive in this configuration because it causes high current peaks to the LEDs.

The other way is toe short circuit the LEDs. The LEDs go OFF, the current is limited by the SMPS (if fast enough) and the dissipated power should be low.
The the swich becomes high impedance... the SMPS risies the voltage until the LEDs draw the expected current.

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I recommend to use a true LED supply with dimming function.
They are cheap with good availability... and made for LED brightness control.

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Btw:
150W .. 280W is not a good dimming range. It is just 1:2. I recommend to use a dimming range of at least 1:10, better 1:50.

Klaus

I didn't think that I would need to dim lower than 150W, so I chose the driver version featuring an internal potentiometer that can be adjusted from the outside, in hindsight I should have chosen the one which featured a input to which one may connect a 0V - 10V signal, a PWM or a potentiometer to dim the LED's.

The maximum output voltage isn't much more than what the LED's demand, I was actually concerned it might be a little to low but it worked fine.

I will contact Mean Well to ask them about this.

Thanks for your advice.

you can dim a constant current led power supply (or rather the leds) with pwm dimming...but you just have to zero the current demand during pwm off intervals.......you also have to inject a control voltage into the feedback loop (again during pwm off intervals) such that the error amplifier does not rail high during pwm off intervals........the alternative way is to do it like the linear.com led driver chips, where they actually "freeze" the error amplifier output during pwm off intervals.

Dig into the datasheet of the driver.

Klauss covered the basic options well. A shorting switch in parallel with the LED seems like a plausible option to me. Another would be to PWM the input power of the driver though I suspect a driver of this power range can't just start and stop at that type of rate (many linear current sources would handle this fine).

My understanding is that PWM is the preferred method of dimming because color temperature can vary over different currents.

My understanding is that PWM is the preferred method of dimming because color temperature can vary over different currents.

Click to expand...

This is facsinating for me, because for years and years i have looked for a visual demo which prooved that the light and color quality is better with pwm dimming as opposed to analog dimming, and yet i have never found a single demo which prooves this.............we played around with it at dyson.....and showed people leds which were pwm dimmed, then analog dimmed, and asked them which looked nicer......you wouldnt believe it, something like 90% said the analog dimmed ones looked nicer. (they didnt know which was which)