Efficiency: PWM vs CC

Hey folks,

I had a pretty specific question to ask:
Does the efficiency of LED's depend on instantaneous current or average current (heat or something)?
I understand they work better at lower currents.

If it depends on instantaneous current, I think I should strap a capacitor across the LED to keep the current flowing through the LED and to keep it more efficient.
If it's average current i'll have an advantage if I just use straight PWM as persistance of vision should make it appear brighter.

Is this true?

Just some background info - I'm trying to use 4 P4's to light my room, i'm using these as regulators with an arduino for the PWM.

Thanks and any input would be much appreciated.

charithjperera said:

I had a pretty specific question to ask:
Does the efficiency of LED's depend on instantaneous current or average current (heat or something)?

Click to expand...

That's two questions, and the answer is yes

Generally speaking there are two effects - a drop of efficiency with increasing temperature (and hence average power) and 'droop' - the drop of efficiency with increasing instantaneous current even for a perfectly heatsinked LED (or an LED only being briefly pulsed, with no chance of heating up).
Both effects happen, but for a given thermal setup and output brightness, it's probably only droop that significantly affects things.

At decreasing instantaneous currents, there's a point at which colour output can start to shift, and manufacturers often recommend using PWM for low output levels, rather than CC even if it might be slightly less efficient.

charithjperera said:

If it's average current i'll have an advantage if I just use straight PWM as persistance of vision should make it appear brighter.

Click to expand...

I thought that at a high enough frequency to avoid flicker, the eye responds pretty much just to the average brightness?

uk_caver said:

That's two questions, and the answer is yes

Generally speaking there are two effects - a drop of efficiency with increasing temperature (and hence average power) and 'droop' - the drop of efficiency with increasing instantaneous current even for a perfectly heatsinked LED (or an LED only being briefly pulsed, with no chance of heating up).
Both effects happen, but for a given thermal setup and output brightness, it's probably only droop that significantly affects things.

At decreasing instantaneous currents, there's a point at which colour output can start to shift, and manufacturers often recommend using PWM for low output levels, rather than CC even if it might be slightly less efficient.


I thought that at a high enough frequency to avoid flicker, the eye responds pretty much just to the average brightness?

Click to expand...

uk_caver is right that there are two effects causing decreased efficiency at higher current. However, the relative importance depends on the LED, the drive current, and the heatsinking.

Poor heatsinking will increase the importance of output vs. temperature, making average current more important.

High currents increase the slope of voltage vs. current exponentially, causing rapidly reduced efficiency, thus making peak pulse current more important.

I do recall reading that back in the dark ages (when LED watches were the rage and white LEDs were wishful thinking), it was found that an LED operating at 10X current for 0.1 duty cycle appeared brighter than it's counterpart operating at DC. But applying ancient knowlege to modern technology is risky at best. While the human eye probably hasn't changed, LEDs have, particularly in the area of output vs loss. You may find that where the human eye is more sensitive, light vs. current curves have reduced that effect.

I guess what I'm saying is that there its't a simple answer to your question.

D

Innocent enough question, and as pointed out, not quite so simple to answer.

I guess as a first step, if you are after good efficiency from AC power, then I would use an LED driver designed for the job. There are several AC - LED drivers on the market, some quite reputable. Advacned Transformer comes to mind, but thee are others.

If you first do an AC : DC converter, then use that rather inefficient DX converter, you will be lucky to get 60 - 70% efficiency just in power conversion, never mind LED efficiency.

There are lights built, both with LEDs and Incans, that rely on regulation with just a pure on / off pulse mode of operation. In the case of the incans, the filament tends to stay hot, so the pulse is not noticed. If you cycle the LED fast enough (perhaps 500 KHz or so) then it isn't usually noticed either.

I have seen some rather drawn out threads, with multiple very knowledgeable people contributing various points of view on which was "better" for efficiency. The difference in efficiency between the two approaches is small for a room light. You are more likely to notice tint shifting before you will notice the difference in efficiency, assuming you buy a decent driver.