PWM for constant voltage

[Hobby73]

I understand the advantages of PWM vs analog for variable voltage applications (e.g., dimmers, speed controls), but is there any advantage of PWM when the application uses a constant voltage that does not vary? 

For example, we have outdoor garden lights (LED's) with a fixed brightness that are supplied with a constant voltage.  I thought it was odd that they are PWM. 

[Infraviolet]

Sometimes it is simply easier to do PWM from an avaiable voltage to "fake" a lower voltage than it is to actually provide a constant voltage rail at that reduced voltage. Lower component count involved, and PWMing the full voltage may often be more power efficient than having the circuitry required to step down the full voltage to a lower one.

[langwadt]

Sometimes it is simply easier to do PWM from an avaiable voltage to "fake" a lower voltage than it is to actually provide a constant voltage rail at that reduced voltage. Lower component count involved, and PWMing the full voltage may often be more power efficient than having the circuitry required to step down the full voltage to a lower one.

and it is linear

[radiolistener]

advantage of PWM is that such solution is cheap, disadvantage is that output is noisy and not stable.

For a constant voltage there is no sense to use PWM. It's much better and more reliable to use linear regulator.

The only exception is when you're planning high power load on that line, for example 100-1000 Watt. PWM allows to increase efficiency of DC/DC step-down regulator, but as a side effect its output is very noisy.

[gnuarm]

I understand the advantages of PWM vs analog for variable voltage applications (e.g., dimmers, speed controls), but is there any advantage of PWM when the application uses a constant voltage that does not vary? 

For example, we have outdoor garden lights (LED's) with a fixed brightness that are supplied with a constant voltage.  I thought it was odd that they are PWM.

LEDs are not powered by a constant voltage.  LEDs are current driven.  The brightness is easy to adjust by varying the current.  However... the easiest way to vary the current is with a switching circuit using an inductor and capacitor.  Or, I should say, a switching circuit without the inductor and capacitor. 

If your garden lights have a fixed brightness, why would they use PWM???  Are you talking about a switching regulator perhaps? 

[ArdWar]

advantage of PWM is that such solution is cheap, disadvantage is that output is noisy and not stable.

Another disadvantage is that for most LED operating at max current rating is less efficient than operating it at lower current.

For example below, operating the LED at 1000mA drive current is 20% more efficient than doing 50% PWM of 2500mA drive current. It usually also last longer too.

[gnuarm]

advantage of PWM is that such solution is cheap, disadvantage is that output is noisy and not stable.

Another disadvantage is that for most LED operating at max current rating is less efficient than operating it at lower current.

For example below, operating the LED at 1000mA drive current is 20% more efficient than doing 50% PWM of 2500mA drive current. It usually also last longer too.

In most LED applications, this doesn't matter.  Also, the fact that the LED is being PWM'ed does not mean it has to be used at it's maximum current rating.  If long life is required, the current can be reduced with more LEDs. 

I think automotive tail lights are an example of PWM that I don't care for.  Maybe my eyes are a bit different from many, but I can see the blinking, when I move my head or eyes.  Instead of a smear, I see multiple tail lights as my field of vision moves.  The first time this happened to me, I was entering a high speed road where the merge lane was non-existent.  I had just a moment to look over my shoulder before I would have to hit the brakes hard to get stopped.  As I turned my head back, the taillights of the only car suddenly looked like several cars!  I had to panic stop as I thought I had not seen the other cars! 

Now, it is more an annoyance, but still a bother.  It's all because they are saving the cost of the coil and cap to smooth the current so the LED doesn't blink.  They could also fix it by increasing the frequency. 

[radiolistener]

I think automotive tail lights are an example of PWM that I don't care for.  Maybe my eyes are a bit different from many, but I can see the blinking, when I move my head or eyes. 

High frequency flickering of vehicle stop light is very bad, especially when it uses very bright LEDs. I hate it. Because it it is very unpleasant to look at such lights, their high intensity light leads to blind and constant flickering does not allow the eyes to adapt and distracts from more important details. You have to look away to avoid that unpleasant blinding flicker, which makes them hard to follow.

[Zero999]

advantage of PWM is that such solution is cheap, disadvantage is that output is noisy and not stable.

Another disadvantage is that for most LED operating at max current rating is less efficient than operating it at lower current.

For example below, operating the LED at 1000mA drive current is 20% more efficient than doing 50% PWM of 2500mA drive current. It usually also last longer too.

In most LED applications, this doesn't matter.  Also, the fact that the LED is being PWM'ed does not mean it has to be used at it's maximum current rating.  If long life is required, the current can be reduced with more LEDs. 

It's still generally more efficient to reduce the drive current, than PWM, which is normally done to make the design simpler. There is an exception when PWM is better: fo LED who's colour shifts as the current changes.