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I can think of some reasons why modern LED streetlights would likely be pulsed;

  • Efficient voltage conversion starting from line voltage would likely include an AC or switching step higher than 60 Hz.
  • Highest efficiency operation of LEDs often occurs at a current greater than can be sustained continuously due to heating issues.
  • Conversion back to steady DC (edit: at line frequency 50/60Hz) would require additional components that could fail, and would have no benefit that would offset the reduced efficiency operation.

There is a short section in Wikipedia about pulsed LED operation but it just introduces the concept without addressing how widespread pulsed operation is in the field.

As long as the frequency were sufficiently high that there were no chance of flicker perception, it seems to me that LED streetlights would be pulsed - or at least the blue LEDs used to excite the phosphor. The phosphor could have a long enough half-life to make most of the spectrum of the resulting emitted light steady even if the LEDs were pulsed.

Because some white light LEDs rely much more on LED's primary blue light than others, I'm going to ask my question primarily about the LEDs themselves, rather than the emitted light.

Are LEDs in modern streetlights usually pulsed? If so, roughly what frequency? 100 Hz, 1 kHz, 10 kHz? While there could be substantial variation in some regions, I'd expect in regions where cities are implementing widespread conversion from gas (mercury, sodium) to LED, there must be some commonalities or general trends/convergence in design.

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    \$\begingroup\$ Borderline "too broad" unless you give an example of a streetlight because there will obviously be all kinds of solutions, but still an interesting question. \$\endgroup\$ – pipe Jun 14 '17 at 7:37
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    \$\begingroup\$ @pipe I understand the concern, but in this particular case at this time, there is a good chance that standardization or standard practices have set in. How about we give this one a day or two to see if a clear answer emerges? If not I'll edit it. I'm after a good answer here so I'll keep an eye on what happens. In the mean time, if you can suggest or even make a helpful edit, that would be great! Anything short of insta-closing would be appreciated. \$\endgroup\$ – uhoh Jun 14 '17 at 7:39
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    \$\begingroup\$ Roughly between 50Hz and 50kHz. \$\endgroup\$ – PlasmaHH Jun 14 '17 at 7:45
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    \$\begingroup\$ I seem to recal a lot of people from the engineering comunity arguing against LED streetlighting, as it has a number of dissadvantages when compared to cold sodium-vapour (for example, I believe professional astronomors like SV lights more because they can just use a filter to block out the two spectral lines and get rid of a lot of the issues with light pollution. SV lights are also very efficient in terms of percieved light intensity, perhaps even better than LEDs although with the modern increase in efficiency that could no longer be true) \$\endgroup\$ – Joren Vaes Jun 14 '17 at 8:12
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    \$\begingroup\$ Efficacy goes DOWN with increasing current. The Vf vs I and Lm vs I curves are not straight but slightly curvy. PWM is used because it's a fair bit easier to implement a constant voltage source than a variable constant current one. So you set the output voltage to produce the maximum current and pulse that to achieve brightness control. \$\endgroup\$ – Barleyman Jun 14 '17 at 8:52
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You're making a wrong assumption that the efficacy goes up with higher power level. The opposite is true, to any meaningful power levels the efficacy decreases whenever you increase the current.

PWM is used because it's very easy to implement. If you set your current to the maximum you want to use, you can have linear brightness control by just adjusting the duty cycle. Adjusting current has nonlinear response requiring calibration table if absolute accuracy is important (it often is not). enter image description here

As you can see from this 1W white led Lumen vs current curve, doubling the current does not double the light output. Whether this is important depends on your application. If you're dealing with a >1kW advertising backlight, the electricity bill easily exceeds the up front cost of the display module. There are also thermal considerations, with better efficacy you have less waste heat on your system.

enter image description here

To make things worse is that efficacy drops even more with higher junction temperature. This graph shows ambient temperature but essentially junction temperature works in similar fashion. They're just being difficult about it. Now PWM will average the heat output but still, worse efficacy requires higher average current which means higher junction temperature..

One downside of a PWM is that the load is nasty from SMPS point of view, you're effectively imposing constant radical transients to the poor thing. At the very least you need a large output capacitor to buffer the voltage dips and peaks at edges.

A problem with constant current driving is that it's more complicated, especially if you want adjustable output current. There are further complications with local dimming applications as Vf varies with output power level so your current regulator has to dissipate the difference.

Edit added bit about junction temperature.

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    \$\begingroup\$ Wow! To double check, is the plot really showing DC current, not PWM average current? And an LED that can handle 400 mA DC continuously is still at least locally linear all the way down to around 5 mA DC (give or take)? i.stack.imgur.com/dSQbw.png This is all really measured, and not just a plot/extrapolation of some parameterization? \$\endgroup\$ – uhoh Jun 14 '17 at 10:15
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    \$\begingroup\$ @uhoh Yes, that is DC. With PWM the active pulse would behave much the same way. \$\endgroup\$ – Barleyman Jun 14 '17 at 10:35
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    \$\begingroup\$ There's also a second order effect, light output goes down with junction temperature. So PWM with higher current will run with worse efficacy to start with. It will also run hotter for the same light output that makes things worse. So it's definitely better to use constant current from system efficiency point of view. \$\endgroup\$ – Barleyman Jun 14 '17 at 10:44
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    \$\begingroup\$ @uhoh The only reasons to use PWM is that if you need very high instaneous light output (optical sensors for example) or you want to control brightness. For lighting there's no reason for PWM if you don't want a dimmer. \$\endgroup\$ – Barleyman Jun 14 '17 at 11:09
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    \$\begingroup\$ But the efficiency that matters is the efficiency of the power supply + LED and power supplies ussually have increased efficiency with higher power. So is the "cascade" of both efficiencies that matters in the end, and that could be a design decision as well. \$\endgroup\$ – Andrés Jun 14 '17 at 11:10
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LEDs used for street lighting usually employ a DC/DC converter of some kind, with tight current control at its output. So, providing a steady current isn't reducing efficiency nor does it add unnecessary components which could fail, nor does it reduce the lifetime of the LEDs.

It's the simplest and most efficient way to drive a high-power LED array. Steady current, provided from a "pulsed" source.

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    \$\begingroup\$ The circuit of most DC/DC converters has a coil on output, which —despite the pulsed nature of its working— automatically produces a more-or-less steady current. That's the very core function of such a DC/DC converter. \$\endgroup\$ – Janka Jun 14 '17 at 8:20
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    \$\begingroup\$ I'll go do some reading and try to find some data. Starting from very low voltage (where there is heat generated and no light) and ramping up, the efficiency starts low, since it starts at zero. There is only non-radiative recombination. At you increase the voltage and the current increases, the ratio of radiative to non-radiative recombination improves, as do other aspects. I had thought that the plateau in efficiency occurred at a point beyond where heat could be removed for continuous operation, and so for the most watts of light per watt of electricity, it was better to add LEDs and pulse. \$\endgroup\$ – uhoh Jun 14 '17 at 8:32
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    \$\begingroup\$ Efficiency at low voltages is low because you cannot have light emmission if you don't jump over the band gap directly. But as soon you do, there is no sense in increasing the voltage any more — the overshoot energy is simply turned into heat. \$\endgroup\$ – Janka Jun 14 '17 at 8:40
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    \$\begingroup\$ @uhoh, the current at which the efficiency stops increasing is quite low. Efficiency is then almost flat before it falls off at least partially due to slef-heat (including heating from contact resistances). I went to a conference on this and other III-nitride matters a few years back: It was true then and it's more true now, that once you've got useful amounts of light coming out, driving them harder makes these LEDs less efficient. \$\endgroup\$ – Chris H Jun 14 '17 at 12:27
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    \$\begingroup\$ @ChrisH one person's then is not necessarily the same as another person's then. My then is farther back than most people's then. I did some further reading in the last few hours and I can see that the more recent stuff looks just as you mention, a really broad plateau followed by a very gradual drop at much higher currents. There was a lot of work done over the decades; better crystal growth, then sapphire substrates, substrate lift-off too, then better MBE technology, plus better heat flow and solid-state modeling, etc. They don't give out Nobel Prizes for nothing you know. \$\endgroup\$ – uhoh Jun 14 '17 at 12:33
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To summarize: They wouldn't do it because it's not efficient, wouldn't keep the lights in safe spec, and is not viable as a way to control a large group of lights (owing to the distance and lack of versatility).

In street lighting, the name of the game is efficiency.

LEDs are inherently a difficult customer, since at their high-efficiency ranges, they are too non-linear** and most be driven by a constant-current power supply.

Operative word: "constant".

Since they already must drive it with a constant-current supply, if they also wanted to do PWM, that would add unnecessary complexity. And there's a much better way to dim LEDs using the constant-current supply already present. Here, look at this datasheet on Page 11. Forward voltage vs forward current. Note this graph is very distorted, for normalized, look at my endnotes.

enter image description here

If you're driving the LED at 3000ma and want to dim it, cut current to 1000ma, and voila. Of course it doesn't quite drop by 2/3, look at "flux vs current", same page.

enter image description here

At 1/3 the current, luminous flux drops from 235% to 95% of spec. It's much more efficient at the lower current. Voltage drops too, which nibbles a bit of the efficiency difference, but not by a whole lot.

Would someone deliberately use more emitters to improve efficiency? Absolutely. Many commercial and industrial customers are looking at total life-cycle cost, and emitters are a small part of that. If $100 more emitters saves $300 in electricity over the life of the fixture, it may be a smart move. I had a guy who specced three LEDs at redline max 1400ma. It gave the needed light. However heat was the key issue. I respecced using the datasheet "normal" current of 350ma and seven emitters. Got the same light at half the heat.

Now that I've positively shown lower power is more efficient for LEDs, you can see where PWMing them is not efficient. Running 3000ma at 33% PWM is worse than running 1000ma continuous.

Why would anyone PWM then?

In a perfect world, all dimming would be via something like the 0-10 volt signal widely used commercially, and each LED module would use the "adjust the output of the constant-current supply for perfect dimming" method. However.. that does not work everywhere. Fact is... PWM is an efficient way to propagate a dimming signal.

Consider the lowly "LED strip". A narrow strip of PCB, every 50mm (2") it has a CUT line, three LEDs and a resistor. Or for an RGB strip, three RGB LEDs and three resistors. And with RGB, of course, they want to dim each channel individually. How do we get three dimming signals down to hundreds of little segments? Cost makes it impossible to put adjustable-output constant-current power supplies on every 50mm segment. The only workable dimming method is PWM.

It gets better. PWM is both the power and the signal. If the PWM controller can only drive 3 amps, and you want to run seven 6A strips, you can use an amplifier: it receives the controller's output as a signal and uses it to gate its high-current outputs, tapping out PWM in lock-step. The versatility is hard to beat.

And this works for any of a huge variety of LED lighting (whose purpose is notably, not efficiency.) Nobody really cares about the lumens per watt here:

enter image description here src

Why not street lights, then?

It's not entirely unreasonable to dim LED street lights. They could ease on at dusk, burn in excess of legal requirements to 11pm, then roll back in the haunting hours when hardly anyone is out. But they wouldn't use PWM. The signal won't propagate well over an installation the size of a town.

An LED street light takes high voltage (240-277V or even 480V which they tap off the nearest power line without metering, that means that PWMing the power line is right out)***. Internally, a street light has a sensible number of large emitters - ideal for series connection to a high voltage constant-current supply. This would be best dimmed by current adjustment. They would either use radio - or if they were hardwiring an expensive signal wire, they would use it for a lot more stuff than dimming. They might work with the power company to power-line-encode a data signal similar to how power companies can remotely shut off smart meters. Adding $20 a unit for the transceiver is not a "deal breaker" on a $1000 street light.




** Incandescents are linear once lit, so sending 120V to them will reliably produce 60W. Discharge lighting (fluorescent, neon, low/high pressure sodium, mercury vapor and metal halide) is totally non-linear: once struck, they are a dead short and must be current-limited by a ballast/driver. In the case of LEDs, their voltage-current curve is quite steep, You remember the Voltage vs Current chart from This data sheet page 11. Look again: The scale is distorted, and volts don't start at zero. If corrected, the graph would look like this:

enter image description here

That's what you call non-linear. Remember, this line moves a little depending on temperature, age, binning, etc. and when the line is that steep, a little is a lot. Send 3.05V and who knows what'll happen! The manufacturer only guarantees what'll happen if you send 2500ma. Every other chart in the datasheet is based on current, for that reason.

*** The power company and the city agree how much power a normal street light draws, and the power company simply multiplies by the number of lights, and bills them.

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  • \$\begingroup\$ This is an interesting perspective, thanks! You might consider adding a tl;dr at the top. It is a yes/no question and I think you have a definite conclusion on that, why not add a "yes" or a "no" somewhere in the beginning of the answer as well? \$\endgroup\$ – uhoh Jun 14 '17 at 18:06
  • \$\begingroup\$ 2" is 50.8mm to pick some nits, You memorize some common imperial values fairly quickly.. Chinese companies (a metric country) always reply to me in mils and inches when I specify everything in metric as a random observation. WRT PWM being complex to implement, not really. A humble MOSFET in series with the LEDs will get the job done. Put it on the negative side and you don't have to deal with high voltages either. WRT $20 extra expense, you underestimate how far beancounting does. Extra $2 expense on a product costing something like $5k gets frowned upon. WRT remote control, GSM would do it. \$\endgroup\$ – Barleyman Jun 15 '17 at 9:00
  • \$\begingroup\$ I'd implement PWM control by transmitting desired current value OTA and using locally some 8/16-bit cheap microcontroller to produce PWM if I wanted to use PWM control to start with. See my answer with similar content. \$\endgroup\$ – Barleyman Jun 15 '17 at 9:01
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In general, there are two methods of dimming LEDs, PWM dimming and Amplitude dimming. What you refer to as DC dimming is amplitude dimming. In professional lighting applications, PWM is no longer used for dimming , mainly due to health concerns over the generated flicker. With street lighting, another issue is the stroboscopic effect. You will find today that virtually all professional LED drivers including street lights use amplitude dimming. You can read more about flicker and dimming here.

Update: In response to some of the comments, I would like to extend my answer. By professional lighting applications, I am referring to constant current dimmable >20W LED drivers such as these, not cheap and nasty halogen or bulb replacements or computer backlight applications.

There are two causes of flicker, one is caused by the mains ripple propagating to the output. Cheap single-stage LED drivers such as those used in bulb replacements suffer from this phenomenon.

The second type of flicker is caused by PWM dimming. This may be perceptable or imperceptable. The IEEE PAR1789 is a recommendation of how high the PWM frequency needs to be for it to be considered imperceptable. That said, you will find in industry that high-quality LED drivers for professional applications almost exclusively use amplitude dimming (DC dimming).

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  • \$\begingroup\$ PWM is most definitely used in in professional display backlight applications. Constant current is an exception. Flicker is usually a non-issue once you have high enough frequency. 90 to 360Hz is typical range. \$\endgroup\$ – Barleyman Jun 14 '17 at 9:46
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    \$\begingroup\$ @mr js good article about flicker .I hate cheap nasty flickering leds . \$\endgroup\$ – Autistic Jun 14 '17 at 10:18
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    \$\begingroup\$ @mr_js: The article you link to is almost entirely about flicker due to the mains supply (which is at a rather low frequency, usually 50-60 Hz, typically giving rise to flicker at 100-120 Hz). Professional lighting applications do use PWM for dimming, but will typically use a much higher frequency (tens of kHz). \$\endgroup\$ – psmears Jun 14 '17 at 10:58
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    \$\begingroup\$ PWM is broadly used. It just isn't used much anymore at mains frequencies (100-120Hz effective) partly because switching supplies are cheaper than copper windings these days. Unfortunately, General Motors didn't get the memo, and GM car taillights are simply the brake lights PWM'd to a "dimmer" light level, and in a visible range. In fact they are as bright as brake lights when they're on, and as your eyes sweep-scan the road, they leave tracks across your corneas. Maddening! \$\endgroup\$ – Harper Jun 14 '17 at 15:47

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