Are LEDs in modern streetlights usually pulsed? If so, roughly what frequency?

Question

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.

Answer 1

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).

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.

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.

Answer 2

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.

Answer 3

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.

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.

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:

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:

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.

Answer 4

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).