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There are many cases of LED lights failing by beginning to flicker intermittently. I am not talking about the intentional duty-cycle modulation utilized in many driver circuits. I am referring to visually evident strobing on the order of 10 Hz. This failure mode is common to flashlights, desklamps, and ceiling lamps. What is it about the electronic circuits contained within commercial LED lamps which causes this to be a common failure?

Is this something fundamental to the way white LEDs behave when they overheat? Do they intermittently go open-circuit?

Is this due to the action of a safety circuit in the driver?

It is not due to a failure in the switching supply, because it is often observed in battery-powered flashlights.

It is not due to a failure in the dimming circuit, because it occurs in lamps not powered through a dimmer.

It appears that there is some common oscillatory or astable failure mode in which component values on the "hairy edge" (especially capacitors?) combined with:

increased resistance in connections due to corrosion; -AND/OR-

increases in current drawn by LEDs as their temperature rises

shift the effective value of components used in the circuits, leading to this flickering failure mode often being induced in the lighting circuit.

Will someone "in the know" please cast some light (so to speak) on the apparently common shortcoming in the electronic design of commercial LED driver circuits which is the root cause of this ubiquitous problem?

Thank you very much.

godot
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  • Schematics for the safety circuit you mention would help. – Brian Carlton Mar 03 '17 at 22:38
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    This isn't about LEDs but about the power supply included in the LED bulb. Only. When your LED bulbs have this problem, the components inside the power supply are of the cheapo kind. Most likely the capacitors losing capacity too early due to heat, which makes the supply only function in an intermittend way. – Janka Mar 03 '17 at 22:47
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    Theres no way to tell, its dependent on the driver circuit and if the LED's are in series\parallel. LED's pretty much do what the voltage you feed into them does, if you see an LED flickering it is most likely due to its voltage source. – Voltage Spike Mar 03 '17 at 22:52
  • Without providing further clarification or a specific example, this question is unanswerable as it stands. – Voltage Spike Mar 03 '17 at 22:52
  • You may consider it as an instability of a mismatched source that has poor regulation under low load conditions. Other than some triac dimmer activated AC LEDs at lowest settings and motor-boating of insufficiently loaded SMPS , I've never see or heard of this LED flicker problem in my usage. Often regulators are fast rise and slow decay causing this motor-boating effect with noise with poor gain margin. – Tony Stewart EE75 Mar 03 '17 at 23:25
  • Not an answer. Just commentary: It turns out due to a few *laws* about human vision (you can read about them here [1]) you are best able to see flicker at around \$10\:\textrm{Hz}\$. Also, I've never considered LEDs particularly stable due to experiences you can read about here [2]. But I can't say these apply to your observations. [1]: http://electronics.stackexchange.com/questions/267674/pwm-frequency-for-led-dimming/267741#267741 [2]: http://electronics.stackexchange.com/questions/102751/highly-accurate-constant-current-source/102756#102756 – jonk Mar 03 '17 at 23:28
  • As I am sure you all know, schematics are often hard to obtain for commercial products. I am unaware what circuitry (if any) is hidden in the "head" of the failing flashlights on the tiny PCB upon which the LED arrays are mounted. I have reverse engineered the ckt for the driver circuitry "downstream" of the secondary of the transformer contained in a "wallwart" driver for a desklamp which suffers this malady. The failure seems to correlate with LEDs being HOT. But, according to datasheets, LEDs DRAW MORE CURRENT when they are hot. "Unstable" doesn't explain time-constant oscillation. – godot Mar 04 '17 at 20:40
  • Tony: Search on LED flashlight flicker, and you find find many people claiming to repair the oscillation through cleaning contacts. Why would increased contact resistance cause LEDs to oscillate in a simple circuit with a battery? I assert that something fundamentally surprising is going on here. (Can some situation cause LEDs to intermittently go repeatedly open-circuit? I doubt it.) – godot Mar 04 '17 at 20:51
  • Characteristic I-V characteristics dictate that, contrary to your guess that the driver circuit might fail when "insufficiently loaded," observation shows that this flicker failure occurs when the LEDs are HOT and drawing more current: I.e. the driver supply is more heavily loaded. While there are R's and C's in this circuit, (and even some L in the form of a small xformer), it is not at all clear why this circuit would go unstable when the load INCREASES. In the desklamp case, the chopper is entirely upstream of the transformer primary, w/ only rectified filtered DC at output. – EE since '76 – godot Mar 04 '17 at 20:56
  • Most likley crappy driver but in some rare occasions it's the bonding inside the LED capsule which looses contact when it heats up -> cools down -> regain contact -> heats up -> looses contact an so on. At least for lower frequencies. – winny Mar 09 '17 at 16:04
  • It would be interesting if you could describe precisely the LED lightbulb that was flickering at 10-18 Hz. Was it mains powered or using a 12V supply, etc. – bobflux Mar 09 '17 at 16:23
  • You stated "This failure mode is common to flashlights...". The LED flashlights I own have no solid state devices, only batteries and a resistor. Now if **THOSE** LEDs flickered and the batteries were good, **only then would I blame the LED.** – Mike Waters Mar 09 '17 at 16:34
  • @MikeWaters: More likely the mechanical connection for the batteries. – Oskar Skog Mar 09 '17 at 17:16
  • @OskarSkog The OP stated "visually evident strobing on the order of 10 Hz". That is not likely to be caused by a bad mechanical connection. – Mike Waters Mar 09 '17 at 17:29
  • I used to work for a company who specialized in LED lighting and while we *did* see flickering that *was* do to LED failure, it was on the order of several seconds to minutes not tens of Hz. Repeated thermal cycling (15mins on, 15mins off looping for months) eventually induced some kind of thermal failure in the LEDs themselves, you would see some of the individual dies drop out and come back as the unit heated up or if you gave it a good flick (note these were all 5+ watt multi-chip modules). We'd see this effect in something like 20% of the LEDs after 10-20k cycles (peak temps ~80'C). – Sam Mar 09 '17 at 22:51
  • Peufeu, et al.: The flashlights I know for sure exhibit this failure were from a special sale at Home Depot in downstate Illinois. They were from bubble-pack collections of ~4 lights for ~$4. They were ubiquitous there. All my friends had them, etc. I bought 10 of them: 4 headband-mount, and 6 hand-held. I found some of the packaging: E.g.: No brand. Made in China. UPC: 8 20909 94037 1. "Distributed by Home Depot USA...Atlanta GA." All lamps consist of small LEDs in the typical 5mm hemispherical-domed package. (2) have 1 LED; (4) have 3 LEDs; (1) has 6; (3) have 9. There is a small PCB in the – godot Mar 11 '17 at 04:39
  • (cont.) head of the flashlight to hold the array of LEDs, but there are no components evident on the PCB in addition to the LEDs. Once they were removed from the packaging, it was not obvious where the others in the area came from, but, in retrospect, it is possible they all had a common origin. When I saw apparently the same failure mode in a friend's NEW desklamp, consisting of a wall-wart and 6 apparently Cree ~1W LEDs at end of cord in lamp, I surmised there must be a common cause, and determined to identify it. Again: Other types of lamps do not oscillate when they fail. Why do LEDs? – godot Mar 11 '17 at 04:53

2 Answers2

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The problem is not in the LED. The problem is the LED is misunderstood.

Almost everything associated with an LED is thermal related.

I do not know where you gathered your claims on the various devices but I doubt what you describe is the result of a single phenomena. Nor do the issues manifest in an identical fashion.

I can assure you it has nothing to do with the LED and everything to do with the LED driver and a design flaw.

If an LED device does not have a dimming feature it is still likely the LED driver will be dimming the LEDs to regulate the current.

The most likely causes would be over voltage and under voltage protection built in the driver chip or a code error when the LED is controlled by a micro-controller.

LED present a load characteristic that is much different than digital devices and other loads that require constant voltage.

Forward voltage characteristic is not just a function of forward current. Looking at the datasheet the prominent I-V curve is shown with voltage as if it is an independent quantity. The designer looks at this curve to get the voltage that matches the desired current to be used.

The forward voltage is not just current dependent but temperature and varies with the fabrication process from one LED to the next even coming off the manufacturing line one after another.

When LEDs are strung in series the forward voltages vary unpredictably. It is not unusual for a sting's forward voltage to vary outside the intended design parameters. Increasing the number of LEDs in a string will increase the potential difference between Vmin and Vmax.

A constant current source must operate over a wide range of output voltage and maintain the current. The min max rage is used to select the regulator topology, the IC, and the passive components.

When designing the regulator with typical values, the input voltage will have it's variance maybe 5%, headroom is designed to be tight, and the output voltage a buck regulator will lose regulation with a small increase in forward voltage. A buck regulator designed for the typical Vo will be unable to control the current when V out max exceeds minimum input voltage.

Dynamic resistance is used to select the output capacitance for the desired ripple. In a voltage regulator the load resistance is simply Vo/Io. With LEDs load resistance is replaced with dynamic resistance. Using typical values for Vf and If will lead to incorrect results that are 5 to 10 higher than the true Ro value. The dynamic resistance is derived from the datasheet's I-V curve. The I-V curve is only one typical instance.

UPDATE

I have some new information. First I will address your comments.

WRT battery-life, buck regulation would appear to be a poor choice for a flashlight.

A flashlight is likely to use a buck regulator. Battery voltage rarely will match forward voltage. A buck regulator is the most efficient, lower cost, less real estate versus a boost driver. A linear regulator would be too inefficient.

Isn't the forward voltage of an LED, as with all solid-state diodes, determined by the band-gap of the junction materials? How can this vary substantially from one device to the next?

Variance in the forward voltage is a characteristic of the manufacturing process. The manufacturers will bin LEDs by forward voltage.

The following is a quote from LumenHub

The coating processes (epitaxial growth and phosphors) create significant inherent variations that impact the lumens, color temperature and voltage of the LEDs. Even with all of the R&D efforts underway and the billions of dollars spent within the semiconductor industry to minimize this production variation, the end result is a process that is not capable of producing highly consistent and tightly controlled production of LEDs. So, in an effort to maximize yields (and with a knowledge that the lighting industry has a wide range of needs), LED manufacturers sort their production into lumen, color and sometimes voltage bins.

You also say, "When LEDs are strung in series the forward voltages vary unpredictably." I think you may be confusing series with parallel.

When LED are strung in series the individual forward voltages are additive. I make strips of 16 LEDs and the forward voltage of each strip varies significantly. One strip may get mostly LEDs that operate below typical while another above typical.

It is the above typical that is the bigger problem with a poorly designed driver with insufficient overhead.

Forward voltage is a function of manufacturing process, forward current, and temperature. Each one of these factors has its own intrinsic variations from one LED to another. These variations will alter the I-V curve significantly. Yet an engineer will use the one typical I-V curve with one temperature (25°C), test current (350mA), and the typical forward voltage for their design parameters.

The I-V curve in the datasheet is typical and there is nothing typical about an LED. That datasheet I-V curve is in a different ball park than real life.

Many of the characteristics in the LED datasheet are spec'd at 25°C. No LED Light Bulb is going to operate at 25°C.

New

There is flicker associated with all lighting. This flicker is in the 100-200 Hz range from poorly designed and PWM dimming LED drivers. Although there is some talk about saccadic eye movement where the eye can see a harmonic frequency of the flicker from PWM regulation.

Output capacitance and inductance affect the PWM switching and ripple. Many drivers do not use an inductor and just turn the LED of an on. It is like the blind leading the blind out there. Somebody comes up with a bad idea, posts it on the Internet, and other follow.

It is possible you are more susceptible to seeing flicker. There are some neurological factors in the way flicker is perceived.

The following is an article about flicker, saccadic eye movement, and poor LED designs at the cause. How to design an LED circuit to not have flicker.

Designing to Mitigate the Effects of Flicker in LED Lighting

UPDATE II

So you want SUPPORTING EVIDENCE?

Why did the original post say about 10Hz and later you say 15-18Hz was measured?

Supporting evidence is not there in the form that flicker is caused by under voltage lock out in the buck converter. It is much more vague.

Example this article Buck Regulators Make Driving High Brightness LEDs Easy says "You can overvoltage the switch, you can overvoltage the current-sense pin, and basically have anything from a little bit of LED flicker to a destroyed device."

Example: The best IEEE study on flicker A Review of the Literature on Light Flicker:... could do was citing another study: Rand , D. , Lehman, B. , and Shteynberg, A. (2007) Issues, Models and 30 Solutions for Triac Modulated Phase Dimming of LED Lamps , Proc. IEEE 31 Power Electronics Specialists Conference

The Rand study looks only at the AC/DC rectification stage points to a problem in the DC/DC with almost nothing said about what actually causes the flicker. On page 1402 they just say "Thus, simple buck derived, low cost, systems are primarily utilized after the rectifications. There
are numerous IC drivers with dimming capabilities on the
market, yet these systems typically have difficulty with phase modulated dimmers."

In the press you will find variations of this: LED Magazine, Understand the lighting flicker frustration and this: LED Journal, Reducing Flicker in LED Lighting

Then there is technical papers that say nothing like this: Flicker Parameters for Reducing Stroboscopic Effects from Solid-state Lighting Systems

Even a Cree White paper is just a rehash of the same, you will see the diagrams in this paper in many of the published articles: Cree, Flicker happens. But does it have to?

The bottom line is, as I first said, it is NOT the LED, it is design flaws in the driver. LEDs do not fail very oven in the first 50,000 hours if not abused by heat. An LED's flux output degrades with use but opens and shorts are very rare. So rare Lumileds white paper makes a case that their Rebel LED will never fail in the first 80,000 hours.

The best I can do is recreate it in the lab (which I have done) as I described and video the flicker.

Of all of the above there is very little talk about flicker in the 10-20hz range. I do not know where you got this "Many have seen..." but that I have to dismiss without supporting evidence. But I know it is real because I can reproduce it in the lab. I see lots of very bad LED designs and I do see low frequency flicker in the Lab.

I do not understand those that ask for citations supporting what I say. I read stuff I remember. I do not catalog the citations of everything I read. I connect the dots. I share that information I have gathered over the past 40 years and all I get is "personal conjecture and guesses and no supporting evidence" Personal conjecture, some, but not Guesses! There is nothing I said above that is a guess, it is all based on real facts.

UPDATE III

I have a 15 second video. For the first 10 seconds I am increasing the amps from a Meanwell HLG-60-54B Constant Current supply from 700mA to 1.25A.

It is driving two parallel strings of 16 Cree XPE Red and Blue LEDs.

Each string is connected to a LM3466 linear current regulator. The regulators ensure the forward current between the two strings remains the same.

The Red string has a forward voltage just under 30 volts and the blue just over 44 volts.

At about 1A the red triggers the under voltage and 3 seconds later is locked out and remains off. At 700mA the two boards will run all day.

Video is 720p (1280x720) in 2 formats, webm 4MB and mp4 20MB

LINK to HTML: 15 second MP4 video of flicker HTML5 Video Player webm and mp4
LINK to 10MB mp4 file: 15 second MP4 video of flicker, mp4
LINK to 4MB webm file:15 second MP4 video of flicker, webm

Misunderstood
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  • WRT battery-life, buck regulation would appear to be a poor choice for a flashlight. In the yrs I spent as a lighting engineer, w/ the exception of MECHANICAL bimetallic-strip fluorescent starters, I never saw any other form of lamp – incandescent, arc, or discharge – fail by entering into visible oscillation. Maybe you are onto something WRT dynamic resistance of LEDs. Since LEDs increase current and power output with increased temperature, this indicates their dynamic resistance in the operating region is opposite of all other types of lighting. Perhaps this leads to the oscillatory behavior – godot Mar 04 '17 at 21:12
  • You say, "The forward voltage...varies with the fabrication process from one LED to the next..." Isn't the forward voltage of an LED, as with all solid-state diodes, determined by the band-gap of the junction materials? How can this vary substantially from one device to the next? – godot Mar 04 '17 at 21:43
  • You also say, "When LEDs are strung in series the forward voltages vary unpredictably." I think you may be confusing series with parallel. In series, each junction is independent of the performance of its neighbors; this is why many lamps employ series LED configurations – even at the cost of providing a DC supply for an unusual voltage. In parallel, slight variations in the devices causes current-hogging, which, in turn, leads to an unpredictable power dissipation from individual devices. Because LEDs INCREASE the current they draw with temperature, this can easily lead to thermal runaway. – godot Mar 04 '17 at 21:56
  • It is NOT that I am sensitive to flicker. Many other people see exactly the same thing. I have measured the frequency of oscillation with a frequency meter as being from 15 to 18 Hz when the lamp enters this flickering mode of failure. Due to lack of support evidence, I must dismiss much of the rest of what you say as consisting only of your own personal conjecture and guesses. WHAT COMPRISES THE LOW FREQUENCY OSCILLATING CIRCUIT??? – godot Mar 08 '17 at 00:30
  • RE: personal conjecture and guesses. I find that insulting. I did not realize you required supporting evidence. See my Update II. RE: "Many other people see.." not really, otherwise it would be reported and talked about. The only flicker being talked about is the 100-200hz PWM dimming and triac dimming in the range you say. The reason I know is because when designing and evaluating circuits, I see it in the lab. I do not see it outside the lab. – Misunderstood Mar 09 '17 at 16:05
  • Very interesting answer. I will add that some people also hate higher frequency PWM flicker (ie, in the kHz range): people who use video or still cameras with rolling shutters. This kind of lighting will create all sorts of weird effects on the recorded media. Also, for my bicycle, I hate PWM on the headlight. Even at hundreds of hertz, it produces a weird and disorienting stroboscopic effect on fast-moving objects. – bobflux Mar 09 '17 at 16:26
  • Hundreds of hz is very common and many have complained about that and there is much talk about it. Quite often the PWM switching frequency is in the 100-200hz range and that is the problem. But the 10-20hz flicker is a poor design, typically the under voltage lock which can be triggered by many characteristics of the driver design. – Misunderstood Mar 09 '17 at 16:44
  • I had a 4 watt incandescent (FOUR WATTS) fail on me, couple weeks ago, with oscillation for 15 seconds right before it failed. – analogsystemsrf Mar 09 '17 at 17:43
  • @analogsystemsrf sounds like a power supply failure where the input voltage dropped triggering the under voltage protection. Name brand? – Misunderstood Mar 09 '17 at 18:35
  • @godot added video, update III – Misunderstood Mar 09 '17 at 21:14
  • Misunderstood: 1st vid won't play in my browser, 2nd vid gets 404 error, but the 3rd vid is very interesting. This would appear to mimic the oscillatory failure mode. In this case, you are speaking at a much higher level of abstraction, saying it "triggers the undervoltage" (of your LM3466 regulator). The datasheet says, "an internal thermal shutdown circuit activates at 150°C to disable the LM3466." This indicates that it is the LM3466 which overheats, not the LEDs. All the same, it would explain the oscillation. Are such regulators present in flashlights? – godot Mar 10 '17 at 03:25
  • It is in the LED String Disconnect and Reconnect section of the datasheet. The Red string was found to be inactive, after I increased the current 50% slightly changing the forward voltages. The point is the setup was on the fringe. If an engineer does not understand and test properly, a product can ship with a flawed design. An example is this guy http://electronics.stackexchange.com/questions/291119/pwm-noise-led-driver In one of his comments he adjusts PWM and it flickers and is stabbing in the dark for a solution and others are suggesting a band-aid approach rather than design review. – Misunderstood Mar 10 '17 at 14:24
  • I've isolated one LED on a 12 LED panel that flickers when driven with a constant current supply. As far as I can tell, the IV curve appears to be changing, sometimes rapidly. – rsaxvc Jan 16 '21 at 14:50
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Thermal TimeConstant of 3mm copper wire is approximately 0.1 second.

Thus a LED about to permanently detach from its "flag or paddle" in the package, thus heating and cooling and being ON then OFF, needs to dump heat to the PCB traces.

A cubic meter of copper has 9,000 second thermal tau.

A cubic decimeter (0.1meter) has 90 second thermal tau.

A cubic cm has 0.9 second thermal tau.

A 3mm cube has approximately 0.1 second thermal tau.

Silicon thermal tau, at 11,400 second per cubic meter, is quite close to CU tau.

analogsystemsrf
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  • ASRF, you say, "a LED about to permanently detach from its "flag or paddle" in the package, thus heating and cooling and being ON then OFF..." Are you implying in this statement that you are familiar with the design of typical LED dice and chip carriers, and that their configuration is such that overheating can cause some of them to temporarily "detach from," and then "reattach to" the conductive carrier substrate, thus causing a temporary open circuit? And, furthermore, that this occurs with power dissipation just slightly less than would cause them to permanently detach (i.e. burn out)? Tnx. – godot Mar 04 '17 at 21:26
  • I considered the possibility of thermal overload causing a flicker. If the LED would trigger a thermal shutdown and the recovery were fast enough, possible, not likely. When the LED is turned off there is too much of a delay, greater than 100mS (10hz), for the LED temperature to drop and the driver to respond. The thermistor is mounted on the circuit board and the heatsink will hold the temp. The probability of a wire bonding open or Catastrophic Failure is very low. Probability of catastrophic in 80,000 hours of the Rebel LEDs I use, is near zero. – Misunderstood Mar 06 '17 at 15:20
  • I said nothing about a thermistor. There is no evidence that there is a thermistor present. – godot Mar 08 '17 at 00:20
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    I asked if you were really familiar enough with the fabrication process of LEDs to know whether there was a "flag or paddle" which separates from the conductive substrate when the temperature of the LED die exceeds some threshold. You provide no additional information to support your statement, so I assume it is only a guess. Rather than knowledge, it appears you are providing us with hypothetical guesses as to what you, with questionable experience, think may cause oscillation. – godot Mar 08 '17 at 00:23
  • LEDs need a path to dump heat. Standard silicon ICs also need a path to dump heat. There is differential thermal expansion between the silicon and the flag/paddle. Eventually that will stress and separate, over enough decades of cycling. – analogsystemsrf Mar 09 '17 at 17:41
  • Just to say that a decAmeter is 10m, while a decImeter is 0.1m. – Sredni Vashtar Mar 09 '17 at 21:44