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  1. #1
    *Retired* NewBie's Avatar
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    Default Constant Current vs. PWM dimming Revealed

    This chart is the CIE color chart with the Constant Current dimming and PWM dimming, with the Luxeon bins shown:




    Here is an exploded view of just the data, note the dimming % is listed, % for PWM is the PWM duty cycle, % for Constant Current is the % of the max current:



    In the above chart, take note of the percent where the constant current and PWM dimming split, about 50% for the Constant Current. A person could dim with constant current to 50%, then utilize PWM, to minimize color shift, yet pick up the efficiency increase.



    Here is the efficiency difference:





    Yet that chart only shows half the story, here is the efficiency gain at each step, I was rather surprised here:




    Basicially it all boils down to Current Dimming is alot more efficient than PWM dimming (so you get considerably more run time on down to 0.5% current dimming, but PWM dimming results in less color shift. A combination of the two approaches could sure be interesting indeed.

    FYI, the current level this Luxeon was ran at is 380.1 mA = 100% Also, the Luxeon emitter slug had a hole drilled into the side and a type-K thermalcouple inserted, the slug temperature variation over the whole test was a maximum of 2.3 degrees Celcius (yes, a large aircooled heatsink was utilized for this). The same emitter is utilized for all sets of data.

  2. #2

    Default Re: Constant Current vs. PWM dimming Revealed

    WOW!

  3. #3
    Flashaholic* PeterB's Avatar
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    Default Re: Constant Current vs. PWM dimming Revealed

    Cool! Awesome data!

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    Default Re: Constant Current vs. PWM dimming Revealed

    Thanks fellas.

    I hope to get more info at 1 amp current levels today.

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    Default Re: Constant Current vs. PWM dimming Revealed

    Neat!

    Is the CC color shift perceivable by eye?

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    Flashaholic* modamag's Avatar
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    Default Re: Constant Current vs. PWM dimming Revealed

    Awsome!

    What's circuit size? Will it fit inside Mag AA, C, or D host? [img]/ubbthreads/images/graemlins/naughty.gif[/img]

  7. #7

    Default Re: Constant Current vs. PWM dimming Revealed

    wow, totally missed the thread development.

    plenty of useful info, need to catch up on the reading .

    Never liked PWM, but i guess it has its own application

  8. #8
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    Default Re: Constant Current vs. PWM dimming Revealed

    WOW... a blast from the past!

    certainly one of Newbie's best contribution to cpf and always worth another read.

  9. #9
    *Retired* NewBie's Avatar
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    Default Re: Constant Current vs. PWM dimming Revealed

    Thanks for the comments.

    I had a lot of fun doing it!

    BTW, did you ever try the speaker thing I mentioned before?

  10. #10
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    Default Re: Constant Current vs. PWM dimming Revealed

    I was looking thru some old pictures and realized I never posted this one:

  11. #11
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    Default Re: Constant Current vs. PWM dimming Revealed

    Sorry for my lack of understanding here, but I'm wondering how everyone's distinguishing PWM from Constant Current. Here's my understanding...

    A CC power supply varies the output voltage via a sense resistor to keep the current (the voltage at the sense resistor) the same. The way to vary the output voltage is via a PWM (or PFM) signal that adjusts the on-time (and thus average voltage and average current) at the output. The output is then fed through a filter (cap typically, and the inductor in a buck or boost (or sepic) topology acts as a filter also) so the output is not on or off, but a rippled voltage. That's the only method I know of on how to make a CC PS (by using PWM or PFM). Any dimming would just be a change in the feedback signal, reference voltage, or pulse width (or a combo thereof).

    Unless there's another way to control the reduction of voltage/current, I don't see how PWM and CC is being distinguished. And I also don't see how people could be seeing flicker from PWM/SMPSs unless the output's not being filtered at all (but that's just a sloppy design if there's no output filter, unless space is really so constrained, but even a small cap would reduce the ripple to better than just on and off).

    TIA for the education.
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    Default Re: Constant Current vs. PWM dimming Revealed

    The PWM we're describing does not have anything on the output to smooth the signal - so no cap, inductor, etc. Each pulse is a full current pulse to the LED, followed by some off time where the LED emits no light at all.

    Adding a cap/inductor and some feedback gives you a switching DC-DC converter which gives you constant current on the output.

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    Default Re: Constant Current vs. PWM dimming Revealed

    evan - thanks for the clarification. All PS designs I've ever seen have output filters (not for flashlights tho). I'm surprised filters aren't used for flashlight drivers...the cost of a cap is minimal.
    Streamlight: TL-3 Xenon, Vital Stream Scorpion-FB1, Scorpion LED SSC P4, KeyMate Silver (Nichia CS)
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    Default Re: Constant Current vs. PWM dimming Revealed

    This is a great thread. Thanks for bumping it

    But i think a basic point is missed in all this analysis. Of course, PWM is less efficient than CC dimming talking in radiometric units.

    But it not necessarily mean it is less efficient in the whole visual effect.

    Think you are in front of the flashlight, and at 100% power, you receive and perceive 50 cd. With CC dimming at 50%, you receive and perceive, say for example, 30cd (due to increase efficiency).

    But, if you dim at 50% by PWM and the frecuency is enough high, you receive in average, 25cd (using the corresponding 100% power current in the light pulse). But you still notice 50cd of light, with flickering (depending of the frecuency used). This is why PWM was so widely used in the past, specialy with red leds, wich increase light output in a near linear way with increasing current, so the efficiency loss is only due to higher voltage applied during the pulse.

    So a well designed PWM may be more efficient, in terms of human perception, than CC dimming.

    Hope you understand what i want to mean with my reduced english.
    Last edited by Kinnza; 12-14-2006 at 12:45 PM.

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    Default Re: Constant Current vs. PWM dimming Revealed

    But, if you dim at 50% by PWM and the frecuency is enough high, you receive in average, 25cd (using the corresponding 100% power current in the light pulse). But you still notice 50cd of light, with flickering (depending of the frecuency used).
    No, this is not true. You still percieve 25cd. Above a certian frequency your eyes integrate the total output into a non flickering beam equal in brightness to the average of the pulses and off times. Everyone keeps saying that your eye see the maximum brightness of the pulses, but it is simply not true.

    Think about this - if at lower duty cycles, PWM still appeared as bright as a single pulse, just how would PWM achieve dimming at all? Ponder that question and you will realize that your eyes average everything out.
    Last edited by evan9162; 12-14-2006 at 01:50 PM.

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    Default Re: Constant Current vs. PWM dimming Revealed

    Good reasoning, evan9162, you convinced me.

    As you said, everybody says it, and its wrong. I read it so many times i believed it without questioning.

    Its possible its one of the false things with, due to repetition, sounds true.

    Or maybe it depends of the frecuency used?

    (im going to research in this topic a bit more, as im not sure human eye averages out all, in my understanding, eye response isnt always linear)
    Last edited by Kinnza; 12-14-2006 at 03:06 PM.

  17. #17
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    Default Re: Constant Current vs. PWM dimming Revealed

    The effect of CC dimming is even more dramatic if you go on down towards 3% dimming, where you get 240% more light, if the LED I used was driven at 1140mA (data from that example on page 1).

    So, if we assume you had 50 candelas at 1140mA
    -PWM would give you 1.5 candelas
    -CC would give you 3.6 candelas

    For the same power consumed.

    Or another way to do it is to dim both down to 1.5 candelas, and consume 58% less power.


    Another piece of the puzzle is the effect on battery efficiencies when you take a full blown current pulse out of the battery (PWM), vs. sipping ever so lightly on the cell. The battery will deliver quite a bit more energy if you just pull a light current off of it, instead of yanking a full 1140mA pulse out of it.


    Now, a typical switcher starts getting less efficient down at those levels. Most chips these days have burst modes/pulse skipping, which will greatly increase the efficiencies, and if you have a proper sized capacitor, you can keep the ripple down at 0.8%. Another technique is to use two phase switchers, and just switch off one phase. If you think a little deeper, you can put heavy MOSFETs (higher gate charge, lower on resistance) on one phase, for the higher currents, and put light MOSFETs (low gate charge, higher on resistance), and keep the efficiency of the switcher up. You can also lower the frequency, to reduce losses due to gate charge and gate drivers at light loads. Often you will save additional energy on the very light load side, if you replace the one of the MOSFETs (depends on buck or boost), with a schottky diode.

    As you start going up in PWM frequencies, you start taking additional losses due to the 700-800pf of capacitance in the LED. Adding just a capacitor on the output of the PWM, without an inductor, will greatly increase the losses in the transistor.

    CM-

    The paper on the PWM thing was done by Henry of HDS, where he used PFM to get around the PWM LED flashlight dimming patent.


    Another technique is to dim with CC, on the upper end, where efficiency is important (high power draws), and then switch to PWM/PFM of the CC mode, where you convert down to lower the current, but then pulse that.


    One note about dimming of LEDs, is that if you use something like an X1/WF bin, the green tint will become pretty obvious and re-inforced at the lower currents. Of course, this also depends on where in the X1/WF bin it is actually at- within the X1/WF bin.

    Both PWM and CC have tint shift issues when dimming, and there is a chart on page one demonstrating the effect. PWM has a bit less shift, but notice how when CC dimming, it started shifting back towards the cyan direction at the very low levels.
    Last edited by NewBie; 12-19-2006 at 01:27 AM.

  18. #18
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    Default Re: Constant Current vs. PWM dimming Revealed

    I did some research about the possible enhancement of photometric effect using PWM, but i was unable to find any solid info at the time this thread was active.

    But today ive read this article:

    "A research group at Ehime University developed a pulse drive control method to make LEDs look twice as bright by leveraging the properties of how people perceive brightness. The group was led by Masafumi Jinno, an associate professor of Dept of Electrical and Electronic Engineering at Graduate School of Science and Engineering of Ehime University.
    When a short-cycle pulse voltage with a frequency of approximately 60Hz is applied to an LED at a duty ratio of about 5%, the LED looks about twice brighter to human eyes than that driven by a direct voltage, the research group said.
    Based on an evaluation test using subjects, the group reported that a blue LED looks 1.5-1.9 times brighter while green and red LEDs look 2.0-2.2 and 1.0-1.3 times brighter, respectively.
    "With this method, the brightness of LED with a luminance efficiency of 100lm/W can be simulated by using a 50lm/W LED," Jinno said.
    The test result was unveiled at the "New Technology Presentation Meetings by Four Universities in Shikoku Region" sponsored by Japan Science and Technology Agency (JST).
    There are two principles, the Broca-Sulzer effect and the Talbot-Plateau effect, involved in how human eyes perceive brightness. The Broca-Sulzer effect refers to a phenomenon in which light looks several times brighter to the eye than it actually is when exposed to a spark of light, such as a camera flash.
    In addition, the Talbot-Plateau effect is a principle where human eyes repeatedly see flashes and sense the average brightness of the repeated lights. Thus far, "it has been believed that, due to the Talbot-Plateau effect, the brightness perceived by human eyes would not change even if an LED is pulse driven," Jinno said.
    "The Talbot-Plateau effect is a principle found in the days when fluorescent mercury lamps and other light sources driven by a power supply with a longer voltage cycle of about several hundred milliseconds were used," Jinno said.
    Thus, the group decided to drive the LEDs using a power supply with a shorter voltage cycle of about several hundred microseconds. As a result, the group discovered that, when a pulse voltage with a frequency of approximately 60Hz is applied at a duty ratio of about 5%, the impact by the Broca-Sulzer effect becomes greater than that of the Talbot-Plateau effect so that the light emitted from the LED looks brighter to human eyes.
    The LEDs in three colors used in the evaluation test were all manufactured by Nichia Corp. The model number of the 464nm blue LED is NSPB500S, the number of the 520nm green LED is NSPG510S, and that of the 633nm red LED is NSPR510CS."



    So finally, the enhancement of photometric effect has been proved. Human eye is an imperfect integrator of light. From the article, i take the conclusion the effect is more noticiable when using short pulses, below 1ms.



    The problem then deciding what is more efficient photometrically, if PWM or CC, depend on if the enhancement is higher than the radiometric losses, wich have been well caracterized along the thread (we miss you, Newbie ). As the article report different enhancements depending of the wavelenghts tested, i think its going very difficult to do accurate maths about it, almost impossible.



    It would be required to do a function wich takes into account frecuency, pulses duration, lux level of the pulse (perfectly integrated) and wavelenght. Too difficult, IMO.



    Ive found too a graph wich explain brightness enhancement perception for different lux levels and pulses duration:





    It seems 60 ms is the limit of pulses duration to get noticiable effects and that the effect is way higher as higher is the flux level. And from the previous article, that human eye integrates better as longer the wavelenght (so strong bluish spectrum benefit more than reddish).

  19. #19
    Flashaholic* MrAl's Avatar
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    Default Re: Constant Current vs. PWM dimming Revealed

    Hi there,


    First off, this is interesting. It was known that the eye integrates light but
    it was always suspected that there may be some combination of pulse and
    frequency that could upset this characteristic. Apparently, if what they say
    is true, we have something new now.

    There are some problems however.
    The first one i notice is that 5 percent is a pretty low duty cycle.
    The second one is that the effect is different for different wavelengths.

    The difference with wavelenghts is going to cause color shift, which will
    shift white into the sky-blue color. This isnt good, unless the white can
    be first shifted into pink, but that would require different LEDs than
    the white that we have now. I guess it's doable though.

    The duty cycle problem is a little harder to get around. With a 5% duty
    cycle and a pulse 20 times as high as normal, there will be great efficiency
    problems that will reduce the light anyway, so even if there is a twofold
    increase at low currents that doesnt mean the same at high currents.
    Also, a pulse 20 times as high as normal means the manufacturers specs
    will be exceeded for max pulse amplitude.
    Ok, so say we drive it with a pulse 5 times as high as normal to stay within
    (some LED) specs. At 5 percent duty cycle that means we're down to
    1/20th of the normal light output, then increased it my 5 times times the
    eff factor, increased by the magic factor (of the article). This leads to
    a rough equation like this:
    E2=E1/20*5*e5*m
    where
    E1 is the light output of a white Nichia at 20ma,
    E2 is the new light output
    e5 is the efficiency decrease factor for driving at 5x nominal current
    m is the magic factor, a vector really.

    To get an idea in percent, we can set E1=1.
    e5 would be around 0.75
    m is 2 for blue and green, and 1 for red (approximately)

    This gives us:
    E2=1/4*e5*(2+2+1)/3
    so
    E2=1/4*0.75*5/3
    so the result is:
    E2=0.3125
    which means the max light output is 31 percent of what an LED is when
    run at nominal 20ma, and the color is sky blue.

    Now if we tint the LED plastic we also reduce light output, so that's a
    problem too. Thus, we would end up with white light, but even less
    than 31 percent of the full brightness.

    The average current, however, would only be 5ma. This is 1/4 of the
    nominal current. The new efficiency over the old eff could be expressed
    as:
    Eff2=Eff1*4*0.31
    so after normalizing the old efficiency to 100 percent (1) we get:
    Eff2=1*4*0.31
    so
    Eff2=1.24, which means 124 percent.
    Thus, we have gotten a little more light out of it, at that light level.

    The final analysis:

    The max light level is 31 percent of normal brightness (about 1/3 of normal).
    The color is tinted sky blue.
    The efficiency is better, meaning about 20 percent less energy is expended
    in lighting the LED, at 1/3 brightness.
    The small Nichia LEDs could be driven like this, but the larger high powered ones
    are not rated for over 2x nominal current so technically speaking, they cant be
    powered in this way.
    The LED can not be lit at full brightness without going way over manufacturers specs,
    and this may not even be possible due to serious efficiency issues at *much* higher
    peak currents.
    Last edited by MrAl; 05-12-2008 at 11:15 AM.
    Take care,
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  20. #20
    Flashaholic* uk_caver's Avatar
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    Default Re: Constant Current vs. PWM dimming Revealed

    Wouldn't a light with a pulse frequency of 60Hz actually be potentially flickery, especially for low-level lighting where some dark/peripheral vision could be expected to be operating?

    In the study, I wonder if the people were looking at the LEDs, or using the LEDs to actually light up a scene.
    Personally, looking at coloured 5mm LEDs, I've found it very hard to make meaningful brightness comparisons. Having identical LEDs side-by side running at significantly different currents, apparent differences (if they exist at all) are much smaller than the differences in drive currents. The LED is the brightest thing around of its colour, and the end result is that it seems to practically saturate a spot on the retina +/or brain.

    I'd wonder if pulse vs. constant brightness results from looking at (relatively very bright) LEDs would differ from looking at (relatively much darker) scenes lit by LEDs?

  21. #21
    Flashaholic* MrAl's Avatar
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    Default Re: Constant Current vs. PWM dimming Revealed

    Hi again,

    In addition to my rough analysis above i'd like to add the following...

    60Hz is pretty fast. It's not super fast, but it's usually fast enough to fool
    the human eye, or at least mine (chuckle) because i have built white LED
    lights that run from half wave rectified waves and they seem ok to me
    (although i prefer 120Hz for other reasons). If you turn your head fast though
    you can see the pulsing, so i guess it depend on how you are using the light,
    if it will work for you or not. Even displays are sometimes run at 50Hz,
    and some monitors operate the vertical sweep at 60Hz, although i dont like
    them because you do notice some slight blink effect of some kind. But then
    i dont know the phosphor decay properties anyway.
    I'd also be very surprised if they were looking right at the LED, as that would
    not do much for anybody, although i dont know for sure.
    Take care,
    Al
    LED's vs Bulbs, the battle is on.
    My bumper sticker: "I Brake for LED's"
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