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Off-line LED drivers

Discussion in 'Electronic Design' started by Adrian Jansen, Jan 27, 2013.

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  1. Many sources now for these devices, but the few I have inspected all
    have relatively large electro caps on the input side, to make DC from
    which the current controller PWM draws its supply for the LED drive.

    I strikes me that with the published figures for LED lifetimes of 50,000
    hours, the electro caps in these are probably going to fail long before
    the LEDs and other associated circuitry.

    Has anyone investigated sensible ways of running the PWM circuit direct
    off the ( full-wave ) rectified line, and accepting the fact that for a
    few msec, the input voltage will fall below the 10 volts or so needed to
    drive the LEDs ?

    Seems to me that would extend the potential lifetime of the driver
    considerably.

    Any thoughts ?
     
  2. mike

    mike Guest

    And accepting the fact that line transients can exceed the limits of the
    device.
     
  3. Phil Allison

    Phil Allison Guest

    "Adrian Jansen"

    ** Why is that ?

    It is common for electros to be fine after 50,000 hours of continuous use.

    One example is DC plug pack supplies left on 24/7.

    Temperature is the issue and long as the electro is not subjected to more
    than say 45C it can have a very long life.



    ..... Phil
     
  4. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** The kind that have iron transformers .


    ..... Phil
     
  5. Fabio_78

    Fabio_78 Guest

    Many sources now for these devices, but the few I have inspected all have

    Hello, if the working temperature is low enough and
    the capacitors good enough (pricy?) they should
    last enough :)
    And also the optocouplers tend to age.

    There are many design examples that lack a large
    electro cap on the input. AFAIK this is done mainly for
    high power factor, but as a side effect the large high
    voltage electro disappears. The single stage High PF flyback
    is simple to do for low power requirements, and moves the
    cap from rectified mains to led output side.
    For higher power one could use the usual PFC+flyback
    (or PFC+resonant) and use high voltage film capacitors
    as storage.
    Check appnotes for ST L6561/2, powerint linkswich-PH,
    if I remeber correctly also fairchild and ON should have
    appnotes on the subject.
     
  6. Jamie

    Jamie Guest

    Sure, a linear current sink will work just fine,
    if you don't mind a small amount of energy loss in
    thermal on the transistor?

    You could do it with phase firing and that won't lose
    too much, but that involves the use of a cap but not a large one,
    one that most likely would last much longer...

    Jamie
     
  7. E

    E Guest

    I don't quite understand why they don't use good old inductive ballast (or
    current-limiting transformer) in those led things?
    Cheap shit ballast is way more reliable than any cheap shit switchmode psu.
     
  8. Guest

    If you try to put some 50 Hz inductances into some E27 lamps, the
    thing would be quite heavy and existing luminaries could not handle
    it.
    Please read the LED specs carefully.

    If the LED is marketed by (absolute) maximum current of say 1000 mA
    and hence marketed as "3 W" LED, the life time (after LED and phosphor
    degradation) is a few thousand hours.

    However if respectable manufacturers only specify the lumen output
    (with 1000 mA abs max) at 350 mA (1 W) and the 30.000-50.000 hours
    might be believable.
     
  9. MrTallyman

    MrTallyman Guest


    The right move for the entire industry would be to place the DC source
    conversion IN THE LAMP and make the "light bulbs" just the LEDs and
    current limit elements and attachment socket stub.
     
  10. We already have that, and they have much the same problems of CCL lamps
    - the temps generated by the lamp itself impact on the electronics.
    Much better to separate the LED and its heat from the electronics.

    With the long life ( quoted ) on the LEDs, they essentially become
    fixtures in the building, rather than replacable elements like the
    tungsten lamps we have all used. So it makes more sense to have the
    entire lamp installation permanent. That is, if the rest of the driver
    can also be made long life.

    I rather doubt that inductive ballasts can supply the relatively
    stringent constant current requirements of LEDs, although it is
    tempting. Certainly the simplicity is attractive. And weight is not
    really an issue in permanent fittings. Copper and iron cost maybe.
     
  11. Tim Williams

    Tim Williams Guest

    I've got it!

    We'll build a high-reliability circuit, conservatively rated transistors,
    big transformer/coils, lots of heatsinking, and leave the inevitable
    electrolytic hanging out the side, mounted in a screw base! Patent
    pending!

    Ya know, the scary thing is the plausibility. They need to hurry up and
    make high voltage aluminum polymers work. Then this'll (mostly) go away.

    Tim
     
  12. josephkk

    josephkk Guest

    Gosh, i am seeing LED lamp and luminaire ratings of 50,000 hours and up,
    even claims of over 100,000 hours. You would need damn good PSUs for
    those.

    ?-)
     
  13. I too am very sceptical of those 50,000+ hour ratings. One can only
    presume they do some sort of high temp accelerated test to base the
    figure on. Unless its just purely marketing guesswork.

    However even if they are optimistic by a factor of 5, that still makes
    the LED regardable as a fixture, rather than a replaceable item, in the
    average domestic/business installation.
     
  14. Guest

    you never know, I just read of a guy making a fuss with safety
    agencies
    and the manufacturer because his PH lamp fell down when the plastic
    gizmo
    it was hanging in broke. It had been hanging there for 27 years!


    -Lasse
     
  15. E

    E Guest

    For domestic that may be considered fixture, but for business use 50000
    hours
    is just 2.5 times fluorescent tube lifetime ie. about 5 years. Lack of
    replaceable
    *standardized* bulbs is big drawback for led technology. Leds are damn
    expensive to begin with and the need to replace the whole luminaire every
    five years means much more installing work (expensive).

    Also at 50000 h they typically quote something like 70% light output
    remaining.
    It means that to achieve some minimum lighting level at end of life, you
    must
    design with 30% extra power initially so most of lifetime you are making
    more
    light than necessary. Fluorescents drop only by 10% so not much a problem.
    Of course if you have feedback on lighting level, then it is not any
    problem.

    Energy saving of leds compared to modern well designed fluorescent lighting
    is anyway questionable at best. Of course if you compare leds to some old
    non-designed system with 18W halophospate tubes with D-class ballast,
    plastic diffuser and no reflector you get impressive numbers to put in ads.

    Even if you spend big bucks for led lighting you do not necessarily save any
    energy over fl. lighting. But using conventional light sources, natural
    lightning and good design does not give you any "Green PR" like leds do.
     
  16. josephkk

    josephkk Guest


    Some Sites/pages to consider:

    http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/t8/05-t8-lamp-life.asp

    http://www.lrc.rpi.edu/programs/futures/LF-LampLife/

    http://en.wikipedia.org/wiki/Electric_light

    http://www.lithonia.com/micro_webs/electronicballast/source.pdf

    http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/hwcfl/HWCFL-life.asp

    It seems that fluorescent life varies a bit over the range of 10,000 -
    20,000 hours, LEDs range from 50,000 - 100,000 hours. I thus challenge
    your only 2.5 times the life.

    Depending on usage LEDs can deliver more lumens per watt, near equivalent
    CRI, dimmability (something fluorescent does not handle well at all) and
    some other useful trade offs. For some applications LEDs rock, for others
    there are plenty of lamp technologies.

    Induction lighting in particular is still giving all other technologies
    fits in several applications.

    ?-)
     
  17. Guest

    This is just a design parameter.

    The lifetime is defined when the light output is reduced to a specific
    amount (say to 70 %). The rate of reduction can be measured with
    relative short (1000 h) tests and extrapolated from there.

    Those LEDs that dim in a few thousand hours are usually driven with
    Imax, i.e. a LED with Imax=350 mA ("1 W") at 350 mA. This causes
    severe thermal problems (high junction temperatures), heating the PSU.
    Running at such high current also reduces the efficiency [lm/W].

    Running the LED at Imax/3 will slow down the degradation considerably,
    making 50000+ hours believable. The junction temperature is much
    cooler, even when running three Imax/3 LEDs on the same heatsink as
    one at Imax (Rth j-c is in parallel). It might even be possible to
    reach over 100 lm/W claimed efficiency. Of course, to get similar
    initial light output as the one LED at Imax, you need nearly three of
    those devices at Imax/3, which of course increases the cost
    significantly.

    For even longer lifetime, reduce the current even further and add more
    LEDs, but of course this drives the costs up and is only justifiable,
    if the replacement costs are high.

    Look carefully at the LED specs, optical characteristics are more or
    less useless at Imax, many manufacturers specify the optical
    characteristics at some more usable fraction of Imax.

    Those over 100 lm/W efficiency claims are usually measured below
    Imax/10 and at 25 C junction temperature (just when turned on :). In
    addition the highest outputs only apply to "cool white" i.e. those
    with a very strong blue spectral line.
     
  18. josephkk

    josephkk Guest

    Nice trim.

    The 60% number is averaged over all other technologies without frequency
    of use weighting. Thus incandescent bend the figure a lot.

    ?-)
     
  19. Guest

    A better approach for the industry would to standardize some
    constant_currents_ (DC) such as 20/50/100/350/1000 mA so that any
    light panel from any manufacturer rated for a specific constant
    current could be plugged into a series string, just like christmas
    tree lights.

    In a system driven by a constant current, each load would have the
    same current, but the voltage could be different, depending of the
    power levels needed. This is very similar as the Pxxnn series tubes
    used in old European TVs, in which the filaments were in series across
    the 220 Vac mains (with some inrush current limiting).

    For larger LED panels it would be mechanically feasible to use
    multiple constant current generators with separate strings with two or
    more of those standard currents through the sockets. The panels would
    only have pins for the required current and the socket would short
    circuit the unused current lines, maintaining loop continuity. Some
    open/short circuit detection at each constant current source could
    take out unused current loops.
     
  20. Guest

    Wouldn't replacing the output voltage feedback divider at pin 1 with a
    high side load (LED string) and 100 ohm to ground and feeding the
    voltage across the resistor to the pin 1 (feedback) produce a 20 mA
    constant current source ? This should be able to drive a string of 100
    LEDs.

    Thus only one switching transistor and one inductor would be needed
    for both PFC as well as constant current regulation.

    Of course, with such highly integrated chips, this might cause havoc
    e.g. in startup functions.
     
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