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Possible High LED current with long life time

Discussion in 'Electronic Design' started by Pubs, Jan 30, 2008.

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  1. Pubs

    Pubs Guest

    Hi,

    I have a application where my LED currents are switched as in a
    matrix. My current LED on time is 60 us and off time is 15240 us. The
    intensity is problem thus I pump a higher current in that 60 us
    (applied potential is arround 9V in that period of time ) and that
    gives the required intensity. LED doee not heat up since LED current
    duty cyle is extreamly small (rms current is also very small too). Is
    this method will reduce the LED life time ?.
    Could some one refer some articles on LED life time dependability with
    current.

    Thank you.

    Pubs
     
  2. WJLServo

    WJLServo Guest

    LED die shouldn't mind the high peak currents, as long as thermals are
    within bounds. Where you will see some lifetime degradation is in the
    bonds between the die's bonding pads and the bond wires. The
    phenomenon you want to research is referred to as "electro-migration."

    W Letendre
     
  3. Wow. I don't like to go much beyond 8:1 to 10:1. You're doing 250+:1.
    I don't think you'll get away with it unless the peak current is not
    much more than 5-10x the allowable DC current (meaning the LEDs will
    be relatively dim).


    Best regards,
    Spehro Pefhany
     
  4. Pubs

    Pubs Guest

    My design works prefectly. IT gives almost like a continoues current
    because that current pulse is repeatedly applied. My concerened was
    with the life time.

    Pubudu
     
  5. Pubs

    Pubs Guest

    Hi Letendre,

    Thank you very much for your advice.

    Pubs
     
  6. Yes, yes, I know, 'prefectly'. So what does the LED data sheet say
    about the conditions you are subjecting it to? For example, here is a
    data sheet that specifies maximum PEAK current ("Limits of Safe
    Operation") at 25°C of 150mA (only 5 x the maximum DC current) for
    pulses of < 10 *microseconds*).

    http://www.lumex.com/pdf/SSL-LX5063ID.pdf

    It should be derated above 25°C, obviously.


    Best regards,
    Spehro Pefhany
     
  7. Pubs

    Pubs Guest

    Thank you very much for your help.
     
  8. Pubs

    Pubs Guest

    So the peak current is 150 mA at less than 10 us pulses for this LED
    type. Anyway my drivers provide arround 90 mA per LED. I think I am in
    the safe region.
     
  9. Pubs

    Pubs Guest

    My LED data sheet is shown below.
    http://www.jameco.com/Jameco/Products/ProdDS/183222.pdf
    According to it, peak current is 500 mA for pulses less than 50 ms. I
    am well withing the region.

    Thank you for your help.

    Pubs
     
  10. Fred Bloggs

    Fred Bloggs Guest

    Is this a bargraph?
     
  11. For pulses less than *1* ms, not 50.

    Actually, the datasheet is strange. It gives a max DC current of 20mA,
    but a max pulse of 500mA at 1/20 duty. Which is a *25* mA average, and
    probably much more than that in terms of effective heating.

    And is has a "continuous forward current" of "100mW"!
     
  12. ehsjr

    ehsjr Guest

    50 us, not 50 ms, but you are still ok in my opinion
    at 60 us with such a low duty cycle (1/255) and 90 mA.
    Trouble is, 1/255 (60/15300) seems wrong.

    Anyway, your question was about LED lifetime. The paper
    at this url may help:
    http://www.lrc.rpi.edu/programs/solidstate/pdf/ProjectingUsefulLife.pdf

    Ed
     
  13. Very impressive part, Jameco must have access to some very high
    technology suppliers. Compare a similar Avago part:
    http://www.avagotech.com/assets/downloadDocument.do?id=2579

    Applications
    • Commercial outdoor signs
    • Automotive interior lights
    • Front panel indicators
    • Front panel backlighting

    Max DC current is 30mA (derate above 50°C) rather than 20mA (?)
    Peak current is only 100mA vs 500mA for the Jameco thing.
    Max Pd is 111mW vs. 100mW for the Jameco wonder.

    Of course it could just be a sloppy translation and layout from a
    sloppy Chinese data sheet from a Guangzhou back-alley assembly outfit
    that hardly knows what they are doing, but that's just conjecture.

    Best regards,
    Spehro Pefhany
     
  14. RHRRC

    RHRRC Guest

    The whole thing is strange - maybe "pub' writes his own datasheets for
    leds as the other data offered makes just about as much sense.

    With a 1in 255 ratio and 90mA led current the average led current is
    only 353uA yet the stated led voltage for this current is 9V!!
    (unless it is meant that the average led current is 90mA whence the
    led 'on' current is almost 23Amps)

    Clearly the fellow has no idea what he is doing else he would not have
    chosen such a ridiculous ratio to start with
     
  15. Pubs

    Pubs Guest

    My current driver can handle upto 100 mA Max (typically 90 mA) with
    the duty cycle I mentioned above. Data sheet is strange to me
    too ..... but I am in the safe region according to my duty cycle. I
    did not check my current pulses since my instruements giving some
    problems at this tiny duty cycle. But what I observe is that my LEDs
    are realy bright and I dont want to go beyond 9 V at all. My problem
    is with the life time. I working on measuring the current pulses.

    My design is a LED matrix with 48 rows and 254 columns. I am using
    column scanning instead of row scanning. The way I designed it, it is
    much cheaper than using row scanning and also less over head for the
    uP.

    Can some one tell me how do I theoretically calculate the average
    current in a such system ?
     
  16. So you're not limiting the current, just hitting them with 9V? In that
    case, think you'd probably getting around 250mA peak current, which
    would be more like 5mA average, which would be fairly bright with a
    modern LED (you can compare with a DC current and see). The color may
    also shift substantially with such abuse.
    Since you aim to have 12,000+ blue LEDs, it will be a bummer if/when
    you lose a whole bunch of them.
    Multiply the theoretical *average* current per "on" LED times the
    number of remaining LEDs which should be on.

    Best regards,
    Spehro Pefhany
     
  17. Fred Bloggs

    Fred Bloggs Guest

    MCD lists an Albuquerque, New Mexico address:
    http://www.mcdelectronics.com , just another site sparse on technical
    information.

    It appears the signage silicon market is dominated by start-ups.
    Macroblock seems to have a global presence and a product line that
    pretty much covers everything:
    http://www.mblock.com.tw , very impressive stuff, the big semis are just
    plagiarizers by comparison.

    From his responses, it is apparent the OP's question is not worthy of
    consideration, let him take his confusion to the Basic ng...
     
  18. Pubs

    Pubs Guest

    I dont just apply 9V directly. It has a constant current driver. So it
    can not have 250 mA peak currents in anyway. max is 100 mA. Design has
    a safety circuit which cuts power to LEDs in case "on" time chages by
    +/- 5 us.
    Thank for the information. My deisng is continouesly running for
    couple of month till now without seeing any intensity changes so far.

    Thanks for your unformation.

    Pubs
     
  19. I would like to add that LED datasheets tend to be conservative in terms
    of short high current pulses.

    At your own risk, you must do "sufficient reliability testing", but I
    give good odds of success with situations that the datasheet does not say
    is good.

    Slight caution - the LED may have a design change not reflected in the
    datasheet and such design change could reduce ability to survive
    conditions that the datasheet never supported. Those who abuse LEDs are
    "on their own", although so far I have yet to blow an LED with hours of
    low-duty-cycle pulsing with peak current that according to the datasheet
    "should blow the LEDs sky-high" (my words).

    I would like to add:

    Look at the datasheet for both peak current and maximum pulse duration
    and also duty cycle.
    So far, I have yet to blow an LED with higher peak current as long as
    pulse duration is reduced in inverse-square relation to increase in peak
    current according to the datasheet, the average power dissipation does not
    exceed what the datasheet supports as allowable (even with higher voltage
    drop at higher current), and the RMS current does not exceed what is known
    safe.
    As in: For most LEDs rated 30 mA max continuous, 100 mA max peak at 10%
    duty cycle and for 10 milliseconds on-time (cycle period 100 milliseconds):
    RMS current tends to be allowed to be up to 32 mA or so according to the
    datasheet, though not documented when peak current gets past 100 mA.

    I would extrapolate to pulse duration at current past 100 mA being 10
    milliseconds times square of ratio of 100 mA to actual pulse current, and
    allowable duty cycle 10% times is square of ratio of 100 mA to actual
    pulse current.

    So far, I have yet to bloe LEDs with such conditions with pulsing to an
    amp or so. However, I do warn that doing anything outside the datasheet's
    box is at your own risk!

    One more thing - LEDs are nonlinear. Most white, blue, and
    non-yellowish-green ones are more efficient at lower peak currents!
    Same story at least with "low current red" - as in most red ones of a
    30-plus-year-old chemistry but having nominal peak wavelength of 690, 697
    or 700 nm (believe me, their bandwidth is on the high side for
    phosphorless LEDs).
    Most high brightness and high efficiency ones other than the above tend
    to have efficiency maximized when peak current is in or near the range of
    from half of maximum rated continuous current to double maximum rated
    continuous current.

    One more thing with pulsing LEDs: Watch for higher voltage drop, and do
    not exceed maximum rated power dissipation (I find that is usually easy to
    obey with lower average current when peak current is high and a maximum
    RMS current is obeyed).

    RMS is not 1.11 times average for most pulsed waveforms - for
    rectangular pulses, RMS is peak times square root of duty ratio. For
    example, 1 amp at duty ratio of .1% or 1/1,000 has RMS current of 31.6 mA.

    - Don Klipstein ()
     
  20. RHRRC

    RHRRC Guest

    If your led current is 100mA and the duty ratio is 1 in 255 the
    average led current is 392uA.
    Allowing nothing for the reduction in efficacy due to the high
    current, the observed 'brightness' of the led will be the same as if
    it was powered with a constant current of 392uA.
    Quite clearly this would not result in the 'bright' leds you report
    and one is led to conclude that your currents are much higher than you
    state.
     
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