Connect with us

High brightness white LEDs damaged by custom switcher

Discussion in 'Electronic Design' started by Paul E. Schoen, May 4, 2007.

Scroll to continue with content
  1. Some of you may recall my discussions about a 12 VDC powered switching
    converter to drive strings of 7 or 13 high power white LEDs, in posts from
    1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
    problems). We built the circuit, found a few problems, then tested it and
    it seemed OK. Basically this is a switching boost regulator using a 10 uH
    inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice simulations
    as well.

    The circuit is somewhat unique in that the PIC is programmed to produce
    alternate high and low brightness modes, at about 700 and 100 mA, when
    power is applied and removed. The PWM is controlled by the voltage on a 1
    ohm sense resistor, and it is designed to ramp up slowly and cut back
    quickly. There are also undervoltage input and overvoltage output sensing
    circuits to shut down the PWM until the next power on reset.

    My friend had three different 7-LED clusters (made by Cree), of low,
    medium, and high brightness. While testing the medium-brightness cluster,
    he noticed a less than normal difference in brightness, and found that two
    of the LEDs were not lit. They tested open, but apparently the voltage was
    enough to bridge the gap and provide a current path for the remaining 5 in
    series.

    The manufacturer examined the LEDs and stated: "#1 appeared to have an
    evidence of damaged chip which looked to be a burnt metal traces around the
    bond pads, it also had a blown wire bond on the Anode side which had an
    appearance of a fused wire. The lamp #4 had the blown off bond pads on the
    chip side." They concluded it was a high current surge.

    My recommendation to my friend was to perform a deliberate destructive test
    on one or two lamps to see what level of current would cause such damage. I
    think he had previously tried up to 1.5 or 2 amps for a few seconds without
    sign of damage, although possibly not with these exact model LEDs.

    Without going into more detail for the sake of brevity, I would like to
    discuss possible scenarios, testing suggestions, and design improvements to
    minimize the risk of more failures in the field. Terry Given was kind
    enough to analyze my circuit and layout, with many suggestions for
    improvement, and I plan to implement them in the next iteration of the
    design. However, someone else was adamant that a PIC would never be as
    reliable as a pure analog design, and suggested an off-the shelf Linear
    Tech or other part, which would involve a complete redesign and loss of the
    dual brightness feature.

    Thanks,

    Paul
     
  2. D from BC

    D from BC Guest

    How about asking Microchip Inc. if PIC's are just as reliable as
    dedicated smps PWM controller IC's.
    D from BC
     
  3. I'm sure they would "stand by their chips", but I think the concern was
    that the software could go awry and damage could be done before it could
    react. That's why I am considering a separate fail-safe mechanism, such as
    a fast acting fuse, or an NPN transistor that would turn on, when the
    voltage on the 1 ohm sense resistor reached Vbe, and turn off the MOSFET.
    Maybe an SCR would be better, requiring a complete power down to reset.

    Paul
     
  4. rebel

    rebel Guest

    (snip history)
    Did the manufacturer examine the entire string, and if so what were their
    conclusions (if any) about the diodes that *hadn't* failed? (It is easy to say
    that a "current surge" blew the shit out of the failed ones., just like most
    house fires always seem to be caused by an electrical fault.)
     
  5. D from BC

    D from BC Guest

    Arrghh..I'm forgetting lots of my PIC stuff..
    Then there's the PIC watchdog timer...
    IIRC ..it's to help with "getting stuck in the hole" type glitches"...
    The uP will recover however I can imagine that the recovery period
    might still cause some smps external damage.

    The method of OC protection will depend on worst case over current
    rate of rise. MegaAmps/picosecond??

    Alternatively, Polyswitch devices (PTC) are used for overcurrent
    protection but I don't know if I'll protect LED"s in your app.
    It's a datasheet exploration...
    D from BC
     
  6. Perhaps for development you could use a power NPN
    that directly did a limiting clamp of the current
    through the LED strings. Also a sensor that flags
    when the NPN is conducting.

    This would allow any fault to remain long enough
    for investigation, without damaging the LEDs.
     
  7. rebel

    rebel Guest

    Why not use something like an LM317 in the (series) feed to the LEDs as a
    programmable current source. That would facilitate (a) avoiding possibly
    damaging current surges (sorry, tongue in cheek), and (b) allowing the stepping
    of the LED current through 0/100/700mA or whatever steps you want.

    Pierre (who loves 34063's and 317's)
     
  8. Joel Kolstad

    Joel Kolstad Guest

    That's a little naive. There are plenty of switching power supply out there
    being run by microcontrollers these days, and on ones where it's preventing
    catastrophic failure is important, you certailny to see various current
    limiting/fail-safe devices... ***just as you do on analog designs, since
    there's plenty to fail on them as well***.
    You don't necessarily lose the dual-brightness feature -- somewhere an analog
    design still has a reference voltage (or similar) that it's trying to match,
    and you can usually find some means of changing that reference, PWMing the
    output, etc. to get variable brightness.
     
  9. Phil Hobbs

    Phil Hobbs Guest

    I usually use two-transistor current limiters--you know, the classical
    two-terminal ones--rather than ICs for protecting sensitive devices like
    diode lasers. I'm much more confident that I understand their transient
    response than the IC's, especially in areas like coming out of thermal
    limiting.

    Cheers,

    Phil Hobbs
     
  10. Thanks for the suggestion, but the power required for the clusters is at
    least 21 watts for 7 and 39 watts for 13. Even if the device could handle
    the voltage and power, it would be inefficient and unusable in the
    application, which is a diving flashlight. The efficiency appears to be in
    the order of 90%, and 4 watts is not excessive, especially when the water
    acts as a huge heat sink. However, efficiency and battery life are
    important.

    I think the additional transistor or SCR will work. I can't add too many
    parts, as the entire board is only 0.95" x 2". The transistor would
    effectively form a linear current regulator, but would subject the MOSFET
    to excessive power. If I can get a small SCR, or make one from two
    transistors, it would latch the MOSFET off, but then the 12 volt gate drive
    would be applied across the 20 ohm gate resistor.

    Maybe I will use the TI UCC27321 MOSFET driver which has an enable line
    with a Schmitt trigger. Luckily it is an active High with internal pull-up,
    so the saturated NPN transistor would disable the drive. Adding a capacitor
    should hold the output off for long enough to produce a "flashing" effect,
    which would be very noticeable.

    It needs a redesign anyway, and I'm a little leery of the Microchip
    drivers. The TI products have a combination MOSFET and bipolar output
    designed to switch more efficiently through the on threshold.

    Additional suggestions are welcome!

    Paul
     
  11. Hmm, are you trying to maximize thermal-gradient stresses?
     
  12. I was trying to minimize the possibility of overshoot. In the simulation,
    with 12 VDC input, an 80% duty cycle produces a peak of 2 amps at 55 volts
    in 600 uSec. The desired 700 mA is reached in about 300 uSec, and the first
    100 uSec is used to pump up the voltage of the 47 uF output capacitor. This
    is with a simulated 13 LED cluster. The 7 LED cluster also takes about 600
    uSec to reach a peak of 3.8 amps, and reaches 700 mA in about 250 uSec.

    A 45% duty cycle, on the 7 LED cluster, reaches and holds 700 mA at 27
    volts in about 210 uSec, and a 65% duty cycle PWM reaches and holds 700 mA
    at 46 volts in about 1.2 mSec.

    The circuit is certainly capable of generating enough current to damage the
    LEDs, particularly the cluster of 7. However, I am reading the output
    current at a 1 kHz rate, so I think I may have discovered the problem. The
    PWM starts at 0, and can only increment by about 1/63 per mSec, so it
    should take at least 30 mSec for the PWM to reach the 45 or 65% level
    needed for the target output current. However, if somehow the PWM is set
    too high, it will take less than 600 uSec for the current to shoot up to as
    much as 4 amps, which would probably cause the damage seen.

    I could easily increase the sampling rate, at least for the output current,
    to 100 uSec, as the conversion time is 51 uSec. It seemed like the slow
    rise of PWM would have been enough, but this is very likely the culprit.

    Another simple thing I could do is generate an interrupt when my proposed
    NPN overcurrent sensor indicates more than 700 mA, and then shut down the
    PWM (as well as lowering the duty cycle) until it clears. I will probably
    want to change the sense resistor to about 0.68 ohms so that it will only
    trip over about 1 ampere, which is still safe. The same logic signal could
    be tied to the enable line of the driver as well.

    OK, back to the drawing (and coding) board...

    Thanks,

    Paul
     
  13. Guest

    Guest Guest

    is device to device parameter variations being accounted for?

    most of these LED's are not anywhere near being matched pairs/triples or
    other, unless you get the manufacturer to custom mount them from the same
    die and then do a parametric test on each and then sort them and mark them,
    (Very costly)

    even clusters are assembled from random devices!

    MIL specs may offer you some leeway, but even controlled groups will be bulk
    tested only for basic params.

    if spiked current is failing some devices, they may have beeen borderline to
    begin with, as most will just heat up and dim before catastrophic failure
    occurs. usually over time.

    your blown bonds and similar symptoms tells me that you had EXTREME current.

    rise time between units may force some to go into failure befor the adjacent
    units absorb the available current also.

    as with all design, the simplest is the best. trying to parlay your
    education of PICs and neatsy circuits willl yield expensive and hard to
    manufacture systems.

    take some lessons from the chinese,tiawanese and others, cheap simple and
    basic.

    stay within some easy set standard limitations and you may have good
    results.
     
  14. Fred Bloggs

    Fred Bloggs Guest

    I don't recall seeing the previous posts on this project so really don't
    know what your circuit looks like. The most reasonable current limit
    topology for almost any kind of switching power control is one which
    truncates the power flow upon over current. In your case this would mean
    a small sense resistor + comparator monitoring the source current of the
    MOSFET and truncating the gate turn-on pulse as qualified by a fixed
    threshold corresponding to a level somewhat below the peak current
    capability of the LEDs. This will provide protection against an
    instantaneous over-current while the PIC will handle the averages. It is
    most likely that your design should include protection against the PIC
    in addition to the usual analog protection measures. On the analog side
    of things there could be several causes of the over-current if this is
    in fact a failure mode. Some possible causes are: 1) the inductor
    transiently saturates for some unanticipated reason having to do with
    the PIC or possibly a bad rating selection, 2) the filter capacitor
    displays too much ESL and ESR relative to the LEDs+ 1 ohm sense resistor
    resulting in higher peak current transients being driven into the LEDs
    than planned, 3) too high a gain of dIleds per unit percentile of duty
    cycle corrupting the PIC control algorithm into a hunting condition or
    resulting in a faulted steady state which is an alias of a fault
    condition. This last may require that you employ some sort of dual
    modulus PWM scheme for the duty cycle generation to obtain better
    resolution on your control. After reading your other replies, it looks
    like your sample rate is too low by a factor of ten. There is such a
    thing as an effective Nyquist criteria that must be satisfied for
    applications such as this.
     
  15. default

    default Guest

    Hmm . . . saw something similar in my own experience. Bought Cree one
    watt leds and connected them in series and blew one up - linear supply
    with a dropping resistor.

    Problem turned out to be the little aluminum heat spreader pad they
    were mounted to and multi layer board. I used something like a #6
    machine screws to fasten it to the heat sink and the screw threads
    contacted an exposed bit of conductor on the white insulating board.
    Apparently the copper multi layer board uses large traces to aid heat
    removal and there's no provision in manufacture to prevent the edge of
    the copper from contacting mounting screws. Once the screws were
    tightened down the copper deformed and made a permanent short to the
    heat sink - the problem was solvable in the others (that weren't
    destroyed) by taking a drill bit and reaming the hole a little to
    break the bond, then using smaller insulated screws.

    Check for shorts to the heat sink?
     
  16. Al

    Al Guest

    Amen!

    Many years ago I did some analysis one a power switching circuit which
    used two transistors in the primary of a center tapped transformer. The
    transistors would switch alternately to produce a square wave on the
    secondary which was then rectified to produce a higher voltage.

    Modules which failed had one blown transistor on them. As it was blown,
    it was not possible to read the parameters. Analysis of stock
    transistors showed that their characteristics, although within spec,
    varied. When the characteristics of the transistors were matched and
    they were used in matched sets, the problem disappeared.

    The problem disappeared until a couple of years later, when a new
    manufacturing guru wondered why they were going through the expense of
    checking all the transistors. He abolised the practice, and voila,
    problem reappeared.

    Al
     
  17. Jim Thompson

    Jim Thompson Guest

    [snip]
    Of course there was no "engineer" there to ponder WHY? And fix the
    frigging problem the right way. Selecting parts is the weenie's way
    of "engineering :-(

    ...Jim Thompson
     
  18. Fred Bartoli

    Fred Bartoli Guest

    Al a écrit :
    Bad design to start with.
    Bad answer to go on: for the cost of matching, you could easily pay for
    the components cost to fix the problem.
     
  19. Fred Bartoli

    Fred Bartoli Guest

    Paul E. Schoen a écrit :
    What's your circuit? In details...

    With 7/13 leds you're having about 28V/52V from your strings of white leds.
    Your circuit is a boost, so you're reasonably safe from some POV, except
    this:
    at 100kHz, L=10uH, E=12V, and Io = 700mA you're in CC mode with:
    @ Vout=28V : duty ratio= 0.57, Imin=1.3A, Imax=2A => Irms=1.1A
    @ Vout=52V : duty ratio= 0.77, Imin=2.6A, Imax=3.5A => Irms=1.5A

    depending on how you've filtered (which kind of bypass cap) your output
    the led might see a big chunk of the pulsating current: the 47uF may not
    be as good as you think and the dynamic impedance of your leds will be
    very low at 700mA.
    Thus the Leds bonding might really see an effective 1.1/1.5A rms, not
    even speaking of any regulation problems involved by the low loop
    crossover frequency (for ex. what will happens if you have a small +0.5V
    step on your supply once the circuit is running full power?).

    And why messing with a pic (everything implying a pic is a mess, anyway)
    while you could easily do this with a cheap dedicated SMPS controller?
     
  20. Guest

    Just so I'm reading this correctly, you are not paralleling strings of
    leds. That is, you only drive one string of leds. If that is not the
    case, I would expect problems.

    When I think of the effort it takes to make a bullet-proof DC/DC chip,
    I just shake my head at the idea of doing it in software. In a chip,
    events take place simultaneously, while a uP is a step at a time using
    polling.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-