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  • Author Author: Gough Lui
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Click MR16 LED Globe Failure Post-Mortem

Gough Lui

One of my regular blog readers submitted an LED globe for post-mortem analysis, having failed well before the expected lifetime. I take a look inside, determine the configuration and analyze the relatively "anonymous" product which seems to show several design shortcomings which I discuss and think might be relevant to other readers at the element14 community.

 

The article is posted at http://goughlui.com/2016/06/01/failed-click-ltmr5w3k-5w-led-mr16-replacement-globe/

 

What really gets me is that many products are still designed using multiple-parallel LEDs without any current balancing mechanisms (e.g. remember emitter feedback resistors when paralleling BJTs?) and that leads to potential for a single string to be overstressed continually, fail early, and begin the cascade of failure as other LEDs are forced to soak up current in excess of their ratings, and operate in less efficient regimes resulting in greater heat production and degradation of the semiconductor material. Of course, if you went all-series, you would have an all or nothing situation and require higher voltage driver electronics, but I personally would gravitate to this arrangement for reliability reasons, or go with multiple independent channels if reliability is demanded.

 

Any comments, or stories related to LED globes or LED light designs are very much welcome.

 

- Gough

  • in reply to Gough Lui

    Yeah I agree, things could change over time (I don't really know enough about Vf behaviour over time). It is interesting though that even well-known brands seem to do this, e.g. this Cree lamp teardown shows no resistors in sight, and apparently this is two parallel strings:

    image

    Given that every engineer knows (or should know!) about Vf and the variation of it, I suspect it is highly likely that the binning goes on (for reputable manufacturers at least), else it doesn't explain why so many different manufacturers do this practice.

    Probably this is just for consumer lamps though, they can afford more exotic circuits for industrial lamps I guess.

    You're right that we just don't know about the quality and binning as part of that, since failures take quite a while to occur.

    Also, as a digression, Cree has some high-end lamps too, which have clever tricks to make sure the light looks "nice" for (say) cosmetics counters etc. Some of their lamps have white and red LEDs, and the brightness of the red LEDs are adjusted over time as the lamp warms up, to compensate for the changes in color that occur by using the phosphor method for white LEDs.

    I bought a bunch of those to use in my lab, but not fitted them yet. Although with more circuitry, their failure rate may be just as bad/good as the simpler LED lamp designs (I've no idea : (

  • in reply to shabaz

    You are right - thanks for that link.

     

    I did say that if the manufacturer did some work to "match" the LEDs, then it gives it a fighting chance in real life, especially if both strings have the same path over the MCPCB and thus can equalize their temperatures as well. The lack of any feedback, be it active (e.g. current mirror, etc) or passive (resistor) is a big drawback as even if you get the LEDs sufficiently close at beginning of life, you can't guarantee that the LEDs would always degrade at the same rate over time which adds uncertainty to the lifetime rating. Of course, with lots of LEDs, you get other failure modes, such as failure in the MCPCB itself and in the soldered connections between chips, so I do like single-series circuits with a moderate number of higher-powered LEDs but this then presents optical challenges in preventing hot-spots which cause glare.

     

    It's a big balancing act when it comes to design, but when you come across many globes with up to five-string parallel configuration without a feedback/balancing resistor in sight, you really have to wonder if and how well those LEDs had been binned and how long it really lasts. After all, some of these globes do have more pessimistic projected lifetime (e.g. 15,000 hours vs 25,000 hours) and that might be because of this failure mode rather than any intrinsic lumen depreciation due to phosphor burn-out or accumulated junction damage.

     

    At least it seems LEDs mostly fail more gracefully than CFLs which have occasionally smoked, sparked, popped and hissed. That being said, I'm not sure it's a great idea to have a lot of potential "strobe lights" hanging around - it could potentially trigger photosensitive epileptic seizures, or at the least, be a big annoyance to end users. Just another way things could be improved over time.

     

    - Gough

  • Hi!

     

    Interesting analysis. It would be good to examine the Vf of each LED too, because there is a school of thought that it is ok to parallel up LEDs without resistors under certain conditions.

    Although the Vf variation can be large between LEDs, it is possible to get them binned from the manufacturer so that the variation is much smaller. In that case, the series resistance can be

    much smaller. A point can arrive (in theory) where there is enough effective series resistance in the LED characteristic curve itself to be sufficient that the current delta (even under

    junction temperature deltas due to the imbalanced current) is small enough to be within the rated spec for each LED.

    It is hard finding this information though, it most likely needs to be tested. The best link I could find is here:

    The Nuts & Bolts of LEDs—Part III | EDN

    Notice however that still their conclusion is that so-called active control (i.e a constant current source) is preferred, but then the author does work for a semiconductor manufacturer - he's still

    right of course, active is good.

    However, that doesn't mean the series resistor method is bad, it just means a lot of care is needed, i.e. more expensive LEDs (since you're paying for the fact they are binned) and some calculations.

    For example if you can bin to within tens of mV (this should be feasible, especially if they all came from the same wafer) and can adequately heat sink, and your LEDs happen to have

    ESR in the right ballpark, then that means you could reduce or even eliminate the external series resistor. A lot of 'if's of course, but worthwhile the manufacturer doing if it saves them costs

    and simplifies the circuit if they are manufacturing hundreds of thousands.

    It could be that to save costs the low-cost China models are not binned, but if the LEDs are manufactured close-by then maybe they can purchase a whole batch, so that consecutive LEDs are

    all from the same wafer and therefore naturally very well 'binned'. This is speculation though..

  • in reply to Dudley

    Thanks for clearing that up the doubt and for the green light, and thanks for taking the time to pop in here and let me know image.

     

    - Gough

  • in reply to Gough Lui

    We have absolutely no problem with you or anyone else posting links to blogs on external sites, as long as:

     

    • It's of interest to engineers
    • It's not spam

     

    Obviously we'd rather you blogged here, but we're aware that's not always practical, you might already have a blog before you came here, or you might have incentives to blog elsewhere, or just prefer the experience elsewhere. And should you have a blog elsewhere, we have no problem with you linking to it.

     

    Our primary goal is to encourage discussion of electronics and engineering in general, and you linking to your blogs does exactly that. So stop worrying about scorn from us and get posting! image

     

    I think it's a given as well that on your external blog you might occasionally mention some of our competition, and that's fine too, as long as it's sensible and not blatant promotion.