Click MR16 LED Globe Failure Post-Mortem

We should have a section at element 14 for discussing different lighting product experiences and feedback. Although a lot of home lighting experience could be subjective though, so hard to compare.

Much like mcb1 I too am in no hurry to swap conventional lighting at home for LED. But kind-of giving LEDs a trial currently in one room. The previous lights were incandescent, 40W or 60W (depending on what is available off-the-shelf - the 60W are getting rare in the DIY stores) x 5 lamps, which reflect off walls, not down spotlights.

I went for these ones, the R50 version in the photo below: RefLED Blown Reflector | Sylvania Lighting Solutions

These ones are not perfect for all rooms, they are not suitable for dimming with an incandescent lamp dimmer.

The feedback so far is semi-neutral to positive depending on who I speak to.

The consensus is basically it is definitely brighter. Those who are neutral overall say the color feels too white/unnatural, and the color or effect (could be CRI related or color temperature related, I can't decide) has a slight feel of ugliness like some economy CFLs have. I can see that too.

I get the feeling that without mixing LED colors*, this problem would be hard to get rid of. I'm not sure though, and will try some more lamps.

I might also swap back to incandescent for a bit, and see what the feedback is then..

* maybe separate color lamps, e.g. dimmable red, to mix with white LED lamps could be an option too, to try to correct things. More awkward though.

From what I can see, some of the heatsinks they used were so "devoid" of metal (because it's expensive) that the LEDs ran warmer than they really needed to, solely to optimize the physical size of the globe, the weight, the material cost and "just" meet the necessary lifetime.

It does, however, bring me back to an observation I made in the CFL days, in that their designs were starting to become "optimized" in the sense that they wouldn't last too much past their rated lifetime (or risk being run when the lamp is so inefficient that the energy savings are reduced), and the failure modes were varied (e.g. glass envelope failure near the electrodes, depletion of emission mix, ballast electronics failure including transistor shorted, capacitor blown, etc) that it seemed they took some effort not to make any ONE part too good, because ultimately it would go to waste when any one component in the system had failed.

I suspect this mindset may be taken over to consumer grade LEDs, to optimize costs while meeting "expected" lifetimes in the majority case (e.g.lifetime hours is sometimes stated to lifetime for 80% survival, sometimes 50%). It does lead to a potential waste of components, but I suppose at the end of the day, as long as the specs are met, nobody really cares even if the specs (15k hours) are a lot less than the theoretical lifetime of the LED chips or drivers (50k hours and upwards in some cases).

- Gough

Hi Gough!

Agree, the different topologies would all have different behaviour. There are enough parameters (and mostly non-linear) that a manufacturer likely models them on a computer, so they can see the effect of all the variations (Vf, temperature, failure, etc) to optimise for their desired scenario, e.g. perhaps optimise the topology for longest life, or optimise topology for least brightness variation after a single LED failure, etc. and the effects their heat sink will have on temperature and plug that into their model too. Or maybe some manufacturers have a 'suck it and see' approach! Some cheap lamps (like the flat low-profile round ones) clearly have just a stamped metal enclosure that doesn't really seem like a heat sink.. probably just 'good enough' for that particular LED, topology and power supply!

Another thought your image bought up is how the parallel-series combination is done can have an effect on the imbalance itself and its magnitude, as well as a failure result.

For example, inverting the structure, so instead of having four INDEPENDENT strings of three in series, we have instead three series connected sets of four LEDs in parallel - such an arrangement would:

- on failure of one LED, result in just that one LED failing and current will be redistributed among the remaining parallel three (which, if there is enough margin, should allow things to operate a little longer).

- on failure of two LEDs from two different parallel sets, results in the same thing above, but only two LEDs are lost and six LEDs are under greater strain.

- on failure of two LEDs from the same parallel set, likely will result in over-current in the two remaining LEDs which will likely fail in short order and take the whole array out.

In some sense, swapping the topology around might actually be a nicer way of approaching the issue, combined with running the LEDs below absolute maximum rating to permit say one failure in each parallel set.

- Gough

I suppose as Cree are LED manufacturers, they get the first pick about what goes into their lamps, so binning on tight tolerances is indeed a big possibility. As you mentioned earlier, if it's close enough, maybe you can get away with it.

I have a sneaking suspicion they may be relying on the residual resistance in the relatively thin PCB traces to help reduce the imbalance as a form of "inbuilt" feedback resistance.

Aside from that, my experience with tearing down Philips (professional series) luminaires is that they tend to design differently - even if they have two strings parallel, they're often made of Luxeon chips which may be rated and specified at both 350mA and 700mA each with an absolute maximum of 1.5A per string, but in practice, they're actually running the two strings with a combined forward current of either 700mA or 1A. This means that the margin for imbalance is quite big, as say with 1A combined current, if one string was at 700mA, the other would only have to be soaking 300mA and everything's still "in spec" although there will be obvious differences in brightness. Better still, if one string fails, the cascading failure takes more time, because the resulting current still remains below the absolute maximum.

Having two strings seems fairly common even amongst quality lights - that is consistent with my experience - but the other thing I suppose is that two strings is easier to balance than three, four or five. The reason I say this is because the "margin" between the full driver output (i.e. worst case, every other string is dead) and the absolute maximum current of a single string of LEDs changes and in some cases, the designs are set such that a single string failure is ultimately catastrophic (i.e. no margin at all). The integrity of the luminaire depends on the combined reliability of every LED element - so I suppose the other benefit is that fewer strings often also coincides with fewer LEDs and fewer solder joints etc.

The paralleling business is likely to ensure the output voltage remains low enough - for example, mains powered "double insulated" or SELV globes say for countries running on 110-120v AC, they will need the globe to be operational down to about 84v AC in the worst case and giving the driver loss leaves us with maybe 80v to play with. At 3.5v/white LED, that leaves a maximum series string of 22 LEDs. If built with 5630 0.5W modules, that gives us about 11W of output only. Of course, larger LEDs will mitigate this, but I suspect another possible reason boils down to the insulation in the LED and MCPCB assembly itself given the substrate is metal and electrically conductive. If they want the output to maintain SELV status and stay under 48v (even with step-up) then that cuts the number in half.

Overall, I have no doubt that they probably know what they are doing. After all, as a very young child, I managed to run LEDs in a "voltage mode" rather than current mode than I should have by ensuring the series combination had a voltage drop somewhat above the maximum anticipated voltage and living with reduced power output. It's not a good idea, but that doesn't mean it can't work for almost "forever" (e.g. 7 x series green 2.1v LEDs = 14.7v nominal operating Vf, run from a 12v (maximum) supply with a 2.7v margin and accordingly reduced output).

- Gough