From: Sean Breheny <sean.breheny@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 8 Nov 2021 11:13:28 -0800
Subject: Re: [EE]: Sony LED TV repair update KD-60X6700E. 50 backlight LEDs
 in	series.
Thread-Topic: [EE]: Sony LED TV repair update KD-60X6700E. 50 backlight LEDs
 in	series.
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Interesting problem!

I suspect you are correct about capacitance causing the flash.

I once worked on a product which used blue LEDs as indicators and it was
important that they had a consistent brightness (so that they didn't look
different from each other when all were on, which was the normal state -
off indicated a problem). We had a problem with these LEDs either failing
entirely or going dim after some time in service. In the end it turned out
to be an ESD problem during assembly but before we knew that, we had a
question about whether the problem could have been caused by temporary
overcurrent on the LEDs because we had a problem early-on where we had been
running them at too high a current before we swapped out the driver board -
and this had affected hundreds of units in the field so we couldn't just
preemptively replace the LEDs themselves.

To answer whether the observed misbehavior could have been caused by
overcurrent, we set up an experiment where we ran 10 LEDs each at 5
different current levels (from my memory): 100% nominal, 150% nominal, 200%
nominal, 300% nominal, and 400% nominal. Nominal was the maximum
recommended continuous current. We continued to run the experiment for
about a year - even after we had solved the fundamental problem with ESD
precautions. 100% never failed. 400% all failed completely within hours of
each other after about 3 or 4 days. But the really interesting thing was
the tremendous variation in the fate of the intermediate units. Some even
up to 200% never failed. But some failed open. Some stopped producing light
but still drew current (I don't recall what the voltage drop was). Some
became dim while still drawing the same current. Some became intermittent
where wiggling the leads would cause them to flicker (surprising that
stress could be so easily transmitted through the epoxy body). Some were
intermittent where no external influence seemed to start or stop the
intermittency - it was something purely internal.

I will say that I don't think we ever saw one with any type of failure
which "fixed" itself permanently. Some stopped producing light and then
started making light again, but there was always intermittency later, too.

Sean


On Mon, Nov 8, 2021 at 1:55 AM Justin Richards <justin.richards@gmail.com>
wrote:

> Thought this may be interesting or someone may have experienced
> similar with self repairing LED's
>
> Has anyone experienced strange failure modes of LED's where they
> randomly open circuit but then randomly behave like normal LEDs after
> perhaps welding the bond wires short again.
>
> I initially expected to find shorted LEDs but after doing a complete tear
> down all the LED's are now operating normally with volt drops ranging fro=
m
> 2.98v - 3.45v.  And purposely shorting out LEDs  has no effect.  i.e. The
> system continues to work fine, the compliance voltage simply drops by 3v
> each time implying the PSU is doing a good job as a const curr source.
>
> I think there is a LED or two that has failed open but when hit with a
> sudden voltage rise from 0 - ~200v welds itself shut then after some
> heating/cooling cycles it or another one fails.
>
> Is that plausible?  Many videos doing similar repair only ever seem to fi=
nd
> failed LED strips and works fine once they have been replaced.
>
> Another interesting observation is after disconnecting one of the strips.
> As mentioned they are all in series, however, in this mode they briefly
> flash when powered on which should not be possible.  I am guessing the
> capacitive effect of the cabling provides for a small current to flow wit=
h
> such a high dv/dt.  This brief flash looks very similar to the flash when
> the TV fails to power on and hence the reason for my crazy theory.
>
> Other info ..
>
> The TV worked for a short time with a NTC thermistor in series with the
> LEDs.
>
> As it failed again I have dis-ssembled expecting to find some failed LEDs=
..
> However, during testing it randomly powered on and has since been rock
> solid.
>
> I have taken many measurements but here are the key parameters while its
> running ok
>
> 50 LEDs all in series with a compliance voltage of 165VDC and a current o=
f
> 540mA.
>
> The compliance voltage increases and current is roughly stable at around
> 540mA.taking measurements while increasing a  variable resistor in series
> with the LEDs.
>
> Justin
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