LG 47LN5790 badly burnt out LEDs

[budm]

Got this one in today, the backlights flashed on for seconds.
It looks like bad design to allow that much damages.

[ReeceyBurger123]

Re: LG 47LN5790 badly burnt out LEDs

Wow thats mental ! Wonder if they seen it set fire ? Does it smell bad ? Also what you think caused it to burn that bad ?

[fredaelsadany]

Re: LG 47LN5790 badly burnt out LEDs

hallo
thank you MR budm
what happened to the LCD ITSELF can have photo for any bad damage for it.
it is concentrated in certain place can be some water came inside .. strange .
thank you MR budm for inf

[tom66]

Re: LG 47LN5790 badly burnt out LEDs

More cheap LEDs from China failing at high temperature.

The most likely cause was a resistive device failure. The LED failed in such a way that its power dissipation increased massively (Vdrop increased.) Controller did not detect a fault because usually it only detects open circuit LEDs. This probably didn't start a fire because the set will use flame retardant plastics, however it could cause a fire if next to curtains or something similarly flammable. Not to mention the toxic fumes from burning plastic.

The fact that so many LG "LN" series sets use the same LEDs as Vestel crap ought to say something about LG's supplier standards.

[Diah]

Re: LG 47LN5790 badly burnt out LEDs

no way, this is never being LED failure , as it LG , with LN and LM series with this 2 row LED backlight had protection level at power powered, this why its blink now just once, this is out side house burning, would be nice to see if the back cover from the top was in touch

[budm]

Re: LG 47LN5790 badly burnt out LEDs

The damage is only inide the panel, when the backlights flashed on I can see the images, black soot inside, the traces on PCB the LED in that section burnt.
It has more than one set of the strings so the good string still lit up before going into shutdown, the burnt LEDs has open circuit now, so basically it burnt open.
It smells bad inside the panel but I did not smell anything until I open up the panel.
The person that drops the TV said no the images on the screen, but still has sound, no sign of damage from the outside or inside.

[Diah]

Re: LG 47LN5790 badly burnt out LEDs

maybe he brought it direct from Alberta fire ?? i wounder now did you take look on the reflection plastic board which sitting over led, it had burning or not ????

[budm]

Re: LG 47LN5790 badly burnt out LEDs

https://www.badcaps.net/forum/attach...2&d=1462647571
The white sheet burnt, the thick diffuse burnt, the 3/4" dia white sheet under the LED LENS also burnt.
https://www.badcaps.net/forum/showth...604#post655604

[Diah]

Re: LG 47LN5790 badly burnt out LEDs

if this set not from Alberta Fire disaster , then tequila or higher drops on, Good that PCB still on

[tom66]

Re: LG 47LN5790 badly burnt out LEDs

I'm 90% sure this is LED chip failure. This is what happens when a lot of power is dissipated in a small device with very limited protection circuits.

I have seen similar things with poorly designed LED lamps. I have ten small LED lamps that have all failed in the same way, small LED chip failure usually with a small burn mark but sometimes causing a little more damage.

[CapLeaker]

Re: LG 47LN5790 badly burnt out LEDs

that's pretty bad.

[ReeceyBurger123]

Re: LG 47LN5790 badly burnt out LEDs

Its the Led for sure, no way does the trace randomly burn like that. Agree with what Tom says the Led acted as a resistor basically and dropped too much power getting very hot and catching the plastic next to it. Dont know if anyone else has done this but that plastic burns very well and fast, only need a small flame or alot of heat before it starts burning and melting. Seems common on these Lgs for the Leds to fail like that never seen it on Vestels though.

[Diah]

Re: LG 47LN5790 badly burnt out LEDs

Tom, with all respect you should know what kind of material built on LED, this LED in set of LG LN are 3.2 V forward voltage at 50 Lm with may 0.450 mA,
All LED made in china, and consider 0.450 mA what it can burn...

[Diah]

Re: LG 47LN5790 badly burnt out LEDs

by the way, why you put the fault on LED and not on PCB which fail to turn off its protection..if as you said LED will be resistors, ... what about its zener diode ? and why now now PCB protection working ?

[tom66]

Re: LG 47LN5790 badly burnt out LEDs

Tom, with all respect you should know what kind of material built on LED, this LED in set of LG LN are 3.2 V forward voltage at 50 Lm with may 0.450 mA,
All LED made in china, and consider 0.450 mA what it can burn...

Yes, under normal operation the LED operates at 3.2V @ 450mA so power dissipation is ~1.44W. Approximately 20% of input power is emitted as light; the rest is dissipated as heat in the chip. 1.2W is a lot of power, but not enough to cause this damage. (Normally these LEDs run at 85C in an ambient of 35C, so 1.2W ~ 50C or 42C/W thermal resistance.)

It is possible that the LED has an internal failure where it becomes very resistive. Still conducting current and may still emit light but no longer dropping just 3.2V. Failure modes can include, but are not limited to: loose bond wires, one or more bond wires failing open circuit, parasitic structure short, or a crack in the LED die.

Hypothetical example: Say one of these faults leads the LED to still work but with additional 50 ohm series resistance. Additional ~25V dropped across LED is not noticed by controller circuit as over voltage threshold is set quite high. Power dissipation in the LED chip is now >15 watts. This power raises temperature to >500C quickly causing LED silicon to fuse into a short circuit, and further increasing power dissipation and the resulting damage seen here.

This is the end result if the LEDs do not have protection/monitor circuits attached to every single LED, usually in simple constant current ballasts like this. (Inexpensive devices exist that can be added to individual LED chips, but most manufacturers do not bother to include them.)

[budm]

Re: LG 47LN5790 badly burnt out LEDs

Where did the heat come from?

[pgram]

Re: LG 47LN5790 badly burnt out LEDs

I had a similar LED failure on the LG 39LN5700 I'm working on although not quite this extreme. The LED in my TV also looked to have caught the white backing on fire and produced a strong burnt smell. I tried to contact LG about it, but they never responded to my email.

[budm]

Re: LG 47LN5790 badly burnt out LEDs

Ahh, thanks! I was looking for that link to show the same failure mode.
Looks like LG needs to own up to this failure mode.

[tom66]

Re: LG 47LN5790 badly burnt out LEDs

Interestingly, the power appears to be dissipated almost entirely along the thin bottom traces, and hardly any power is dissipated in the large copper sections either side of the LED. I had thought these sections were supposed to be heatsinks, but clearly they do not function well.

[SLK001]

Re: LG 47LN5790 badly burnt out LEDs

Is it possible that an LED reflowed and slid down to short the pad out? That might account for the burning of the narrow traces. However, I would have thought that the protection circuit would have jump in somewhere along the line.