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I have some LED modules powered with 12 VDC. Each module has 3 LEDs and 3 resistors all in series. After working for one year, in some modules, one or two LEDs turned off but another one is on.
I wonder because all LEDs are series and must conduct current because one or two led are on but one or two are off.
Why does this happen?
I choose three 27ohm resistors for sinking 30ma current if LEDs fail in a "short" mode, why when I increase input voltage, the off LEDs become on?
I can't speak for the first photo, but for the second one - three resistors & three LED's suggests that it's one LED per resistor, so the LED's in each module are actually in parallel, not series. The modules are in series, but clearly the power & ground connections are pass-through.
Judging by the scorching, and the fact that the most scorched ones have the LED's out, I'd say it looks like the resistors have burnt out on the failed ones. The resistors are clearly under-sized for the power they are required to dissipate.
Diodes like to fail in a "short" mode, and it basically means that your diode's chemical composition changes from being a junction between a P and an N semiconductor to being a "well-mixed through silicon and other elements" kind of conductor.
That happens when you overheat the diode, but not enough to make it explode.
When I look at your picture, I'd say I see multiple badly overheated arrays of resistors. So, you're probably not cooling these modules sufficiently or are operating them at a voltage that is too high.
The only conclusion I can determine is the LEDs have a very short life span. They could have been over driven (e.g. driving a 20 mA LED at 30 mA) or they are poor quality LEDs with low quality epitaxy layers.
Where two LEDs are out, the resistor PCB scorching is worse than the modules where one LED is out. Scorching is least noticeable where all 3 LEDs are still illuminated.
This would lead me to believe the Vf is decreasing (current increasing) as they deteriorate.
Accelerated ageing, i.e. LED efficiency loss within a period lower than expected life, is caused by adverse factors like low quality of epitaxy layers as well as, often, an excess junction temperature due to insufficient heat dissipation. Furthermore, penetration of humidity or other contaminants, latent ESD (Electro Static Discharge) damage as well as an instable power supply can result in an accelerated degradation of epitaxy layers.
Source: LED Failure Modes and Methods for Analysis, LED Professional, Aug 01, 2010
Measure the Vf of the LEDs. Is the Vf of the unilluminated LEDs less than the illuminated?
If the Vf is decreasing with age, then the aging of the LEDs would increase due to the over stress of the excess current.
Your design is wrong:
You've got a desired forward current of 30 mA, and you've got 3·27 Ω = 81 Ω series resistance.
30 mA · 81 Ω = 2.43 V
That means over your three LEDs, a voltage of 12 V - 2.43 V = 9.57 V drops. That makes, per LED, a voltage of 3.19 V.
That's pretty typical for blue LEDs, but not for red ones. Red LEDs are manufactured with band gaps of 2 V to at most 2.5 V.
So, you're driving your LEDs in constant overvoltage. No wonder they fail all the time!
Let's check this: If we have three 2.5 Vf LED, then there's 7.5 V across the three of them, which means that 12 V - 2.5 V = 9.5 V drop over the 81 Ω. That's 117 mA, not 30 mA.
That means the LEDs get 16 times as hot as they should be getting! No wonder they become worse at emitting light, and better at working like a resistor.
