A few years ago we switched from off-the-shelf 7-segment displays to custom LED assemblies in our machines (white LEDs in both cases.) Ever since then, we've had issues, to varying degrees, with dramatic fading of certain segments (the segments that are on the greatest percentage of the time fade the fastest.) The operating environment is roughly 130F (~55C) and that part can't be avoided. We told the company who designed our custom LED assemblies what the operating temperature would be, and they said it would be fine, but now that we're having problems, they just keep saying it's too hot.

Very frustrating.

The old LEDs survived in the same environment, operating the same percentage of the time, indefinitely. We've got machines running over 8 years with them at this point and have no evidence that any of them has ever shown noticeable fading. The new ones are fading enough to be detectable in as little as 3 months, and we have plenty of examples that were dramatic in 6 months or less (like 50-60% of original brightness on some segments and full brightness on others, all within the same display assembly.)

I cut samples of both LED assemblies open to examine construction techniques, thinking maybe one would dissipate heat much better than the other, so maybe the ambient temperature is the same, but the LED junction temperature is dramatically different, but they look remarkably similar to me. I'm wondering if they use different phosphors, and one handles heat much better than the other, but I haven't found any good info yet.

We've been going back and forth for over two years now with out manufacturer, and they've been difficult to say the least. We're definitely open to finding someone new, but I'd really like to know what we're looking for before talking to more people.

Any thoughts on what might be going on? What should I be researching? What questions should I be asking?

 

This might be instructive -

https://www.digikey.com/en/articles/techzone/2013/may/thermal-effects-on-white-led-chromaticity

Regards, Dana.

 

Oh yeah, forgot to discuss the driving conditions:

Old assembly was switched with Darlington arrays and current limiting resistors, configured to deliver about 8mA per segment.

New assembly uses an LED matrix driver. Outputs are current sources, configured for 40mA pulses, but with output multiplexing, max duty cycle per segment is 1/8th, so average current per segment is 5mA. LED specs claim pulsed current up to 100mA is ok at 10% duty cycle, and continuous current of 30mA.

 

This might be instructive -

https://www.digikey.com/en/articles/techzone/2013/may/thermal-effects-on-white-led-chromaticity

Regards, Dana.

Thanks!

I could've sworn I'd read that one already, but maybe not. A lot of the discussion is about color shift which isn't our current problem, but there are a few morsels I hadn't seen about temperature vs luminosity. I especially like this graph:


Still, the real question is why one LED package performs so much better than the other. We're pretty well stuck with our 130F environment temperature, and the physical design of the LED assemblies looks similar enough that I wouldn't expect drastically different junction to ambient specs... although I can't be sure about that last bit!

I want to believe there's a simple solution, like we just have to get them to use a different chip for the actual LED part of the assembly, but that may just be wishful thinking (so far the only change they offered was switching to a different epoxy that doesn't yellow as much over time. That worked, and the yellowing has stopped, but the fading is just as bad as ever.)

 

https://www.digikey.com/en/articles...g-the-cause-of-fading-in-high-brightness-leds

A detailed explanation of why degradation occurs.

Hopefully your drive circuits drive a fixed current, relatively free of
T variation. Another compounding problem.

Some drive circuits over time actually increase current to maintain
brightness, but that in turn adds to lifetime shortening. Thats easy
to do, simple photo cell in a control loop.

Regards, Dana.

 

https://www.digikey.com/en/articles...g-the-cause-of-fading-in-high-brightness-leds

A detailed explanation of why degradation occurs.

Hopefully your drive circuits drive a fixed current, relatively free of
T variation. Another compounding problem.

Some drive circuits over time actually increase current to maintain
brightness, but that in turn adds to lifetime shortening. Thats easy
to do, simple photo cell in a control loop.

Regards, Dana.

Interesting! I believe the LED driver chip has a very simple constant current output, but I'll double check the datasheet tomorrow and make sure it doesn't do brightness compensation, nor have significant current vs temperature variability.