Nisa explains the “second-half syndrome.”
Aside from manufacturing-line variations, consider that other LED lifetime and failure points may affect different LED types and, of course, the applied driving methods can affect endurance. Also, higher ambient temperature, humidity and operating current cause LEDs to degrade faster.
Generally, two types of LED lamps exist in the marketplace: high-power units that use elevated current (200mA or higher) or low-power units which use modest currents (tens of milliamps). Designated LEDs have their own current-voltage requirements and corresponding light-current characteristics. Expectedly, all diodes burn out if you apply excessive current, meaning, you shouldn’t direct high current into low-current lamps.
Because excessive current flow will burn them out, LEDs need to be properly wired. If you accurately choose your voltages, resistors and currents, both series and parallel connections are feasible. If you choose parallel, all the LEDs must be sufficiently identical in relation to operating current, resistance and diode turn-on forward voltage (transistor load time, for practical purposes). However, without the proper binning of equivalent LEDs, such matching is difficult to achieve.
Red, blue and white LEDs inherently require different voltages; therefore, parallel lamp connections may require each LED to have its own series resistance, to limit the current passing through the diode.
High-power LEDs (but not necessarily high-brightness LEDs) require high current, and this causes them to operate at a higher diode-junction temperature, which, in turn, leads to faster aging and a shorter lifetime.
For their various uses, sign manufacturers, and others, require white-light LEDs to produce high lumen output. And, for decent illumination of channel letters, cabinet signs and electronic displays, sign-type lamps should consistently produce several hundreds of lumens throughout their predicted lifespan. However, unlike traditional indicator lamps, signmakers require their lamps to correspond in color rendering and temperature during their entire lifespan. Such requirements inevitably shorten the lamps’ lifetimes.
Most manufacturers of high-power, white LEDs estimate a lifetime between 30,000 to 50,000 hours, to maintain 70% of initial lumen level. This estimate assumes constant operation near 350mA of constant current and a consistent junction temperature not higher than 90° C.
Improving technologies have produced longer-life LEDs which, to some extent, tolerate higher drive currents and operating temperatures. Presently, certain manufacturers offer LEDs rated for 100,000 hours while operating at over 700mA with junction temperatures up to 120° C. I suggest you consult with these manufacturers, to see if such LEDs are available and cost effective, at high volume.
As the LED lighting industry develops, the engineers consider both 70% and 50% lumen-maintenance levels as lifespan definitions, but for different applications.
For example, a traditional lightbulb ends its usefulness when the filament breaks and the light ceases. Oppositely, an LED lamp’s efficiency drops over time, which causes the light output to diminish gradually. When it falls below 70% (or 50%) of the original output, the design engineers say it has expired.
Interestingly, the average person can’t to judge when the LED lamp has reached this point, which is the drawback of this standard.
To better understand LED lifespans, ask your LED lamp provider for lifespan data sheets that describe the gradual behavior of the lamps’ luminous-output degradation over time and under specific operating conditions.
Also ask them to provide test data that relates to the specific LED chip or die, or junction temperature, over the lifespan test period. Not all LED manufacturers publish such data, but such information can help you calculate a specific LED-based sign’s life.
In my next column, I’ll write more on finding application solutions, and LEDs’ thermal designs and analysis.
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