Abstract
The advantages of light emitting diodes (LEDs) over previous light sources and their continuous spread in lighting applications is now indisputable. Still, proper modelling of their lifespan offers additional design possibilities, enhanced reliability, and additional energy-saving opportunities. Accurate and rapid multi-physics system level simulations could be performed in Spice compatible environments, revealing the optical, electrical and even the thermal operating parameters, provided, that the compact thermal model of the prevailing luminaire and the appropriate elapsed lifetime dependent multi-domain models of the applied LEDs are available. The work described in this article takes steps in this direction in by extending an existing multi-domain LED model in order to simulate the major effect of the elapsed operating time of LEDs used. Our approach is based on the LM-80-08 testing method, supplemented by additional specific thermal measurements. A detailed description of the TM-21-11 type extrapolation method is provided in this paper along with an extensive overview of the possible aging models that could be used for practice-oriented LED lifetime estimations.
Highlights
The typical failure mode of light emitting diodes (LEDs), unlike fluorescent and incandescent light sources, is not catastrophic failure
The most apparent manifestation of LED aging is the luminous flux decrease, or seen from another perspective, to what extent the initial value of the emitted total luminous flux of an LED package is maintained. The IES LM-80 family of test methods have been developed and used in the SSL industry to measure LEDs’ luminous flux maintenance
LEDs was unfortunate but not unexpected; in only a few cases the testing junction temperature of the LEDs was close to the allowed maximum value, but in most cases it was far above that
Summary
The typical failure mode of LEDs, unlike fluorescent and incandescent light sources, is not catastrophic failure. The total luminous flux of solid state light sources (SSL) experiences a continuous decrease with the elapsed operating time. (sloppy, every day terminology to denote this property of LEDs is called ‘lumen maintenance’ that we shall rigorously refrain from using; instead, we shall refer to this LED property as ‘luminous flux maintenance’.) The IES LM-80 family of test methods have been developed and used in the SSL industry to measure LEDs’ luminous flux maintenance. Besides luminous flux measurements, measuring the continuously shifting forward voltage may be part of the life tests but it is still not required in LM-80 tests and such measurements or the results reporting is often omitted. The maximum allowed depreciation of the total luminous flux depends on the exact field of the application; the end Energies 2020, 13, 3370; doi:10.3390/en13133370 www.mdpi.com/journal/energies
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