SEMICONDUCTORS
NewchalleNges iN specifyiNg leD lumiNaire lifetime
Is LEDs’ promise of long life and maintenance-free operation just a myth? LEDs are like any other semiconductor component – they can fail By FrançoisMirand, Future Lighting Solutions
Luminairemanufacturers, lighting designers and specifiers have generally been reassured by the claims fromLEDmanufacturers that their devices will typically operate for 50,000 hours. But in fact, the claims of long operating life for LEDs, as for any component, only apply under certain conditions.Moreover, LEDs can and do fail in rare cases and so can other components of the system. So is LEDs’ promise of long life andmaintenance-free operation just amyth?
This article intends to help purchasing professionals
understand the most important factors that affect the operating lifetime of a solid-state lighting (SSL) system. This system includes not only the LED, but also the control circuitry, power supply and optical and mechanical components. It will show that luminaire manufacturers can rate their products for long life with confidence - but only if the luminaire customer takes a system- wide view of the reliability issue.
evolving concepts for leD reliability In the early days of power LEDs, themost frequently quoted lifetime was 100,000 hours. However, no one could really explain what was behind thismagic number; it was probablymore to do withmarketing than science.
In 2003, Philips Lumileds published the first document
(Reliability Datasheet RD25) to explore the subject in depth, highlighting the key factors affecting long-termLED performance, such as junction temperature and drive current and quoting real- life numbers for its own devices.
Later, in 2007, Philips Lumileds proposed a new graphical data
set, known as Bxx, Lyy graphs, to help lighting designers predict power-LED lumenmaintenance in different operating environments.
Leveraging this data, Future Lighting Solutions introduced its
online LED Reliability Tool (
www1.futurelightingsolutions.com/lrt), which produces the Bxx, L70 graph for all types of Philips Lumileds power LEDs and all operating conditions; it also offers a probability distribution selected by the user.
16 | September 2010
standards Lumenmaintenance is now recognized as an important property of power LEDs, and in 2008 the Illuminating Engineering Society (IES) established the LM-80-08 standard entitled “ApprovedMethod: Measuring Lumen Maintenance of LED Light Sources”.
Future Lighting Solutions’ LED Reliability Tool
The U.S. government’s Energy Star programuses LM-80-08 as a
reference in its certification of luminaires. Energy Starmanufacturer’s guide setsminimumlumenmaintenance thresholds at a 6,000 hour measured data point. Philips Lumiledswas again the first manufacturer tomake an LM-80-08 test report publicly available for its LUXEON Rebel Illumination range of LEDs.
Beyond lumenmaintenance The concept of lumenmaintenance, then, has become entrenched to the point atwhich, today, LED reliability is generally thought to be defined by lumenmaintenance. This is, at best,misguiding and atworst extremely damaging to LED luminairemanufacturers.
First, LEDs have other failuremodes than lumenmaintenance:
catastrophic failure and color deviation over time can both be observed in populations of LEDs. Second, the LED is only one among many components in an SSL fixture, and each of these components has the ability to causemalfunctions or failure of the luminaire.
Because LEDs operate so differently fromtraditional incandescent
light sources, it is a commonmyth in the lighting industry that LEDs do not suffer “catastrophic” failure - that is, suddenly go completely dark - in theway incandescent lamps do. In fact, LEDs are like any other semiconductor component in this respect: albeit at very low rates, LEDs do fail catastrophically.
An SSL systemdesigner should be able to calculate the
catastrophic failure rate of the LED he/she is using under given operating conditions; and should adopt design techniques that
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