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There is no standardized method to determine lifetime, but for many electronic systems it can be and is typically estimated using the predicted lifetime of individual components at the anticipated operating conditions, which are then statistically combined. We do not have sufficient information today on all of the components and interactions of a luminaire to make these predictions. However, that should ultimately be our goal because for now, absent that data, the only remaining option to estimate system MTTF or B50 is to do a full LM-79 luminaire test on a population of product, which poses a conundrum. For many manufacturers a full LM-79 test may be too expensive and timeconsuming. We recommend the full LM-79 test to establish lifetime, taking into account all failure mechanisms, but to address this difficulty we also suggest a number of alternative LED Luminaire Lifetime Recommendations, June 2011 Page 7 approaches to provide some indications of reliable design, if not of true lifetime. (See Labeling Recommendations on page 27.) The remainder of this section describes additional aspects of some of the above contributors to product reliability.
RELIABLE DESIGN AND MANUFACTURINGFailures within an SSL luminaire often stem from at least one of four functional aspects of luminaire design and manufacturing: power management, thermal management, optical management, and luminaire assembly integrity.
Figure 2 provides an overview of a contemporary SSL luminaire and the relationships between the various components and materials and design elements.
FIGURE 2. SSL LUMINAIRE COMPONENTS AND RELIABILITY CONSIDERATIONS
SOURCE: APPALACHIAN LIGHTING SYSTEMS, INC.
Design goals and reliability impacts for each of these four functional aspects are described below.
Power Management – ensuring the power delivered to the LED package(s) is appropriately sized and filtered.
Thermal Management – ensuring that heat generated by the LED package(s) and the power system components is removed to minimize LED temperatures so as to maximize • LED performance and lifetime.
Design Goals: A reliable heat-conducting design, be it passive or active, is required to remove heat from the LED package and luminaire, and phosphor, if applicable. The design should assure that the LED package operates below a manufacturer-reported LM-80 measurement o
Optical Management – ensuring that light output from the LED package(s) is correctly and efficiently shaped and directed toward the desired surface.
Assembly Integrity – ensuring that the overall housing design and assembly process(es) provide for sufficient long-term protection from dust, moisture, vibration, and other • adverse environmental effects.
Design Goals: Luminaire housing design and materials must be designed to offer sufficient protection for the LEDs depending on the anticipated environment. Repairable designs should allow simple field replacement of any failed components without degrading the o integrity of the housing or other components.
Reliability Impact: For outdoor or harsh environment applications, housing failure can lead to catastrophic failure of critical light-producing components. In other cases, mechanical failure may result from insufficient protection for internal components. Any assembly o process bears the risk of occasional random manufacturing defect failures that will occur throughout the life of the product but should not seriously affect end-of-life wearout.
SSL luminaire failure modes are often related, e.g., improper thermal management can lead to premature LED lumen depreciation and/or optical degradation and/or power component failure.
The overall luminaire quality—and therefore the probability of satisfactory long-term luminaire performance—is directly related to careful, thoughtful, and integrated luminaire design, component selection, final assembly, testing, and packaging. Furthermore, well-documented installation instructions—and actual installation coordination for complex or large-scale projects—can also have a major impact on initial and long-term luminaire performance.
Even if the SSL luminaire is well designed to address all of the various failure modes, attention to proper manufacturing steps and quality process controls must be clearly documented and carefully LED Luminaire Lifetime Recommendations, June 2011 Page 9 executed. Any of the failure mechanisms inherent in electronic assemblies and other luminaire components may apply to an SSL luminaire.
Figure 3 shows the frequency of various field failure modes that have been documented for a family of outdoor SSL luminaires from a manufacturer’s installed base. For this example of a welldesigned set of products, the overall failure rate is very low and, interestingly, it depends only to a small extent on the LED packages. (Note, however, that this product line example has not yet reached the end-of-life wearout stage, so it cannot be said that LED failure will not eventually have a larger role.)
FIGURE 3. DISTRIBUTION OF FAILURES IN 34M OPERATING HOURS FOR A FAMILY OF
SOURCE: APPALACHIAN LIGHTING SYSTEMS, INC.
manufacturing defects. Such tests can help to uncover root causes of a product’s premature Over-stress testing is recommended as a useful approach for identifying design flaws or demise. Selection of such tests is beyond the scope of this guide, but it may be worthy of further study by the industry and a sharing of best practices to promote the overall market. This information would be especially helpful for smaller manufacturers lacking the means to do extensive reliability qualification.
Based on experience with the CALiPER 1 program, this guide recommends a minimum 1,000-hour burn-in (continuous use) test of a small number of products to verify that there are no serious, immediately apparent design flaws in a new platform. While this recommendation may be seen as a bit vague and in no way guarantees a good design, any failures that occur in this short period of time are a cause of concern that may warrant another look at the design before product release.
For more information on the CALiPER program, see www.ssl.energy.gov/caliper.html.1
LED Luminaire Lifetime Recommendations, June 2011 Page 10 END OF LIFE Defining and estimating end of life for LED luminaires is complicated by the phenomenon of longterm lumen depreciation. For conventional technologies, the “rated average lamp life” is the point at which half the lamps cease to emit light. All sources lose light output (depreciate) during the rated lamp life as defined by complete, “lights-out” failure of 50 percent of the population. However, a well-designed LED package or array typically would not fail entirely for a very long time.
Consequently, the rated life of an LED-based lamp or integrated luminaire can, in principle, be much longer than incumbent technologies. Whether or not this is true will depend on the behavior of other components of the luminaire. It could be that another subsystem, e.g., the driver, has a shorter life than the LED source and therefore controls the system lifetime. Figure 4 illustrates a simple example with two principal failure mechanisms having comparable median times to failure.
Suppose an LED driver with a failure rate normally distributed with a mean of 55,000 hours and a 10 percent standard deviation (SD) is coupled with an LED source (may be multiple LEDs) also with a normally distributed failure rate with a mean of 60,000 hours and an SD of 20 percent. “Failure” of the LEDs could be predominantly lumen depreciation of more than 30 percent, say, but there could also be other catastrophic failures of the source that contribute to this estimated MTTF. The resulting median failure rate (50 percent cumulative probability of failure due to either mechanism) of the lamp would then be about 52,000 hours, dominated by the driver but somewhat affected by the LED lumen depreciation.
LED Luminaire Lifetime Recommendations, June 2011 Page 11 The average light output of a population of such lamps would be a gradual diminution to perhaps 80 percent of the original light output due to LEDs, followed by a rapidly decreasing average light as the drivers begin to fail, extinguishing some lamps altogether. Because of the broader distribution for lumen depreciation, failures before 40,000 hours (few in number) will be mostly due to low light output. But by 60,000 hours, when only half of the LED sources have depreciated below 70 percent of initial light (assuming that is the dominant mechanism), over 90 percent of the lamps (system) will have failed due to the driver. This would be considered a well-designed system, and well-behaved in terms of failures. This behavior is not significantly different from that of most conventional technologies, although the times are generally longer.
In a realistic system, multiple failure mechanisms may need to be considered, the actual numbers may be very different from those in this example, and the distributions may be other than normal.
Yet in trying to describe lifetime, another wrinkle is that some LED products are being designed to maintain lumen output over time by gradually increasing the driver current to compensate for lumen depreciation. Eventual failure in that case would most likely be characterized by rapid lumen depreciation to below 70 percent once the driver is no longer capable increasing the drive current.
Because the light source may have appreciable lumen depreciation over its life, the effect of lumen depreciation may be more significant in a system where the driver can be replaced. LED lumen depreciation will continue even as the driver is changed out, so it is quite possible that mechanism will dictate when the entire system will need replacing (again, an L70 failure). This brings up the issue of serviceability of a fixture, discussed next.
SERVICEABILITY AND LIFETIMELED-based luminaires can, in principle, be grouped into two classes: those that are non-serviceable and those that can be serviced or repaired in the field. Lifetime, per se, is independent of serviceability, but the economics and ultimate replacement time for the entire fixture will be different between the two classes. Serviceability has not received a great deal of discussion in the industry, so there is no commonly accepted definition, which is itself application-dependent.
Nonetheless, the concept is useful, and so for purposes of this guide, the working definition of the term derives from what we perceive to be customer expectations: Simply put, a product is “fieldserviceable” if the job can be performed by the level of personnel that currently services the incumbent-technology lighting fixtures in the field for a given application. A luminaire that has replaceable parts but requires either very specialized skills for field servicing or must be returned to the manufacturer for service should be considered “non-serviceable.”
LIFETIME FOR NON-SERVICEABLE LED-BASED LUMINAIRES
An LED-based luminaire manufactured in a manner that it cannot be repaired in the field will “fail” upon failure of any part, and will require complete replacement. In general, this is contrary to
current expectations about lighting, and should be addressed on several levels:
It may not occur to customers that they may be buying a “non-serviceable” luminaire.
Replacement costs could be well above expectations, leading to a negative reaction to LED •
When a product is designed in a way that allows for field repair, a number of new questions arise
with regard to lifetime. The purchaser is strongly urged to ask them:
Which parts are replaceable? (E.g., driver, LED engine, optics, wiring cables) What are the expected lifetimes for the various replaceable parts and what is the •
How complex is the replacement? For example, is it necessary to disconnect the luminaire from the building structure or to disconnect power? Does it require a qualified electrician?
• Will the manufacturer have the replacement part when it is needed?
How does one determine when the LED light loss has reached the point that the source, and • perhaps the luminaire, is no longer usable?
• Under what conditions (and when) will the entire fixture require replacement?
Another facet of serviceability is the concept of backward compatibility. Having the option of • replacing a light source, for example, with a technologically upgraded version is quite attractive for the rapidly evolving LED luminaire market.
Regardless of whether or not the product design intent was field serviceability, it seems evident from the issues above that an important first step is for vendors to clearly indicate if the LEDbased luminaire is intended to be “field-serviceable” or “non-serviceable.” Currently, a number of LED module products or light engines are on the market, and work is under way to standardize the interfaces within a luminaire, but these are all works in progress and there are no standards for replaceable parts today—thus limiting the usefulness of field serviceability. This situation is likely to change over time, as standard drivers, light engines, interface standards, and other components evolve, leading to multiple sources of supply.
Further discussion of serviceability is encouraged to better define norms for various applications and ways to address the many questions that arise.
While there may be many failure mechanisms in a complex LED luminaire system, a few key issues are worth further discussion: the character and measurement of lumen depreciation, the behavior and specification of electronic drivers, and considerations regarding color changes as they apply to the useful life of a fixture. These are each considered separately below.