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LED LUMINAIRE LIFETIME:
Testing and Reporting
Product Quality Initiative
Next Generation Lighting Industry Alliance
U. S. Department of Energy
Understanding SSL Luminaire Lifetime
What This Guide Is and Is Not
Failure, Reliability, and Lifetime
Reliable Design and Manufacturing
End of Life
Serviceability and Lifetime
Lifetime for Non-Serviceable LED-Based Luminaires
Lifetime for Serviceable LED-Based Luminaires
Key Issues for Reliability and Lifetime
LED Lumen Depreciation
LED Drivers and Controls
Types of Power Conversion
Standards, Regulations, and Protection
Relation of Color Shift to Lifetime
Segmentation of the Luminaire Market
Standards and Measurement Work
Specifying and Demonstrating Lifetime
Determining and Maintaining Specified Lifetime
New Platform Lumen Maintenance
Product Variation of New Platform
Design for System Reliability
The Role of Warranties
Recommended Further Reading
UNDERSTANDING SSL LUMINAIRE LIFETIMESurprisingly to many, the true reliability and lifetime of light-emitting diode (LED) lighting systems is generally not known. Even worse, lumen maintenance values of LED devices are widely used as a proxy for the lifetime of an LED lighting system, which is misleading since lumen maintenance is but one component of a luminaire’s reliability. In fact, quite often the lifetime of a well-designed and manufactured luminaire is not determined by LED lumen depreciation. For many manufacturers estimating the luminaire lifetime using LED lumen maintenance, results can be ascribed to dependence on readily available numbers without developing actual luminaire data. In many cases, neither product providers nor customers are aware of the differences, perhaps in part because the problem has not been sufficiently explored and communicated.
It isn’t just about the LED. Good LEDs can be incorporated into poorly engineered products and turn the Methuselah of lighting into the exponent of “live fast, die young.” The promise of LED lifetime is often presented in terms of hours and years but with little background data. Warranties as well may be misstated because of this lack of data, at the manufacturer’s peril. The statement “100,000 hours of LED luminaire lifetime” is gradually giving way to the realization that there is little consistency, very little published data, and few hard facts around so-called luminaire lifetime numbers. The situation is better at the LED package level, where reputable manufacturers have thousands of hours of data under varying conditions. But this is not enough.
To manufacturers and specifiers in the solid-state lighting (SSL) community, the dawning realization is that we need to work together toward understanding the issues surrounding true lifetime and reliability. We need to begin by cataloguing failures and developing good models for underlying failure mechanisms. This process of understanding and explanation is very common in technological progress—steam engines existed long before deep comprehension of thermodynamic processes. With LEDs, we have a substantial head start on the underlying physics, many years of experience in both lighting and semiconductors, and reliability of related products.
As stated in the first edition of this guide, released in June 2010, there is no reason not to begin this journey and every reason to start. Now, one year later, we continue to pursue better reliability methods and metrics and explore the underlying root causes of failure.
LED Luminaire Lifetime Recommendations, June 2011 Page 4 recommendations for their work in supporting the needs of the SSL community. These organizations will ultimately determine the details of the methods to measure and report the reliability of SSL luminaire products.
This guide covers only luminaire lifetime—i.e., changes over time —and does not address initial performance criteria or product consistency. Initial performance criteria for LED luminaires have been separately discussed in the December 2008 publication Reporting LED Luminaire Product Performance, found on the DOE SSL website at http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/led_productperformanceguide.pdf.
FAILURE, RELIABILITY, AND LIFETIME
“Reliability” and “lifetime” are not synonyms. They are two separate and equally useful values reported by a component or subsystem manufacturer. A “failure” is an event which ends the life of a specific product or component, but may need definition if that end is not immediately evident, as is the case with lumen depreciation. Usually a luminaire or lamp design will encompass a number of interdependent components and subsystems, each with different lifetime and reliability values. It is not normally appropriate to use the worst or best case of these values; rather, the system needs to be evaluated as a whole because there can be inter-device effects (such as thermal impacts) that need to be taken into account.
Lifetime is an estimate of how long any single product is expected to operate as intended, given a specific set of environmental and mechanical requirements. Intuitively, we understand a luminaire’s “lifetime” or “end of life” to be when it no longer emits light. For conventional lighting technologies, the “rated life” of a lamp, for example, is usually considered to be the time when half of the lamps have failed (B50). However, we’ve learned that LEDs fade over time, and so we’ve modified our definition to mean “when there’s no longer enough light,” sometimes defined as “useful life.” But sometimes only lumen depreciation is considered, and then, often, only the lumen depreciation of the LEDs is considered in estimating useful life of the luminaire product. Each is a problem, since failure or degradation of drivers, optics, or other components can lead to either total failure, in the traditional sense, or accelerated lumen depreciation of the LEDs. Lifetime does not consider repair or replacement, either of premature failures or in the course of normal maintenance of a serviceable system, although the ability to service can be a valuable attribute of a product.
It is also important to appreciate that insufficient or no light output is not the only reason a product may no longer be acceptable. Other reasons include excessive color shift or changes in light distribution due to failure of some but not all of the LEDs. Whether or not to include these additional failures in defining lifetime can be a difficult question. In the first edition of this guide, the recommendation was to consider only light output in defining lifetime, but to consider all sources of diminution or failure of light output when arriving at the number. That remains our position but we will address the color shift issue, in particular, in more detail in this document.
Failure, as noted above, is an event pertaining to a specific unit of a product or component. Failure of any part may lead to failure of the whole. Some performance degradation of a driver, even short of ending that component’s life, could, however, result in failure of the LEDs. Such interactions among components need to be considered in estimating system failure rates (reliability). Because LED Luminaire Lifetime Recommendations, June 2011 Page 5 the LEDs themselves are expected to have a long, useful life, all other components, adhesives, and materials should also be long-lived, at least to the extent they do not result in an inappropriately shortened life. This last phrase is key: While it may be possible to design the other components to have equally long lives, or even longer lives than the LEDs, that may not be the most cost-effective solution for the application. The underlying objective of designing for long life is identifying the appropriate balance between statistical certainty and cost. If cost were unconstrained, an extremely long-life, highly reliable system could be designed—although, even in this case, random failures would be expected. Practically, the luminaire should be designed so that there is an acceptable level of failure over the anticipated lifetime of the system for a reasonable cost. Above all, it is important that the claims match the performance and that customer expectations are met.
Reliability, as defined by the Institute of Electrical and Electronic Engineers (IEEE) and others, is the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often reported as Mean Time Between Failures (MTBF) as distinguished from Mean Time To Failure (MTTF). (When the failure times are normally distributed, the mean and median [B50] times to failure are the same, but for other distributions they may be different.) MTBF is an especially useful measure when the system is repairable, as it will determine the maintenance interval. The average time of random failure is calculated by dividing MTBF by a population size. For example, if there are 1,000 devices with an MTBF of 100,000 hours, it is expected that there will be a random failure every 100 hours.
For electronics, the performance of a system composed of a number of components is typically characterized by an initial high failure rate (infant mortality period), followed by a long period with a low rate of random failures, concluding with a high failure rate at the wearout period or end of life. The random failures between initial failures and end-of-life wearout are essentially determined by the tails of these distributions. Figure 1 shows what is commonly known as a “bathtub” curve depicting that behavior.
FIGURE 1. THE “BATHTUB” CURVE ILLUSTRATING TYPICAL FAILURE BEHAVIOR OVER
TIME OF AN ELECTRONIC SYSTEMLED Luminaire Lifetime Recommendations, June 2011 Page 6 A similar behavior might be expected for a luminaire system, but because LED technology is still rather new, we should also be conscious of another contributor to failure: design flaws. Though not really a part of estimating product lifetime, design flaws are a reality of the current state of the art that needs to be addressed. Thus, we have divided reliability issues for discussion in this guide into
three main categories:
1. Design flaws. As the first LED lighting products appeared on the market, many design flaws were evident. The most common, initially, was poor design (or no design) of heat removal from the chips. This problem often resulted in overheated chips for which the luminous output depreciated quickly, leading to short-lived products. It was further exacerbated by claims that were essentially based on the LED lumen depreciation data. Design flaws, while perhaps less common and more subtle than they were only a short time ago, still exist: poor thermal management; using incompatible chemicals which degrade the optics or the chips;
poorly matching the driver to the LED requirements; overdriving the chips; poor seals allowing moisture penetration, and so forth. Problems of this sort should largely diminish as designers become more familiar with the technology, so that claims more closely match performance. Choices of drive current and operating temperature, especially, will affect the design life of a product—an important concept that is appropriate engineering for cost control. Slipshod design leads to unpredictable design life; a product with a predictable design life that is advertised accurately and appropriately priced will satisfy a customer need far better than one with excessive and almost surely unmet claims of “lasting forever.”
2. Manufacturing defects. These will always be with us. Even with a well-designed product, excursions from process control occur from time to time. Usually, these defects result in early failures. They may be partially covered by warranties, but that may still be unsatisfactory if the incidence of failure is too high. At present this does not seem to be an overly serious issue, at least with the major manufacturers, but as the volumes rise and as less experienced manufacturers enter the market, it is important that close attention be paid to quality controls. Factory testing and “burn-in” can also help. These can lead to additional costs, but can also minimize the customer seeing the early failures.
3. End of life. A well-designed system operated under normal conditions within specifications will, nonetheless, eventually fail. There may be two or three critical modes of failure that eventually make the system unusable. With well-made components, the time of this failure should be fairly predictable, at least within a range, and this is what lighting users have come to expect is the “lifetime” of the luminaire. Understanding how to evaluate a system and predict end of life accurately is very important for market acceptance of solid-state lighting. It is the focus of the discussions in this guide.