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LED LUMEN DEPRECIATIONFor an individual LED package, lifetime has typically been considered to be the hours of operation at which the light output has fallen to 70 percent of its original value (L70). LED useful life is usually then reported as the median time to failure of a population of diodes under normal operating conditions, called “B50.” In other words, after this period of time, half of the units will fail due to low light output. While B50 represents a time interval, L70 is the lumen performance level defining a lowlight failure. For some applications, B50 may be unacceptable; designers in these cases might prefer to know when 10 percent of the product has fallen below the defined level. Depending on the target market, therefore, manufacturers may choose to report B50, B10, or some other time for a particular.
L70 is widely accepted in LED lighting, but for non-demanding cases L50 may be acceptable, while in other cases a 30 percent depreciation would be considered too much.
In order to design and specify LED fixture performance effectively, LED fixture manufacturers need performance data from the LED and driver manufacturers. One important data set LED manufacturers will provide is collected per IES LM-80-08 Approved Method: Measuring Lumen Maintenance of LED Light Sources. LM-80 prescribes uniform test methods under controlled conditions for measuring LED lumen maintenance and color shift while controlling LED case temperature (TS) using continuous mode operation for a specified minimum duration. LED fixture manufacturers can then correlate the LM-80 data to the LED TS measured in situ during their LED fixture thermal testing, to predict LED lumen maintenance when installed in the fixture and to assess the degree of potential color shift for their specific LED operating parameters. Note that these results only provide estimates of LED lumen depreciation under specified conditions; they are not sufficient alone to estimate the fixture lifetime, for the reasons outlined above. Apart from entirely independent failure mechanisms other than lumen depreciation, LM-80 data do not take into account any interactions between LEDs in the fixture and other materials or components. For example, if the optics should yellow or otherwise degrade over time from environmental effects, the apparent lumen depreciation of the luminaire would be faster than that of the LEDs alone. If the driver current increases over time, by design or otherwise, the LEDs may eventually be driven harder than the specified LM-80 test current, which will change the numbers.
LED DRIVERS AND CONTROLSLike other parts of the lighting system, the devices and components used to convert line power to direct current suitable to drive and control LEDs affect lifetime and reliability. Capacitors, inductors, transformers, opto-isolators, and other electrical components all have different design lifetimes, are affected by operating and ambient temperature, and are vulnerable to electrical operating parameter variations from surges, spikes, and so forth. An effective LED systemreliability evaluation must take all of these aspects into consideration.
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TYPES OF POWER CONVERSIONIn most lighting systems today there is a need to convert alternating current (AC) power into direct current (DC), incorporate control instructions, and regulate the output power. Based on the grouping of these functions, the devices can be separated into a few broad categories: 2 LED drivers, systems of power supplies and LED control circuitry, and AC LEDs.
LED drivers convert a range of high-voltage AC inputs, and produce either constant-voltage or constant-current for an LED or an array of LEDs in a single device. LED drivers are relatively straightforward to evaluate because the performance, reliability, and conditions of use are consolidated. However, options may be limited, rated power may be constrained, and the physical size of the device may affect LED luminaire design.
A system of AC-DC power supplies and LED controllers offers more flexibility but requires a more complicated analysis. In this case, a common AC-DC power supply provides the power conversion while the LED control circuitry typically incorporates control instructions and regulates the output power. AC-DC power supplies are available in a wide variety of sizes, power levels, and reliability/lifetime ratings. LED control circuitry is available in many configurations and can either be incorporated onto the LED array or exist as an independent device. 3 The term “AC LED” typically refers to a package-level device into which integrates power conversion into the package, thus requiring few additional components. The specific design varies by manufacturer.
The LED driver consists of a power source and LED control circuitry designed to operate an LED package (component), or an LED array (module) or lamp. The specific choice of design topology affects the number and type of components, as well as the degree to which they are stressed. Many reliability issues come down to component stresses as determined by the method of converter operation—hard-switched mode, quasi-resonant or fully resonant, etc.
In general, product lifetime decreases as temperature increases. Product temperature is a function of power dissipation, thermal resistance, and ambient temperature. Power dissipation is directly related to the efficiency of the driver. Higher efficiency and low product thermal resistance can reduce the product operating temperature and improve the lifetime significantly.
To protect the application from various hazards such as over-current, surges, shorts, and high temperatures, protection circuits are needed, but they add more potential sources of failure. Many For definitions, refer to IES RP16-10, www.ies.org/store/product/nomenclature-and-definitions-forilluminating-engineeringbr-rp1605-1013.cfm, or UL 8750, www.ul.com/global/eng/pages/offerings/industries/lighting/lightingindustryservices/standards/ul8750/.
In the electronics industry, a microchip that performs the actual power regulation is also called a driver, 3 which may lead to confusion. Definitions here refer to IES RP16-10.
LED Luminaire Lifetime Recommendations, June 2011 Page 15 components affect driver safety, so they must be selected to comply with several standards, 4 as discussed in the next section. The electrolytic capacitor is probably the shortest-lived component for most LED drivers. The typical life of an electrolytic capacitor is cut to half for every 10°C temperature rise. Depending on the application and working environment, a long-life and highquality electrolytic capacitor should be selected to meet reliability, cost, and performance requirements. Other components also need proper derating with respect to both biasing and temperature to ensure reliable driver performance.
To assist in luminaire design, the driver product qualification should demonstrate the product’s robustness to temperature, humidity, temperature cycle, shock, and vibration stress. A test report should include changes in efficiency and output current over time along with details on test conditions, sample size, and confidence interval.
Component and assembly-related defects are hard to avoid on volume production, but their impact may be reduced by using a burn-in process to identify and eliminate the early failures. The details and stresses (e.g., high temperature, temperature or power cycling, or vibration) depend on the design and degradation mechanisms. Ongoing reliability testing should be done at the predetermined intervals to detect the process and design margin drift, and to uncover the problems with components or workmanship.
With the right topology, thermal design, component selection, and derating, along with product burn-in, the LED driver can avoid becoming the “weakest link” of the lighting system, although for cost-effectiveness it may be intentionally designed either to define an appropriate overall system life or to be replaceable, as discussed above. As one might surmise, the exact test protocol for a driver is design dependent and can be extensive. The specific choice is beyond the scope of this guide, but these comments should guide lighting manufacturers in directions that should be considered to achieve an overall reliable end product.
STANDARDS, REGULATIONS, AND PROTECTION
Depending on the type of power conversion utilized for the LED system, there are several areas of regulatory compliance that may be applicable. Because of the variety of system configurations, often the combination of a variety of component types, it is likely that the overall compliance requirements will need to be separately addressed for each individual component. Exactly what is required of each subsystem or component is defined by the luminaire system designer, and it is important that these requirements are accurately communicated to their respective suppliers so that proper regulatory compliance is assured.
A number of standard requirements apply to driver use: first and foremost, the system needs to be evaluated against Risk of Fire and Risk of Shock (i.e., Safety Compliance). In the U.S., UL 8750, the Standard for Safety of Light Emitting Diode (LED) Equipment for Use in Lighting Products, was For example, EMI/EMC (FCC Part 15; EN61547 Immunity), Safety (UL 8750; IEC 61347-1, IEC 61347-2-13), 4 RoHS/WEEE, ESD and Surge (IEC 61000-4-2; IEC 61000-4-5), RF (IEC 61000-4-6). Other IEC and UL standards provide for dielectric voltage withstand test “Hi-pot” and degrees of protection provided by enclosures (IEC 60259) as required by the application.
Systems typically require evaluation to ensure compliance with electromagnetic compatibility (EMC) industry standards. Depending on the anticipated useful life, criticality of use, and replacement cost of the system, additional protection of sensitive components may be warranted and must be evaluated. For the most part, these requirements are not unique to LED systems, although they may be meaningful differentiators when comparing LED system components.
Examples might be protection for network communications built into the lighting systems, reduced power for operation during periods of high temperatures, and so forth.
Apart from these safety and protection issues, however, luminaire manufacturers strongly recommend that the industry develop standardized ways to characterize drivers for use in SSL.
The concern is that because of different means of reporting, it is difficult for lighting manufacturers to compare products and choose the appropriate one for their design. If possible, a standardized test and data set should be devised for driver component degradation and failure rates. 5 Such data might then, together with LED data, allow an estimate of overall system reliability at least for failures or degradation of these two critical components.
The best way to find the optimal solution for an LED system is through careful specification of the realistic operating parameters. These specific choices will influence system reliability as well as
cost, so there will be design tradeoffs. A representative set of parameters would include:
Input power/voltage range Operating parameters (operating temperature and humidity, dry/damp/wet or IP rating) •
Luminaire system lifetime, as recommended by this guide, refers only to lumen output of the fixture, but it includes failures due not only to systematic degradation of LED output as measured by LM-80, but also to any other mechanisms of overall lumen degradation, encompassing changes and complete failures in components other than LEDs or through interactions with the LEDs. As defined, therefore, “end of life” does not take excessive color shift into account, even though for some applications the user might consider that a failure. The decision to emphasize lumen output reflects For further discussion of this issue, please refer to the latest edition of the DOE SSL Manufacturing R&D 5 Roadmap at http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_manuf-roadmap_july2010.pdf.
LED Luminaire Lifetime Recommendations, June 2011 Page 17 the fact that lumen maintenance is related to safety issues in various applications, while color stability is related to aesthetic concerns. Additionally, system color shift is difficult to define, measure, and project. While great progress has been made by the LED community to improve color stability, this reality could, nevertheless, result in customer dissatisfaction, so we discuss color shift in this section of the guide.
LM-80 recognizes that color is important 6 and, further, requires that the test report include “chromaticity shift reported over the measurement time.” 7 It does not, however, provide any recommendation to project the shift to the end of life, nor does it address color shifts that may be attributable to the luminaire design or manufacturing. Moreover, there is no consideration of color shift effects in remote phosphor configurations. All of these items would seem appropriate for additional study and possible standards in the future.
Experiments suggest that, assuring that the temperature of the LED does not exceed certain limits and that the drive current does not change excessively, it is possible to extrapolate the LED lumen maintenance contribution to lumen depreciation to the luminaire. However, while the IES TM-21 effort to address this issue has suggested means to project limited experimental measurements to the end of life, it does not offer any suggestions as to how to project color shift. Practically, this approach may not work for color shift. While a single test of color shift is not particularly expensive, assembling sufficient data on a large enough product sample to characterize color shift accurately can be prohibitive both in terms of the time required and the resulting total cost.
Color stability, like lumen depreciation but to an even greater extent, is not exclusively determined
IMPACT OF LUMINAIRE DESIGN AND MANUFACTURING PRACTICESby the performance of the LED. Examples of how luminaire design and manufacturing practices will
impact color quality and color shift include: