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1、Life of LED-Based White Light SourcesNadarajah Narendran and Yimin GuAbstractEven though light-emitting diodes (LEDs) may have a very long life, poorly designed LED lighting systems can experience a short life. Because heat at the p-n-junction is one of the main factors that affect the life of the L
2、ED, by knowing the relationship between life and heat, LED system manufacturers can design and build long-lasting systems. In this study, several white LEDs from the same manufacturer were subjected to life tests at different ambient temperatures. The exponential decay of light output as a function
3、of time provided a convenient method to rapidly estimate life by data extrapolation. The life of these LEDs decreases in an exponential manner with increasing temperature. In a second experiment, several high-power white LEDs from different manufacturers were life-tested under similar conditions. Re
4、sults show that the different products have significantly different life values.Index TermsDegradation, life, light source, white light-emitting diode (LED).I. INTRODUCTIONTHE interest for using light-emitting diodes (LEDs) for display and illumination applications has been growing steadily over the
5、 past few years. The potential for long life and reduced energy use are two key attributes of this rapidly evolving technology that have generated so much interest for its use in the above mentioned applications. Traditionally, the lamp life of light sources commonly used in illumination application
6、s is determined by subjecting them to a predetermined on/off cycle until half the number of light sources cease to produce light. Unlike these sources, LEDs rarely fail catastrophically; instead, their light output slowly degrades over time. Even if an LED is technically operating and producing ligh
7、t, at some point the amount of light produced by the LED will be insufficient for the intended application. Therefore, the life of an LED should be based on the amount of time that the device can produce sufficient light for the intended application, rather than complete failure. Based on this argum
8、ent, a recent publication from an industry group defines the life of an LED device or system for use in general lighting applications as the operating time, in hours, for the light output to reach 70% of its initial value .The most widely used white LEDs incorporate a layer of phosphor over a GaN-ba
9、sed, short-wavelength light emitter. Usually, the phosphor is embedded inside an epoxy resin that surrounds the LED die. Some portion of the short-wavelength radiation emitted by the LED is down-converted by the phosphor, and the combined radiation creates white light. Early white LEDs were packaged
10、 similar to the indicator-style colored LEDs, specically 5 mm and SMD (surface mount devices). Although these products demonstrated the concept of a white light source, they did not produce sufcient light for display and illumination applications. Furthermore, these indicator-style white LEDs had a
11、relatively short life, 500010 000h to reach 70% light level under normal operating conditions. To address the higher luminous ux requirements, manufacturers have started to commercialize high-power illuminator LEDs that are presently producing over one hundred times the ux compared to indicator-styl
12、e white LEDs. The higher light output is achieved by using larger dies, higher drive currents, and improved heat extraction methods . In addition, some manufacturers are using better encapsulants to improve the life of white LEDs .There are several studies that have investigated the aging mechanisms
13、 of GaN-based LEDs. During the 1990s, Barton et al. investigated the degradation of GaN-based blue LEDs and showed that light output reduction over time occurred primarily due to the yellowing of the epoxy surrounding the die . In 2001, Narendran et al. observed that indicator-style white LED packag
14、es degraded very rapidly, with the LEDs reaching the 50% light output level within 6000 h 4. In that same study, it was shown that the chromaticity values of the white LEDs shifted toward yellow over time, and it was speculated that the yellowing of the epoxy was the main cause for light output degr
15、adation . Therefore, based on past studies, the primary reason for the degradation of indicator-style white LED packages is the yellowing of the epoxy that is caused by excessive heat at the p-n-junction of the LED. Some of the newer illuminator-style white LEDs use encapsulant materials that have l
16、ower photodegradation characteristics, and therefore have a lower degradation rate. However, there are factors such as the degradation of the die attach epoxy, discoloration of the metal reflectors and the lead wires, and degradation of the semiconducting element that are influenced by heat, and these all contribute to the overall degradation of the white LED. Although the newer high-power white LEDs would have a lower degradation rate compared to the early indicator-styl
