电子信息工程本科毕业中英文翻译.doc

英语原文：Life of LED-Based White Light SourcesThe interest for using light-emitting diodes (LEDs) for display and illumination applications has been growing steadily over the 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 applications 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 light, at some point the amount of light produced by the LED will be insufcient for the intended application. Therefore, the life of an LED should be based on the amount of time that the device can produce sufcient light for the intended application,rather than complete failure. Based on this argument, a recent publication from an industry group denes 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-based, 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 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 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-style 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 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 packages degraded very rapidly, with the LEDs reaching the 50% light output level within 6000 h. 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 degradation. 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 lower 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 reectors and the lead wires, and degradation of the semiconducting element that are inuenced 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-style devices, it is the heat at the p-n-junction that most inuences the degradation. The heat at the p-n-junction is caused by the ambient temperature and the ohmic heating at the bandgap.As stated earlier, long life is one key feature of LED technology that has attracted so many end-use communities. To benet from the long-life feature, it is the nal system that has to operate for a long time, not just the individual LED. As noted in past studies, heat at the p-n-junction is one of the key factors that determine the life of the white LED. Therefore, if systems are not properly designed with good thermal managemen techniques, even if they use long-life white LEDs the life of the nal system would be short. Developing the relationship between junction temperature and life would be very useful for producing long-life systems.Although there are different methods available for estimating the junction temperature of LEDs, they are not very convenient,especially once the LEDs are integrated into a system . Furthermore, these methods are not direct; consequently, they are prone to erroneous results. Alternatively, it is much more convenient and direct to measure the heat at a location external to the LED package that is sufciently close to the junction and where a temperature sensor can be directly attached. The temperature of this point should have a good relationship to the junction temperature. The point where a temperature sensor can be attached for this measurement could be the lead wire (cathode side) for the indicator-style LEDs and the board for high-power LEDs. Most manufacturers can recommend such a point,and we refer to this as the T-point in this manuscript.Since white LEDs in the marketplace are packaged differently, their ability to transfer heat from the die to the surrounding environment is different from product to product. Therefore, it is reasonable to assume that different products have different degradation rates as a function of heat. A graph that shows the life of the LED as a function of T-point temperature is extremely useful for system manufacturers to build reliable, long-lasting systems. By knowing how much impact heat has on the degradation rate or life of the LED, the system manufacturer can select components and drive parameters, including the amount of heat sink and drive current, for a product being designed for a given application.Therefore, the objective of the study presented in this manuscript was to investigate the relationship between the T-point temperature and life of a white LED. A second objective was to understand the degradation rate of different high-power white LED products presently available in the marketplace.To understand the relationship between the T-point temperature and life, one type of high-power white LED that is commonly available in the marketplace was selected. Several of these LEDs were subjected to a life test under different ambient temperatures. The details of the experimental setup are described in the following paragraphs.Because the different LED arrays have to operate at a particular ambient temperature, the arrays were placed inside specially designed, individual life-test chambers. The test chambers had two different functions: 1) to keep the ambient temperature constant for the LED arrays and 2) to act as light-integrating boxes for measuring light output. Each individual LED array was mounted at the center of the inside top surface of a life-test chamber. A photodiode attached to the center of the left panel continuously measured the light output.A small white bafe placed over the photodiode shielded it from the direct light, allowing only the reected light to reach the photodiode. A resistance temperature detector placed on top of the bafe measured the chambers ambient temperature and controlled the heater that provided the necessary heat to the chamber through a temperature controller. The temperature in-side the box remained within 1. The heater was attached to a raised aluminum plate with a matte-white cover that sat on the chamber oor. The temperature was estimated using a J-type thin wire thermocouple soldered to the T-point of white LED. For each chamber, an external LED driver controlled the current ow through the LEDs. All life-test were placed inside a temperature-controlled room. The life-test chambers were staggered vertically and horizontally to ensure that heat rising from the bottom chambers did not affect the chambers above them.The results of this study underscore the importance of packaging white LEDs using proper thermal management to maintain light output, and thereby extend system life. Heat at the p-n-junction is one of the main factors that affect the life of white LEDs. Therefore, knowing the relationship between life and heat would be very useful for manufacturers who are interested in developing reliable, long-lasting systems.Results from the rst experimentconducted under various ambient temperatures to understand the relationship between T-point temperature and lifeindicate that life decreases with increasing temperature in an exponential manner. Results from the second experimentconducted to understand how different commercial white LEDs perform under identical operating conditionsshow a large variation in life among the different packages, indicating that the packages used different heat extraction techniques and materials. As part of ongoing research, we hope to further investigate how the different commercial LEDs are affected by heat and nally develop a family of curves that illustrate the relationship between life and T-point temperature for the different products. 中文翻译：基于LED的白色光源的寿命在过去几年中利用发光二极管(led)显示和作为照明应用的技术一直在稳步增长。较长的使用寿命和节能这两个关键属性使这一快速进化的技术已经对上述应用产生了如此巨大的利益。传统上，在照明应用中常用的光源灯泡寿命取决于他们到预定开/关循环直至一半光源停止产生光。和这些光源不同，LED很少如此轻易损坏；相反，它们的光输出随着时间的推移慢慢降低。即便LED在操作和产生光的技术上有了很大进步,但在某种程度上LED所产生的光量并不能满足特定应用的需求。因此,一个LED的寿命长短应该是基于该设备可以为所需要求产生足够的光的时间的长短,而不是彻底的损坏。基于这个观点,最近出版行业组织的定义规定适用于通用照明应用中的LED设备或系统的寿命是直到工作几个小时后能达到初始值的70%的光输出。最广泛使用的白色LED是将一层荧光粉和GaN基封装在一起的短波光线发射器。通常把混有荧光粉的环氧树脂覆盖在GaN基模具上。LED基片发出的蓝光部分被荧光粉吸收，另一部分蓝光与荧光粉发出的黄光混合，可以得到得白光。早期的白色LED封装类似于彩色led指示灯,特别是5毫米和SMD(表面贴装设备)。虽然这些产品展示了白色光源的概念,但它们没有发出足够的光来显示和应用于照明程序。此外,这些指示灯式的白色LED寿命相对短暂,只能用5000-10000小时，正常工作时光线输出就只剩初始值的70%了。为了解决更高的光通需求,制造商已经开始商业化生产大功率led照明灯了。目前已经能批量生产超过指标式白色LED一百倍光通以上的白光LED了。更高的光输出是通过使用更大的模具,更高的驱动电流,改善热提取方法提取方法实现的。此外,一些制造商为了改善白光LED的寿命使用了更好的密封材料。有多项对GaN基LED老化机制的研究。在20世纪90年代,巴顿等人研究的GaN基蓝色LED降解实验表明光输出随时间减少的主要原因可能是由于周围的环氧模具泛黄。2001年,Narendran等人发现指标式白光LED封装降解非常迅速,LED光输出水平在6000小时内就只剩下初始值的50%,而且在同一研究中显示白色发光二极管的色度值转向泛黄,有人推测可能是黄色的环氧树脂为光输出退化的主要原因。因此,根据过去的研究,指标式白光LED封装退化的主要原因是环氧树脂变黄，造成过多的热量在LED的PN结。一些较新的照明式白色LED使用的封装材料有较低的光降解特征,因此有较低的降解率。然而,有很多因素如退化的环氧树脂模具、变色的金属反射镜和导线,和退化的半导体元素等都影响了LED的散热。这些都将促使白光LED的整体退化。尽管新的大功率白色LED比早期的指示灯式设备降解率更低,但它的PN结高温最能影响降解。PN结过热在带隙引起的环境温度变化和电阻变化。如前所述，寿命长的LED技术是一个关键特性，已经吸引了这么多的最终用途社区。受益于长寿命的特点，最终的系统有一段很长的运作时间。在过去的研究指出，在pn结的热量是确定的白光LED的寿命的关键因素之一。因此，如果系统不妥善设计具有良好的散热技术，即使他们使用长寿命的白光LED，最终系统的寿命仍将是短期的。研究基点的交界处的温度和寿命的关系，将对生产长寿命的系统非常有用