电子信息工程本科毕业中英文翻译

英语原文：

Life of LED-Based White Light Sources

The 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 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 argument, 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-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, specifically 5 mm

and SMD (surface mount devices). Although these products demonstrated the

concept of a white light source, they did not produce sufficient light for display

and illumination applications. Furthermore, these indicator-style white LEDs

had a relatively short life, 5000–10 000h to reach 70% light level under normal

operating conditions. To address the higher luminous flux requirements,

manufacturers have started to commercialize high-power illuminator LEDs that

are presently producing over one hundred times the flux 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

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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 attaché 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-style

devices, it is the heat at the p-n-junction that most influences 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 benefit from the long-life feature, it

is the final 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 final 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

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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 sufficiently 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

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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

baffle placed over the photodiode shielded it from the direct light, allowing only

the reflected light to reach the photodiode. A resistance temperature detector

placed on top of the baffle measured the chamber’s 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 floor. 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 flow 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 s from the first experiment—conducted

under various ambient temperatures to understand the relationship between

T-point temperature and life—indicate that life decreases with increasing

temperature in an exponential manner. Results from the second

experiment—conducted to understand how different commercial white LEDs

perform under identical operating conditions—show 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 finally develop a family of curves

that illustrate the relationship between life and T-point temperature for the

different products.

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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。 5

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。

虽然有不同的方法估算LED的结温，他们是不是很方便，尤其是当发光二极管集成到一个系统。此外，这些方法不是直接的，因此，他们容易产生错误的结果。更方便的直接测量热量的方法是利用一个足够接近路口的温度传感器，这个传感器可以直接连接到LED封装外部位置。这一点的温度应该有一个良好的合作关系结温。利用温度传感器可以测量这个连接点。这个连接点可能是引线（阴极侧）为指标式的LED和高功率LED板。多数厂家可以引进我们所说的这个论文中所指的T点。

由于白光LED在市场上是不同的包装，从产品到产品的能力，从模具转移到周围环境中的热量是不同的。因此，不同的产品有不同的降解率作为热源的功能这个假设是合理的。这里有一个显示作T点温度和LED的使用寿命关系的图表，对建立可靠长期的系统的系统制造商非常有用。知道多大影响热降解率或LED的寿命，系统制造商可以选择的组件和驱动器参数，包括散热器和驱动电流的量，为特定应用而设计产品。

因此，在这个论文的研究目的是调查的T点的温度和白光LED的寿命之间的关系。第二个目标是要了解不同的高功率白光LED产品目前在市场上提供的降解率。

为了了解T点的温度和寿命之间的关系，选用了一个在市场上普遍能买到的高功率白光LED类型。在不同的环境温度下对这些LED寿命进行试验。在以下各段的实验装置中进行详细描述。

因为不同的LED阵列必须运行在一个特定的环境温度下，阵列被放在专门设计的单独的寿命试验箱里，试验箱有两种不同的功能：保持LED阵列所处的环境温度稳定和作为光测量光输出的集成箱。每个单独的LED阵列被安装在寿命试验箱中心的内顶面。在左侧面板的中心连接一个二极管用于连续测量光输出。在光电二极管周围放置一个小的白色挡板用于屏蔽直射光，使光电二极管只能接收到反射光。在挡板上放置一个电阻温 6

度探测器用来衡量试验箱的环境温度和控制加热器来提供必需的热量，在侧框室通过温度控制器使温度保持在±1℃。加热器伸出的铝板与白色隔板连接着。使用J-焊接型细线热电偶来估计白光LED的T点温度。外部LED驱动器控制通过每个分庭的LED电流。所有的寿命测试都放置一个温度控制的房间内。寿命试验箱交错垂直和水平放置以确保热量从底部隔板上涨时不会影响他们之上的隔板。

这项研究的结果强调白光LED包装的重要性，使用适当的热管理以保持光输出，从而延长系统的使用寿命。在pn结的热量是影响白光LED的使用寿命的主要因素之一，因此，第一次实验在各种环境温度下对T点温度和寿命之间的关系进行了研究，通过实验表明对于对发展可靠长期系统感兴趣的厂家，了解寿命和热之间的关系将是非常有用的。第二次试验是寿命随温度指数的跌幅。了解在不同的包装不同的热提取技术和材料的商业白光LED在执行相同作业条件下寿命的差别。

作为正在进行的研究的一部分，我们希望进一步探讨不同的商业LED受热的影响，并最终建立一个体系，说明不同产品之间的生活和T型点温度的关系曲线。

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