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1、附件1：外文资料翻译译文有关LED显示屏设计时的参考摘要：本文将主要集中介绍全彩色LED显示模块在大面积显示看板的应用，随着显示器的总体尺寸在画面质量和均匀性方面的标准、要求。利用全彩色RGB模块显示系统的方法已经比较成熟，在这将对使用单一颜色的点矩阵显示器显示字母、数字做一个相关的讨论、介绍。关键词： LED 恒流驱动 均匀性 电源 1 介绍50年前人们已经了解半导体材料可产生光线的基本知识，第一个商用二极管产生于1960年。LED是英文light emitting diode（发光二极管）的缩写，它的基本结构是一块电致发光的半导体材料，置于一个有引线的架子上，然后四周用环氧树脂密封，即固体
2、封装，所以能起到保护内部芯线的作用，所以LED的抗震性能好。在上世纪90年代初蓝色LED因为中村修二的开拓性工作得到了显示性能的改进，全彩色LED显示屏已经使用了很多年，但这还没达到显示的极限。本文将详细描述要考虑选择什么样的参数驱动LED,并获得所需的性能，采用最先进的数字信号处理技术和校准程序。它也将显示出市场的思考，保持平衡的最佳成本效益和产品生命周期的预期。LED光源由于具有使用寿命长、光效高、节能、无频闪等优点，正逐渐取代传统照明光源而走进千家万户，因此针对LED照明系统驱动电源的研究成为电力电子行业的一个热门课题。LED驱动电源的可靠性问题是影响LED照明系统照明品质和使用寿命的关
3、键因素。提高LED驱动电源的效率和进行变换器的热分析，对提高LED驱动电源的可靠性具有十分重要的意义。2 LED参数人类是对颜色都有不同的感知，这一点是非常重要的，要促进模块整合成一个最终的显示单元，需要客观的、可量化以及颜色均匀性的测量方法。国际照明委员会（CIE）建立量化的颜色测量的XYZ三刺激系统，这个系统是根据三基色红、绿、蓝而建立的。XYZ三刺激值是通过集成的光谱功率辐射的分布曲线和三眼在380-780nm可见波长的响应曲线通过计算而得到的。发光二极管发射光辐射的光谱功率的分布在许多方面不同于其它辐射源的光谱功率分布。它既不是像激光那样的单色，也没有像钨灯那样的宽色带，而是介于这两个
4、极端之间。通常是LED恒流驱动的水平在10或20mA时。然而当驱动LED全彩色视频应用时，由于不均匀性的存在通常这些规格标准是不能满足使用时的标准的。这已经表明，发光二极管的效率可以存在显着的差异，在不同的驱动条件下，光输出和色坐标也存在着变化。发光二极管对热性能的影响是很敏感的。通常情况下，温度从25摄氏度增加到60摄氏度时，发光二极管就会减少10%的发光强度。发光二极管在温度升高的条件下有一个通用的行为：在较高的温度下，它们的效率会变低。例如在60摄氏度时的发光强度减少10%，其发光价值主要体现在25摄氏度，因此解决这个问题就需要知道二极管的具体温度降额曲线。LED驱动电流也是至关重要的，
5、在这一点上。有一个非常严谨的公差规范，比如为了保证显示性能，如果驱动没有指定，由于电流对光输出和色坐标的影响就需要恒流驱动程序供给正确的电流。事实上温度对设计的稳定性是很关键的，这意味着需要从根本上控制不同的输出在同一设备上的耐受性与不同驱动的公差。3 恒流驱动随着LED与使用这些组件的驱动，可以很容易地看到，选择与这些特定的组件的设计是非常关键的。尽管你可以争论这一组件是专门为高性能视频市场的，但他们都各有优点和缺点，下面对大多数重要的参数进行了总结。3.1 输出电流的变化 大多数的恒流驱动器都有不止一种的输出。行业标准是8、16或24的输出。这些输出是要连接到发光二极管上的。如果你想显示均
6、匀的图像，用一个芯片驱动多个LED使他们有相同的电流输出就行了，这一点是很重要的。如果之间存在一个显着的该引脚的变化输出，就会增加其余的LED在另一个电流驱动。再次，这将意味着既使所有的发光二极管具有相同的规格也将得到不同的的强度和颜色。因此，很明显这是导致了变化的非均匀性的来源，在13%范围内变化是首选，0%的变化是明显的成本禁止的。3.2 功率耗散功率耗散通常是一个设计后期才能发现的问题，但同时这个问题也是很关键的。这个问题可能导致恒流驱动在工作时输出的为低电压，低电压会增加功率耗的散和热量的产生。热的产生不仅仅体现在设计上，同时也对LED的光输出产生了一定的变化，具体可能造成不均匀性的问
7、题，这个问题还联系到电源设计的本身。3.3 芯片间的输出电流的变化 因为需要驱动一个屏幕，所以推理的是芯片间多个常数电流的变化。因此电流变化小于2.5%是最好的选择，这也将会得到更高程度的补偿需求。现在不仅仅是LED需要补偿，恒流驱动的变化也需要得到一定补偿才能保证系统得到最好的运行。然而更高的补偿就会意味着将产生更多的“高水平”，这就将意味着需要更快的常数，更加高效率的电流驱动。4 电源的设计R、G和B的发光二极管具有不同的电压，所以为了不浪费任何的能量和过多的热量，因此选择适当的LED驱动电压优化开关电源的输出电压是很重要的。通常情况下，绿色和蓝色发光二极管的阈值电压在3.5 V，而红色通
8、常是约1.9 V.因此电源输出电压为2 V、4.2 V（蓝绿色）和2.6 V（红色），这些电压将使总系统的功率耗散达到最优值。要想显示内容可以立即改变，电源必须能够去从0%至100%的电力负荷一次。（通常，视频在60或50赫兹运行和图像的瞬间的改变电源的响应时间是16-20ms）。大多数电源需要一个（高）小甚至恒荷载以保证可靠性和操作。尽管大型设备房功率因数校正（PFC）校正器的成本过高，但加入PFC将拥有更完整的显示均匀性校正。5 均匀性校正为了纠正不均匀性，需要一些高标准的测量手段。例如，一个高分辨率的相机和光谱仪，用于测量每个单独的LED的x，y和y坐标，这样的测量方法的误差将小于可见阈
9、值（这里所有的测量流程必须在相同的条件下如温度、驱动电流等）。测量的结果可以被存储在易失性存储器里，可以使后续的硬件使用这些测量数据。再次，这些连续的适应参数意味着至少有一个不良的控制器能够实时计算如何适应在一定的温度下的函数的参数、使用寿命、要求的光输出等。下图显示了电子如何建立，没有现成的组件可以发现这样一个高标准的性能要求。因此，最好用可编程逻辑实现计算，如此一来对客户倒是一个优势。当显示标准发生变化或有更好处理算法时，可以很容易地在现场进行升级，使显示器将继续执行最新的创新和图像显示功能。图1显示了在处理路径上的实现方案。每一个LED和快速的处理器接口的亮度控制没有表现出复杂的细节上。
10、顶部的路径主要是红色的处理部分，中间一个是主要的绿色通道和下面的蓝色的核心处理。通常，LED显示屏是内置的模块化，这实际上意味着，这样的一个独立的模块只有一小部分LED帐户。因此，每个系统表现出来的复杂性很高。由于该模块仅需要显示整个画面的一部分，因此分配给一个像素的处理时间数量级高于一个完整的显示情况。这也意味着例如缩放算法可以在模块级别上实现，这是不常见的，在工业上的需要花费高昂电子才能充分显示所需的处理能力。 很明显的,颜色添加量必须在色彩校正范围内,但通常是在100-1000倍小于主色的范围内的，在这范围内才能计算出位深度和规定的LED的光输出。因此，在后面的数学计算中，将在12-16
11、位范围内产生处理路径，与中间不累计的计算进入最后阶段，从而达到最高的水平。缩放计算恒流驱动R on GR on B驱动 缩放计算恒流驱动G on RG on B驱动缩放计算恒流驱动B on GB on R驱动图1 大面积模块显示核心处理路径，显示不同的路径为红（顶部），绿色（中）和蓝色（下）6 总结 本文对有关怎么处理和实现大面积模块显示做了一些参数方面的分析，虽然有的数据不是最准确的，但至少能作为一个参考。我们要考虑的不仅仅是一些视觉参数、电子方面的参数等，还要考虑是不是符合行业的标准，最重要的还是经济性、可制造性，以及一些专业方面的考虑，以上提出的仅仅是个参考， 问题的重要部分还需要在实践
12、中探索，让对这方面感兴趣的人有一个最基本的了解和认知。 参考文献1. Nakamura S (1994) Nichia 1cd blue LED paves way for full colour display. Nikkei Electron Asia 6:65692. Wyszecki G, Stiles WS (1967) Color science. Concept, methods, quantitative data and formulae. Wiley, Oxford3. 4. Chen K-Y, Chen S-M, Hao Z-D (1998) Optical illumi
13、nation system having improved efficiency and uniformity and projection instrument comprising such a system. US Patent 5,755,503, 26 May 19985. Reference to multiple LED datasheets. 6. http:/www.ledlight.osram-7. Schwedler W, Nguyen F (2010) Invited paper: LED backlighting for LCD TVs. SID Symposium
14、Digest of Technical Papers 41(1):10911096 The LED display design referenceAbstract: This article epresents a brief overview of the issues involved in the production and implementation of full color modular LED displays for large-area display and signage applications. The technical issues related to
15、achieving large-area uniformity and visual quality are discussed in terms of the practical device selection, driving and system implementation factors that should be considered.List of Abbreviations: LED constant current drive uniformity power1 Introduction50 years ago, people have to understand sem
16、iconductor materials can produce light of the basic knowledge, the first commercial diodes in 1960. English is the LED light emitting diode (LED) acronym, and its basic structure is an electroluminescent semiconductor materials, placed in a wire rack, then sealed with epoxy resin around, that is, so
17、lid package, Therefore, the protection of the internal batteries can play the role of line, so the seismic performance LED good.This article will focus primarily on full color modular LED displays for large-area display and signage applications, where picture quality and uniformity are important cri
18、teria along with the overall size of the display. Display systems which utilize a RGB module approach for full color are considered in detail, with the discussion also relevant to the use single color dot matrix displays or alphanumerical displays, which are effectively a subset of the outline prese
19、nted. Due to the advantages of long life, high light-efficiency, energy saving, no flicker, etc., traditional light sources are gradually replaced by LED light source in the field of family lighting. More and more attentions are payed on the research and development of LED drivers for researchers of
20、 power electronics.Reliability issue is a key factor affecting the lighting quality and life of LED lighting system. Improvement in efficiency and thermal analysis on LED drivers are particularly important in improving its reliability. 2 LED parametersHuman color perception varies significantly acro
21、ss individuals, hence it is critical that an objective and quantifiable method of measurement is employed to facilitate the integration of modules into a final display unit with the required color and uniformity properties. The Commission Internationale de lEclairage (CIE)established the XYZ tristim
22、ulus system for the measurement and quantification of color, which is based on the assumption that every color is a combination of three primary colors: red,green, and blue. The XYZ tristimulus values are obtained by integrating the spectral power distribution of radiation and the three eye response
23、 curves over the visible wavelengths 380780 nm.The spectral power distribution of the optical radiation emitted by LEDs differs in many ways from other radiation sources. It is neither monochromatic like a laser, nor broadband like a tungsten lamp, but rather lies somewhere between these two extreme
24、s.Usually LEDs are specified at a certain constant current drive level usually 10 or 20 mA. However, when driving LEDs in a full color video application, these specifications do not satisfy the full qualification for usage, due to the potential for nonuniformities in output across a display. It has
25、been shown that the efficiencies of LEDs can differ significantly under different drive conditions, both in light output and in color coordinate variation.LEDs are very susceptible to the effects of heat on performance. Typically, a 10% reduction in luminous intensity results from an increase in tem
26、perature from 25 LEDs have a generic behavior at elevated temperatures: At higher temperatures they are less efficient. The luminous intensity at 60C is reduced by 10%, e.g., of its value at 25C Therefore, one not only needs to know the specific temperature derating curves of the LEDs, but one must
27、be able to cope with this issue.LED driver electronics are also crucial at this point. Having a very tight LED specification does not guarantee display performance if the tolerances on, for instance, the drivers are not as well specified. These constant current drivers need to supply the correct cur
28、rent because current has an influence on both light output and color coordinates. Temperature and design stability are de facto crucial. This means that in-between different outputs in the same devices the tolerance needs to be controlled radically in combination with the tolerance between different
29、 drivers.3 constant current driveAs LEDs are immediately driven using these components, one could easily see that the choice and design of these specific components are very critical. This is certainly true in a video environment. Although one can debate availability of such a component specifically
30、 for the high performance video market, all of them have advantages and severe drawbacks. Most important parameters are summarized.3.1 Change the output currentMost constant current drivers have more than one output. Industry standards are 8, 16, or 24 outputs. These outputs are immediately coupled
31、to the LEDs. If one wants to display a uniform image, it is of utmost importance that the outputs within one chip drivin. a multitude of LEDs all have the same current. If there is a significant variation in-between outputs of the pins, this of course would add to certain LEDs driven at another curr
32、ent. Again, this would mean that those LEDs have different resultant intensities and color, even if all the LEDs have the same specification. It is therefore very obvious that not only LED variation is a source of no uniformity, but also the constant current drivers. Hence, variations in the range o
33、f 13% are preferred, as 0% variations are obviously cost prohibitive.3.2 Power dissipationUsually an afterthought, but certainly critical, is having a constant current driver which operates on its outputs with as low a voltage drop as possible, as this would only increase the overall display power d
34、issipation, and heat generation. Heat generation is not only negative for design reasons, but also regarding LED light output variations. Specific hot spots could become the reason of no nuniformity. This issue now also links in into the power supply design itself.3.3. Interchip output current varia
35、tionThe same reasoning as above is valid for the current variation in-between multiple constant current driver chips, as a multitude of these are used to drive a screen. Hence, trimming toward less than 2.5% variation is also preferred. If both the variations of a and b are higher than indicated, th
36、is would even demand for a higher degree of compensation. Now, not only the LEDs need compensation, but also the constant current driver variation. A higher degree of compensation then means even generating more gray levels as we will see later on, which in return would mean faster constant, more ex
37、pensive, current drivers. This immediately links to the next topic.4 Power Supply DesignR,G, and B LEDs have different threshold voltages, so in order not to waste any energy, and have excessive heat generation, it would be best to optimize the SMPS output voltages to the adequate LED drive voltages
38、. Typically, the threshold voltages for green and blue LEDs are around 3.5 V, while red is typically around 1.9 V. Hence a power supply which outputs2 V, 4.2 V (Green-Blue) and 2.6 V (Red), has advantages with regard to total systempower dissipation5 Uniformity correction In order to correct for non
39、uniformities, one needs high standard measurement means, such as for instance a high resolution camera or spectrometer, for measuring each individual LED for its x,y and Y coordinates, with such an accuracy that the measurement procedure error is below the visible threshold. (The procedure here is t
40、hat all the measurements must be done under the same conditions throughout time temperature, drive current, and so on.)These measurements can then be subsequently stored in a nonvolatile memory preferably close to the LEDs, so that the subsequent hardware can make use of these measurements and can a
41、dapt them through time and usage circumstances. Again, continuous adaptation of these parameters means that at least a performing controller can calculate in real time how to adapt the parameters in function of temperature, lifetime usage, requested light output, and so on. The next diagram shows in
42、 detail how the electronics can be built, keeping in mind that no off-the-shelf components can be found for such a high standard performance requirement. Therefore, the preference goes to implementing the calculations in programmable logic, which in its turn has an advantage to the customer. Wheneve
43、r display standards change or betterprocessing algorithms are developed, these can easily be upgraded in the field, so that the display will continue to perform to the latest innovation and image display capabilities.Figure 1shows an implementation proposal on the processing path. Without showing th
44、e detail of complexity on the individual brightness control of every LED and the fast processor interface for updating the parameters in real time. The top path is the main red processing part, the middle one is the main green path, and the bottom one is the blue core processing.As one can see, each
45、 path has its scaler incorporated. Usually LED displays are built modularly, which in fact means that such a self-contained module has only a fraction of LEDs to account for. Thus, the complexity goes down per system, but performance goes up. Since such a module only needs to display a fraction of t
46、he total picture, the processing time allocated for one pixel is magnitudes higher than in the case of a full display. This also meansthat for instance scaling algorithms can be implemented on a module level, which are not commonly used in the industry because the processing power needed on a full d
47、isplay level would require cost prohibitive electronics. Now, due to the modular system, since a module only has a few pixels to take care of, the processing time per pixel is much higher, so the time spent on optimizing and using extreme performing algorithms is possible. Obviously, the amount of c
48、olor addition must be in the range of the color correction needed, but is usually a 1001,000 times less than the main color. The consequence is in this case that the bit depth calculation and LED light output regulations, must be even 10100 times more, to also accommodate a color correction scale. Thus, calculating back the numbers, this would yield a processing path in