外文资料翻译--单片机---LED显示屏

淮 阴 工 学 院 毕业设计 论文 外文资料翻译 学学 院 院 机械学院 专专 业 业 机械设计制造及其自动化 姓姓 名 名 陈功 学学 号 号 外文出处 外文出处 可视化显示技术 用外文写 Handbook of Visual Display Technology 附附 件 件 1 外文资料翻译译文 2 外文原文 指导教师评语 签名 年月日 注 注 请将该封面与附件装订成册 附件附件 1 外文资料翻译译文 外文资料翻译译文 有关有关 LEDLED 显示屏设计时的参考显示屏设计时的参考 摘要摘要 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 关键词关键词 LED 恒流驱动 均匀性 电源 1 1 介绍介绍 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 2 LEDLED 参数参数 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 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 3 恒流驱动恒流驱动 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 13 1 输出电流的变化输出电流的变化 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 1 3 范围内变化是首选 0 的 变化是明显的成本禁止的 3 23 2 功率耗散功率耗散 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题 这个问题还联系到电源设计的本 身 3 33 3 芯片间的输出电流的变化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 4 电源的设计电源的设计 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 5 均匀性校正均匀性校正 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 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 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 G R on B 驱动 缩放 计算 恒流驱动 G on R G on B 驱动 缩放 计算 恒流驱动 B on G B 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 6 总结总结 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 65 69 2 Wyszecki G Stiles WS 1967 Color science Concept methods quantitative data and formulae Wiley Oxford 3 4 Chen K Y Chen S M Hao Z D 1998 Optical illumination system having improved efficiency and uniformity and projection instrument comprising such a system US Patent 5 755 503 26 May 1998 5 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 Digest of Technical Papers 41 1 1091 1096 The LED display design reference Abstract 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 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 power 1Introduction 50 years ago people have to understand semiconductor 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 solid 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 criteria 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 presented 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 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 2LED parameters Human color perception varies significantly across 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 l Eclairage CIE established the XYZ tristimulus 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 curves over the visible wavelengths 380 780 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 extremes 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 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 temperature 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 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 current 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 drivers 3constant current drive As 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 for the high performance video market all of them have advantages and severe drawbacks Most important parameters are summarized 3 1 Change the output current Most constant current drivers have more than one output Industry standards are 8 16 or 24 outputs These outputs are immediately coupled 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 current 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 of 1 3 are preferred as 0 variations are obviously cost prohibitive 3 2 Power dissipation Usually 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 dissipation 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 variation The 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 this 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 expensive current drivers This immediately links to the next topic 4 Power Supply Design R 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 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 system power dissipation 5 Uniformity correction In order to correct for nonuniformities 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 that 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 adapt 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 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 Whenever display standards change or better processing 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 the 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 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 the total picture the processing time allocated for one pixel is magnitudes higher than in the case of a full display This also means that 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 display 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 color addition must be in the range of the color correction needed but is usually a 100 1 000 times less than the main color The consequence is in this case that the bit depth calculation and LED light ou