外文資料翻譯--有關led顯示屏設計時的參考

1、<p><b>　　淮 陰 工 學 院</b></p><p>　　畢業(yè)設計(論文)外文資料翻譯</p><p>　　注：請將該封面與附件裝訂成冊。附件1：外文資料翻譯譯文</p><p>　　有關LED顯示屏設計時的參考</p><p>　　摘要：本文將主要集中介紹全彩色LED顯示模塊在大面積顯示看板的應用，

2、隨著顯示器的總體尺寸在畫面質量和均勻性方面的標準、要求。利用全彩色RGB模塊顯示系統的方法已經比較成熟，在這將對使用單一顏色的點矩陣顯示器顯示字母、數字做一個相關的討論、介紹。</p><p>　　關鍵詞： LED 恒流驅動 均勻性 電源 </p><p><b>　　1 介紹</b></p><p>　　50年前人們已經了解半導體材料可

3、產生光線的基本知識，第一個商用二極管產生于1960年。LED是英文light emitting diode（發(fā)光二極管）的縮寫，它的基本結構是一塊電致發(fā)光的半導體材料，置于一個有引線的架子上，然后四周用環(huán)氧樹脂密封，即固體封裝，所以能起到保護內部芯線的作用，所以LED的抗震性能好。</p><p>　　在上世紀90年代初藍色LED因為中村修二的開拓性工作得到了顯示性能的改進，全彩色LED顯示屏已經使用了很多年，但

4、這還沒達到顯示的極限。本文將詳細描述要考慮選擇什么樣的參數驅動LED,并獲得所需的性能，采用最先進的數字信號處理技術和校準程序。它也將顯示出市場的思考，保持平衡的最佳成本效益和產品生命周期的預期。LED光源由于具有使用壽命長、光效高、節(jié)能、無頻閃等優(yōu)點，正逐漸取代傳統照明光源而走進千家萬戶，因此針對LED照明系統驅動電源的研究成為電力電子行業(yè)的一個熱門課題。LED驅動電源的可靠性問題是影響LED照明系統照明品質和使用壽命的關鍵因素。提高

5、LED驅動電源的效率和進行變換器的熱分析，對提高LED驅動電源的可靠性具有十分重要的意義。</p><p><b>　　2 LED參數</b></p><p>　　人類是對顏色都有不同的感知，這一點是非常重要的，要促進模塊整合成一個最終的顯示單元，需要客觀的、可量化以及顏色均勻性的測量方法。國際照明委員會（CIE）建立量化的顏色測量的XYZ三刺激系統，這個系統是根據三

6、基色紅、綠、藍而建立的。XYZ三刺激值是通過集成的光譜功率輻射的分布曲線和三眼在380-780nm可見波長的響應曲線通過計算而得到的。</p><p>　　發(fā)光二極管發(fā)射光輻射的光譜功率的分布在許多方面不同于其它輻射源的光譜功率分布。它既不是像激光那樣的單色，也沒有像鎢燈那樣的寬色帶，而是介于這兩個極端之間。</p><p>　　通常是LED恒流驅動的水平在10或~20mA時。然而當驅動L

7、ED全彩色視頻應用時，由于不均勻性的存在通常這些規(guī)格標準是不能滿足使用時的標準的。這已經表明，發(fā)光二極管的效率可以存在顯著的差異，在不同的驅動條件下，光輸出和色坐標也存在著變化。</p><p>　　發(fā)光二極管對熱性能的影響是很敏感的。通常情況下，溫度從25攝氏度增加到60攝氏度時，發(fā)光二極管就會減少10%的發(fā)光強度。發(fā)光二極管在溫度升高的條件下有一個通用的行為：在較高的溫度下，它們的效率會變低。例如在60攝氏度

8、時的發(fā)光強度減少10%，其發(fā)光價值主要體現在25攝氏度，因此解決這個問題就需要知道二極管的具體溫度降額曲線。</p><p>　　LED驅動電流也是至關重要的，在這一點上。有一個非常嚴謹的公差規(guī)范，比如為了保證顯示性能，如果驅動沒有指定，由于電流對光輸出和色坐標的影響就需要恒流驅動程序供給正確的電流。事實上溫度對設計的穩(wěn)定性是很關鍵的，這意味著需要從根本上控制不同的輸出在同一設備上的耐受性與不同驅動的公差。<

9、;/p><p><b>　　3 恒流驅動</b></p><p>　　隨著LED與使用這些組件的驅動，可以很容易地看到，選擇與這些特定的組件的設計是非常關鍵的。盡管你可以爭論這一組件是專門為高性能視頻市場的，但他們都各有優(yōu)點和缺點，下面對大多數重要的參數進行了總結。</p><p>　　3.1 輸出電流的變化 </p><p&g

10、t;　　大多數的恒流驅動器都有不止一種的輸出。行業(yè)標準是8、16或24的輸出。這些輸出是要連接到發(fā)光二極管上的。如果你想顯示均勻的圖像，用一個芯片驅動多個LED使他們有相同的電流輸出就行了，這一點是很重要的。如果之間存在一個顯著的該引腳的變化輸出，就會增加其余的LED在另一個電流驅動。再次，這將意味著既使所有的發(fā)光二極管具有相同的規(guī)格也將得到不同的的強度和顏色。因此，很明顯這是導致了變化的非均勻性的來源，在1–3%范圍內變化是首選，0%

11、的變化是明顯的成本禁止的。</p><p><b>　　3.2 功率耗散</b></p><p>　　功率耗散通常是一個設計后期才能發(fā)現的問題，但同時這個問題也是很關鍵的。這個問題可能導致恒流驅動在工作時輸出的為低電壓，低電壓會增加功率耗的散和熱量的產生。熱的產生不僅僅體現在設計上，同時也對LED的光輸出產生了一定的變化，具體可能造成不均勻性的問題，這個問題還聯系到電

12、源設計的本身。</p><p>　　3.3 芯片間的輸出電流的變化</p><p>　　因為需要驅動一個屏幕，所以推理的是芯片間多個常數電流的變化。因此電流變化小于2.5%是最好的選擇，這也將會得到更高程度的補償需求?，F在不僅僅是LED需要補償，恒流驅動的變化也需要得到一定補償才能保證系統得到最好的運行。然而更高的補償就會意味著將產生更多的“高水平”，這就將意味著需要更快的常數，更加高效率

13、的電流驅動。</p><p><b>　　4 電源的設計</b></p><p>　　R、G和B的發(fā)光二極管具有不同的電壓，所以為了不浪費任何的能量和過多的熱量，因此選擇適當的LED驅動電壓優(yōu)化開關電源的輸出電壓是很重要的。通常情況下，綠色和藍色發(fā)光二極管的閾值電壓在3.5 V，而紅色通常是約1.9 V.因此電源輸出電壓為2 V、4.2 V（藍綠色）和2.6 V（紅色

14、），這些電壓將使總系統的功率耗散達到最優(yōu)值。要想顯示內容可以立即改變，電源必須能夠去從0%至100%的電力負荷一次。（通常，視頻在60或50赫茲運行和圖像的瞬間的改變電源的響應時間是16-20ms）。大多數電源需要一個（高）小甚至恒荷載以保證可靠性和操作。</p><p>　　盡管大型設備房功率因數校正（PFC）校正器的成本過高，但加入PFC將擁有更完整的顯示均勻性校正。</p><p>

15、<b>　　5 均勻性校正</b></p><p>　　為了糾正不均勻性，需要一些高標準的測量手段。例如，一個高分辨率的相機和光譜儀，用于測量每個單獨的LED的x，y和y坐標，這樣的測量方法的誤差將小于可見閾值（這里所有的測量流程必須在相同的條件下如溫度、驅動電流等）。</p><p>　　測量的結果可以被存儲在易失性存儲器里，可以使后續(xù)的硬件使用這些測量數據。再次，

16、這些連續(xù)的適應參數意味著至少有一個不良的控制器能夠實時計算如何適應在一定的溫度下的函數的參數、使用壽命、要求的光輸出等。</p><p>　　下圖顯示了電子如何建立，沒有現成的組件可以發(fā)現這樣一個高標準的性能要求。因此，最好用可編程邏輯實現計算，如此一來對客戶倒是一個優(yōu)勢。當顯示標準發(fā)生變化或有更好處理算法時，可以很容易地在現場進行升級，使顯示器將繼續(xù)執(zhí)行最新的創(chuàng)新和圖像顯示功能。</p><

17、p>　　圖1顯示了在處理路徑上的實現方案。每一個LED和快速的處理器接口的亮度控制沒有表現出復雜的細節(jié)上。頂部的路徑主要是紅色的處理部分，中間一個是主要的綠色通道和下面的藍色的核心處理。</p><p>　　通常，LED顯示屏是內置的模塊化，這實際上意味著，這樣的一個獨立的模塊只有一小部分LED帳戶。因此，每個系統表現出來的復雜性很高。由于該模塊僅需要顯示整個畫面的一部分，因此分配給一個像素的處理時間數量級

18、高于一個完整的顯示情況。這也意味著例如縮放算法可以在模塊級別上實現，這是不常見的，在工業(yè)上的需要花費高昂電子才能充分顯示所需的處理能力。 </p><p>　　很明顯的,顏色添加量必須在色彩校正范圍內,但通常是在100-1000倍小于主色的范圍內的，在這范圍內才能計算出位深度和規(guī)定的LED的光輸出。因此，在后面的數學計算中，將在12-16位范圍內產生處理路徑，與中間不累計的計算進入最后階段，從而達到最高的水平。&

19、lt;/p><p>　　圖1 大面積模塊顯示核心處理路徑，顯示不同的路徑為紅（頂部），綠色（中）和藍色（下）</p><p><b>　　6 總結</b></p><p>　　本文對有關怎么處理和實現大面積模塊顯示做了一些參數方面的分析，雖然有的數據不是最準確的，但至少能作為一個參考。我們要考慮的不僅僅是一些視覺參數、電子方面的參數等，還要考慮是

20、不是符合行業(yè)的標準，最重要的還是經濟性、可制造性，以及一些專業(yè)方面的考慮，以上提出的僅僅是個參考， 問題的重要部分還需要在實踐中探索，讓對這方面感興趣的人有一個最基本的了解和認知。</p><p><b>　　參考文獻</b></p><p>　　1. Nakamura S (1994) Nichia 1cd blue LED paves way for full c

21、olour display. Nikkei Electron Asia 6:65–69</p><p>　　2. Wyszecki G, Stiles WS (1967) Color science. Concept, methods, quantitative data and formulae. Wiley, Oxford</p><p>　　3. http://www.ledsmag

22、azine.com/features/4/8/1</p><p>　　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 5,7

23、55,503, 26 May 1998</p><p>　　5. Reference to multiple LED datasheets. www.nichia.com</p><p>　　6. http://www.ledlight.osram-os.com/tools/fine-white-binning</p><p>　　7. Schwedler W, N

24、guyen F (2010) Invited paper: LED backlighting for LCD TVs. SID Symposium Digest of Technical Papers 41(1):1091–1096 The LED display design reference</p><p>　　Abstract: This article epresents a brief ove

25、rview 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 vi

26、sual quality are discussed in terms of the practical device selection, driving and system implementation factors that should be considered</p><p>　　.List of Abbreviations: LED constant current drive unifor

27、mity power</p><p>　　Introduction</p><p>　　50 years ago, people have to understand semiconductor materials can produce light of the basic knowledge, the first commercial diodes in 1960. English

28、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

29、protection of the internal batteries can play the role of line, so the seismic performance LED good.</p><p>　　This article will focus primarily on full color modular LED displays for large-area display and s

30、ignage 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, wi

31、th 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 </p><p>　　LED parameters

32、</p><p>　　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 in

33、to 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

34、is based on the assumption that every color is a combination of three primary color</p><p>　　green, and blue. The XYZ tristimulus values are obtained by integrating the spectral power distribution of radiati

35、on and the three eye response curves over the visible wavelengths 380–780 nm.</p><p>　　The spectral power distribution of the optical radiation emitted by LEDs differs in many ways from other radiation sourc

36、es. It is neither monochromatic like a laser, nor broadband like a tungsten lamp, but rather lies somewhere between these two extremes.</p><p>　　Usually LEDs are specified at a certain constant current driv

37、e 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

38、 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.</p><p>　　LEDs are very susceptib

39、le 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 t

40、hey 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

41、with this issue.</p><p>　　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 a

42、re 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 cruci

43、al. This means that in-between different outputs in the same devices the tolerance needs to be cont</p><p>　　constant current drive</p><p>　　As LEDs are immediately driven using these components

44、, 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 t

45、he high performance video market, all of them have advantages and severe drawbacks. Most important parameters are summarized.</p><p>　　3.1 Change the output current</p><p>　　Most constant curren

46、t 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 displ

47、ay 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 wo

48、uld add to certain LEDs driven at anothe</p><p>　　3.2 Power dissipation</p><p>　　Usually an afterthought, but certainly critical, is having a constant current driver which operates on its output

49、s 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 outp

50、ut variations. Specific ‘‘hot spots’’ could become the reason of no nuniformity. This issue now also links in into the power supply design itself.</p><p>　　3.3. Interchip output current variation</p>

51、<p>　　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% varia

52、tion 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 dri

53、ver variation. A higher degree of compensation then means even generati</p><p>　　4 Power Supply Design</p><p>　　R,G, and B LEDs have different threshold voltages, so in order not to waste any e

54、nergy, 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 i

55、s 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</p><p>　　power dissipation</p><p>　　Uniformity cor

56、rection</p><p>　　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

57、,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 – t

58、emperature, drive current, and so on.)</p><p>　　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

59、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

60、 function of temperature, lifetime usage, requested light output, and so on. The next diagram shows in detail how the ele</p><p>　　processing algorithms are developed, these can easily be upgraded in the fie

61、ld, so that the display will continue to perform to the latest innovation and image display capabilities.</p><p>　　Figure 1shows an implementation proposal on the processing path. Without showing the detail

62、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 pa

63、th, and the bottom one is the blue core processing.</p><p>　　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 mo

64、dule 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 allocate

65、d for one pixel is magnitudes higher than in the case of a full display. This also means</p><p>　　that for instance scaling algorithms can be implemented on a module level, which are not commonly used in the

66、 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 t

67、ime per pixel is much higher, so the time spent on optimizing and using extreme performing algorithms is possible.</p><p>　　Obviously, the amount of color addition must be in the range of the color correctio

68、n 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 output regulations, must be even 10–100 times more, to also accommodate a

69、color correction scale. Thus, calculating back the numbers, this would yield a processing path in the range of 12–16 bits, with intermediate calculations</p><p>　　even at a higher level in order not to accum

70、ulate rounding errors at the end stage.</p><p><b>　　Fig. 1</b></p><p>　　Core processing paths for large area modular displays, showing the separate paths for red (top), green (middle

71、) and blue (bottom)</p><p>　　6 Summary</p><p>　　This chapter provides an overview of some important topics regarding LED processing for large-area modular display implementation. One needs to t

72、ake into account a wide range of visual and electronics parameters, which are not that obvious in the standard display industry. On top of that, the economics, manufacturability, and requirements here are much more invol

73、ved and are indeed important pieces of the puzzle. The intention of this chapter is to make designers, consultants, and customers aware</p><p><b>　　Reference</b></p><p>　　1. Nakamura

74、 S (1994) Nichia 1cd blue LED paves way for full colour display. Nikkei Electron Asia 6:65–69</p><p>　　2. Wyszecki G, Stiles WS (1967) Color science. Concept, methods, quantitative data and formulae. Wiley,

75、Oxford</p><p>　　3. http://www.ledsmagazine.com/features/4/8/1</p><p>　　4. Chen K-Y, Chen S-M, Hao Z-D (1998) Optical illumination system having improved efficiency and uniformity and projectio

76、n instrument comprising such a system. US Patent 5,755,503, 26 May 1998</p><p>　　5. Reference to multiple LED datasheets. www.nichia.com</p><p>　　6. http://www.ledlight.osram-os.com/tools/fine-w