想象一下20世纪50年代辉煌的漫威漫画书 - 一对毫无防备的夫妇从镇上的一个夜晚走回家，经过一个百叶窗的商店橱窗，随着一阵风刮起灰尘和垃圾。
一盏路灯在舞台上投下了昏暗的灯光，浪费地将一部分光线向上投射到太空的黑暗中。
当那位年轻的女士靠在风中，不知不觉地紧紧抓住他的手臂时，那个男人抓住他的帽子。
在下一个面板中，路灯闪烁并伴随着令人不安的“Zzzt”声音而死亡，这是统计数据和白炽灯或钠蒸汽技术的受害者。
就在这时，一个戴着面罩的徘徊者抓住女人的钱包，把这对夫妇撞到地上。
在第三个小组中，当一个高大的建筑物，严峻地噘起他的嘴唇时，小偷跑开了，笑着一个邪恶的笑声，作为一个激怒的超级英雄（不失一般性，让我称之为Arachnid-man）。
故事的其余部分是可以预测的：一个旋风，一个砰砰声，现在我们看到那个女人感激地接受了一个卡通A-man的钱包，因为戴着手铐和粗暴的强盗在一辆警车上被扯下来。
为了使抢劫者（和超级英雄）脱离生意，世界各地的城市和联邦***都要求改用街道固态照明，也称为LED照明。
在此过程中，他们将电能消耗降低50％，运营成本降低40％。
投资回收期可为1至3年。
更换/改造LED路灯预计将在2020年成为一个价值2B美元的行业。路灯可以提供更好的光线质量，从而将夜间变为白昼，并允许将光线导向需要的地方而不是向各个方向溅射。
它们可以被远程控制，当没有人在周围时它们可以自行关闭，并且当太阳升起时它们也可以完全关闭（或当Cyclops在与其他突变体作战时产生光学爆炸时）。
在灯具中，LED以串的形式出现 - 每个都下降3.5V，整个串设计为从40 V或100 V电源驱动，具体取决于每个灯串中的LED数量。
相同的电流流过给定串中的每个LED，范围从0.35A到1A。一个唠叨的问题是光输出的可靠性：LED有时会失效。
根据它们的结构，它们可以作为OPEN或SHORTs失败。
在OPEN电路的情况下，整个串中的电流变为零，并且所有LED变暗，从而导致光损失并且在先前有光的情况下产生丑陋的暗线。
相比之下，当一个LED作为SHORT失效时，它的光输出变为零，但是弦的其余部分保持点亮 - 一段时间。
最终由于热应力，短路设备无法打开，导致整个灯串变暗。
LED开路的平均故障时间（MTTF）取决于它是使用引线键合还是凸点/焊接连接方案在内部构建。
MTTF还取决于工作电流水平和环境温度 - 正如人们所预料的那样，随着电流水平上升或环境温度上升，MTTF变小。
在现代灯具中，不断缩小的空间预算和每平方厘米更多流明的追求（“流明”是人眼所经历的亮度的衡量标准）不幸地对工作电流和环境温度施加了很大的压力：
LED可以承载的电流，其光输出越大。
此外，散热的空间越小，工作温度越高。
当高性能路灯由于OPEN LED灯串而失去大量光输出时，更换它需要相当大的成本 - 不仅仅是灯具本身，而是更换灯具所需的设备和人员。
使这个问题更加复杂的是对灯泡寿命长的要求越来越高：通常需要10到15年。
防止整个串的损失的一种简单方法是使用安森美半导体的LED旁路分流器 - 简单，紧凑的双端器件，在正常工作期间基本上处于休眠状态。
当LED发生故障OPEN时，相应的分流装置暂时经历高电压，导致其切换到导通状态，从而允许其承载设计的LED电流。
结果，串中的其他LED正常工作，除了OPEN电路LED所在的一个暗点外，灯串保持点亮状态。
因此，LED分流器将图1左侧的图像转换为右图。
图1：单个LED未通过OPEN可以使整个灯串变暗。
安森美半导体的LED旁路分流器允许串中的剩余LED正常工作，从而改变左侧的情况，其光线明显减少，转向右侧情况。
灯泡寿命的延长有助于满足保修要求。
通过为每个LED使用一个分流器可以最大限度地减少光损失 - 这种应用的理想产品是NUD4700 LED分流器，设计用于在瞬间峰值为~7 V时开启，并安装在热量优越的POWERMITE®中
封装允许直流操作高达1.3 A.作为替代方案，安森美半导体还提供更高电压的HBL1015和HBL1025 LED分流器，分别绕过两个和三个LED。
这在下面的图2中示意性地示出。
图2：LED路灯的示意图，突出显示旁路分流器的使用：每个LED（左）一个NUD4700或每对LED（中心）一个HBL1015或每个三个LED（右）一个HBL1025实例。
在后一种情况下较低的总成本可以通过以下事实来抵消：即使只有一个LED发生故障，分流器绕过（并使三个LED）变暗。
图3：用于街道照明应用的LED旁路分流器。
图3显示了我们用于街道照明应用的各种LED旁路分流器产品的比较。
下次你走过一条被风吹扫的街道上的街灯时，看看它是否有LED。
如果确实如此，你可以更轻松地走路，享受夜晚，因为没有灯泡突然失效的危险或者从树梢上摔下来的斗篷十字军。
随着您的光临，请花一点时间感谢许多使LED技术成为可能的科学家和工程师 - 这项技术的原始发明者在2014年获得了诺贝尔物理学奖。
以上来自于谷歌翻译
以下为原文
Picture a Marvel comic book from the glorious 1950s – an unsuspecting couple walks home from a night on the town past a shuttered shop window, as a gust of wind blows dust and litter. A streetlight casts a dim light on the proceedings, wastefully throwing a portion of the light upwards into the darkness of space. The man holds on to his hat as the young lady leans against the wind and unconsciously tightens her grip on his ARM. In the next panel the streetlight flickers and dies with a disturbing “Zzzt” sound, a victim of statistics and incandescent or sodium vapour technology. At this moment a prowler with a face mask runs by, grabs the woman’s purse and knocks the couple to the ground. In the third panel the thief runs away laughing an evil cackle as an infuriated superhero (without loss of generality let’s call him Arachnid-man) looks on from atop a tall building, grimly pursing his lips. The rest of the story is predictable: a swoosh, a thump and presently we see the woman gratefully accepting her purse from a caped A-man as a handcuffed and roughed-up robber is carted off in a police van.
In order to put muggers (and superheroes) out of business, city and federal governments the world over are mandating a switch to solid-state lighting for streets, also known as LED lighting. In the process they will reduce electrical energy consumption by 50% and operating costs by 40%. The payback period for the investment can be from 1 to 3 years. Replacement/retrofit LED streetlamps are expected to be a $2B industry in 2020. Streetlights promise better light quality, literally turning night into day, and allow light to be directed to where it is needed rather than splashed in all directions. They can be remotely controlled, they can turn themselves down when no one is around, and they can also turn themselves completely off when the sun comes up (or when Cyclops generates an optic-blast when battling a fellow mutant).
In lamps, LEDs occur in strings – each dropping 3.5V, with the entire string designed to be driven off a 40 V or 100 V supply depending on the number of LEDs in each string. The same current flows through each LED in a given string, ranging from 0.35 A to 1 A. One nagging problem is the reliability of light output: LEDs do sometimes fail. Depending on their construction, they can fail either as OPENs or as SHORTs. In the event of an OPEN circuit, current in the entire string goes to zero, and all LEDs go dark, thereby causing a loss of light and the creation of an ugly dark line where there was previously light. In contrast, when an LED fails as a SHORT its light output goes to zero but the remainder of the string stays lit – for a while. Eventually due to thermal stress the shorted device fails to an OPEN and in turn causes the entire string to go dark. Mean-time-to-failure (MTTF) to OPEN of an LED depends on whether it is internally constructed using wire-bonds or bump/solder-attach schemes. The MTTF also depends on the operating current level and the ambient temperature – as one may expect, MTTF gets smaller as the current level goes up or the ambient temperature rises.
In modern lamps, ever-shrinking space budgets and the quest for more lumens per square centimeter (a “lumen” is a measure of brightness experienced by the human eye) unfortunately put a lot of pressure on both operating current and ambient temperature: the higher the current an LED can carry, the greater is its light output. Also, the less space there is for heat-sinking, the higher is the operating temperature. When a high-performing streetlamp loses significant light output due to OPEN LED string, a significant cost is involved in replacing it – not just for the light fixture itself, but for the equipment and personnel needed to replace it. Compounding this issue is the increasing demand for long lamp lifetimes: often 10-15 years.
An easy way to protect against the loss of entire strings is to use ON Semiconductor’s LED bypass shunts – simple, compact, two-terminal devices that essentially lie dormant during normal operation. When an LED fails OPEN the corresponding shunt device temporarily experiences a high voltage that causes it to switch into a conducting state, allowing it to carry the designed LED current. The other LEDs in the string perform normally as a result, and the string stays lit except for one dark spot where the OPEN-circuited LED lies. LED shunts therefore transform the picture to the left of Figure 1 into the picture on the right.
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Figure 1: A single LED that fails to an OPEN can render an entire string dark. ON Semiconductor’s LED bypass shunts allow remaining LEDs in the string to function normally, transforming the situation on the left with its dramatic reduction in light, to the situation on the right. The extension of lamp lifetime helps meet warranty requirements.
The loss of light can be minimized by using one shunt for each LED – an ideal product for this application is the NUD4700 LED shunt, designed to turn on when there is a transient peak at ~7 V, and housed in the thermally superior POWERMITE® package that allows DC operation up to 1.3 A. As alternatives, ON Semiconductor also offers higher voltage HBL1015 and HBL1025 LED shunts that bypass two and three LEDs respectively. This is schematically illustrated in Figure 2 below.
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Figure 2: Schematics for LED streetlamps highlighting the use of bypass shunts: one NUD4700 for each LED (left) or one HBL1015 for each pair of LEDs (center) or one instance of HBL1025 for each trio of LEDs (right). The lower total cost in the latter case is countered by the fact that even if only one LED fails OPEN, the shunt bypasses (and darkens) three LEDs.
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Figure 3: LED bypass shunts for street-lighting applications.
Figure 3 shows a comparison of our various LED bypass shunt offerings for street-lighting applications.
The next time you walk past a streetlamp on a windswept street look up to see if it has LEDs. If it does, you can walk more easily and enjoy the evening, as there is no danger of sudden lamp failure or of caped crusaders swinging down from treetops. As you saunter away, take a moment to thank the many scientists and engineers who made LED technology possible – a technology whose original inventors won the Nobel Prize in Physics for 2014.
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