现阶段节能已经成为一种硬性的要求，不是一种选择，而其中一部分需要采用环
保绿色的方式去实现节能。对于照明来说，我们可以很容易的设想全球照明提高
10%的效率后所带来的影响，但是提高1000%呢？最近高效率发光二极管（LED）
可能实现这种效率的改进，并且能够以良好的表现力和高可靠性取代现阶段所用
的产品。这篇文章有四部分，第一部分我们关注LED 的物理结构，发光范围，效
率， 发光二极管驱动和应用。
物理解析
物理层面上，LED 类似于p-n 结二极管。作为p-n 结，当将阳极（p 区域）和阴
极（n 区域）之间加正电压时，电子和空穴向截面移动。一旦电子和空穴复合将
释放出能量，p-n 材料的物理特性决定了释放能量的形式，至于分立电路中用到
的标准二极管，它们释放能量的方式可以是没有辐射的或者以可见光的方式向外
释放。对于LED，它所发射出光的波长（也就是光的颜色）由p-n 结材料的禁带
宽度特性来决定。要想提高性能，LED 材料要有低的反向击穿电压，也就是低的
禁带宽度。
颜色
在20 世纪60 年代晚期，红光LED 在商业上最早得到了应用，但是当时发出的光
非常的弱。尽管有这样的不足，红光LED 还是被广泛的应用到七段显示器上。由
于材料科学上的进步，如今商业上应用的LED 已经可以发出多种色光，而其中一
部分光，如果你直接盯着看的话，能使你的眼睛失明。
蓝光LED 在几年前得到了广泛的应用。将蓝光LED，绿光和红光LED 所产生的光
混合将产生白光，这种可以产生白光的技术可以提供很宽的色域，光动态调谐特
性好，显色性能出色（CRI），非常适合高端背光应用。用蓝光LED 和荧光粉的方
式产生白光可以更简单更经济，它可以使一部分蓝光转换成黄光。黄光可以模仿
眼睛所感受到的红光和绿光，因此蓝光和黄光混合就可以产生白光。这种方案可
以提供良好的显色性能，但是由于制作工艺的差异和荧光粉厚度不同，使得这种
方式产生的白光遭受着色温不一致的困扰。
 效率
对于LED 光源来说高效率是一个术语，当提到照明时，效率被定义为单位功率所
产生的光，因此，在公制中，是用流明每瓦来衡量。最近一些LED 产品在介绍时
承诺效率可以高达150 流明/瓦，比较而言，白炽灯为15 流明/瓦，荧光灯可以
提供70 流明/瓦。因此LED 就会在不久的将来取代白炽灯和荧光灯吗？有可能，
但是不幸的是一些LED 的效率数据只是停留在规格书上。问题在于LED 产生的相
当一部分光在封装材料表面被反射回了LED 芯片，对于这样的事实LED 本身也无
能为力。这部分反射回来的光可能是被半导体材料吸收然后转换成热量一样。
抗放射涂层，和将反射角降到最低都可以减少光的反射量和提高效率，降低反射
角可以通过使用半球体封装，将LED 芯片放在中央来实现。然而这些技术还要取
决于制作工艺的变化程度和比较高的额外费用来确保性能稳定。因此尽管LED
在工业上正在被快速的采纳，但LED 的发展还有很长的路要走。
应用
很多因素使得LED 成为令人关注的高性能现代电子技术。例如，更高的输出效率
延长了电池寿命，这样非常适合便携产品的应用。此外，LED 快速的开关特性在
机动车尾灯的应用上可以得到完美的体现。尤其是刹车灯上的应用，它可以给驾
驶者提供更多的反映时间，提高驾车的安全性。由于LED 不含有铅和汞，在背光
应用上，红绿蓝三原色LED 满足了ROHS 标准。由于具有较宽的色域，LED 照明
使全光谱光源变得容易。LED 的独特之处是它的寿命很长，这一特性使得它可以
应用到对长期的稳定性能有较高要求的地方，例如交通灯。视觉处理系统需要聚
焦，闪光，和单色光源——LED 非常适合应用这种场合。LED 由于它简单易实现，
 光动态调谐特性好，在卧室里，当你要放松时，你可以将它设置成绿光，当要看
斗牛节目的时候，可以设置成红光。
驱动
LED 本质是电流驱动器件，也就是说，它的光强度随着正向电流IF 的变化而变化。
光的颜色同样也是由正向电流来决定，LED 正向电压VF ，也会影响颜色的变化。
于是，为了达到所需要光的颜色和亮度，有必要采用恒流驱动LED。一个简单的
LED 驱动的框架可以由一个电压源和一个限流电阻组成（图2a）。这种方法最适
合于窄输入电源，小电流的应用，这时候的LED 的正向压降略低于输入电压。但
是当输入电压变化或者LED 的正向压降升高时，电流就会变化，因此光的强度和
颜色就会改变。
图2：简单的LED 驱动框图
A．电源源和限流电阻 B.线性稳压电源 C.开关电容 D. 开关稳压电源
线性稳压电源常被应用在降压比率比较小的情况下，对LED 电流进行紧密控制
（如图2b）。在要求小电流升高的情况下，可以利用开关电容电路（图3c）。
至于宽输入范围，大电流应用，像上面的方法实现的简单驱动结构会导致功耗非
常高，效率非常低。所以将需要更高效和相应更复杂的解决方案，像开关稳压电
源（图2d）。开关稳压电源通过切断功率流和控制占空比大小来实现功率控制，
这种方式可以得到有规律振动的电流电压。可以将开关稳压电源设置成隔离或非
隔离的方式来实现电压或电流的降低(buck)或升高(boost),或两种功能同时发
生(buck-boost)。
 总之，设计者在选择开关稳压电源时，在给定功率转换要求下要对成本和性能进
行折中选择。另一方面，为了正常驱动LED，开关稳压电源应当设置在恒流工作
模式。当能提供最佳的成本和性能折中方案时，那一种开关稳压电源拓扑结构可
以简单的设置成电流源？请看文章的第二部分。
英文原文参考
A matter of light, Part 1---The ABC's of LEDs
By Sameh Sarhan and Chris Richardson, National Semiconductor
Conserving energy is now a mandate, not a choice, and part of that
mandate is the need to go Green. When it comes to lighting, we can easily
imagine the impact of globally improving the efficiency of lighting
sources by 10 percent. But what if it could be improved by 1000 percent?
Newly enhanced light emitting diodes (LEDs) have the potential to
achieve these efficiency improvements while maintaining high
performance and reliability that supersede many currently used sources.
In the first part of this four-part series, we look at the LED's physical
structure, range of colors, efficiency, LED drivers, and applications.
Anatomy
Physically, LEDs resemble p-n junction diodes. As with p-n junctions,
electrons and holes flow towards the junction when a positive differential
voltage is applied between the anode (p-side), and cathode (n-side). Once
an electron is recombined with a hole, it releases energy. Depending on
the physical properties of the p-n junction materials, the released energy
can be non-radiative, as for the typical diode applied in discrete circuits,
or in the form of emissions in the optical range. For an LED, the
wavelength of the emitted light (i.e., its color) depends on the band gap
characteristics of its p-n junction material. Performance-wise, LED
materials have relatively low reverse breakdown voltages since they have
relatively low band gaps.
Colors
Red LEDs were the first to become commercially available in the late
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1960s but their light output was very low. Despite this shortcoming, they
were commonly used in seven-segment displays. Thanks to
advancements in material science, nowadays LEDs are commercially
available in a variety of colors with some of them having light outputs
that would blind you if you stared directly at them.
Blue LEDs became widely available a few years ago. Mixing blue LEDs
with red and green LEDs produces white light. This technique of
generating white light provides a large color gamut, dynamic light tuning,
and excellent color rendering (CRI), which is well suited for high-end
backlighting applications. A simpler and more economical way of
producing white light is to use blue LEDs and a phosphor coating that
converts some of the blue light to yellow. The yellow light stimulates the
red and green receptors of the eye, and therefore mixing blue and yellow
gives the appearance of white. This scheme can provide good CRI but the
LED's light output may suffer from inconsistent color temperatures due to
manufacturing discrepancies and varying thicknesses in the phosphor
coating layer.
Figure 1: LED color chart for the basic colors
Efficiency
High efficiency is the buzz word for LED-based light sources. When it
comes to lighting, efficiency is defined as the light output per unit power.
Thus, in the metric system, it is measured in lumens (lm) per watt (W).
Recently some LED manufactures introduced LEDs with promised
efficiencies hitting the 150 lm/W mark. In comparison, incandescent
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comes in at 15 lm/W, and fluorescent provides 70 lm/W. So could LEDs
put incandescent and fluorescent out of business any time soon? Maybe,
but, unfortunately some of these LED's efficiency numbers are subject to
specmanship. The problem is that LED inefficiency has to do more with
the fact that a considerable portion of the produced light is reflected at the
surface of the packing material back into the LED die. This reflected light
is likely to be absorbed by the semiconductor material and turned into
heat.
Anti-reflection coating, and minimizing the reflection angles by using a
half-sphere package with the LED placed at the center, reduces the
amount of reflected light and improves efficiency. However, these
techniques are subject to manufacturing variations and may require high
premiums to ensure consistent performance. So while LEDs are rapidly
being adopted by industry, they've got a long way to go.
Applications
There are many factors which make LEDs eye-catching for
high-performance modern electronics. For example, their higher light
output per watt extends battery life and thus they are well suited for
portable applications. In addition, an LED's fast turn-on/turn-off
characteristics fit perfectly with the needs of automotive tail lights,
especially the brake lights, since it improves safety by providing drivers
more response time. RGB LEDs in backlighting complies with ROHS
standards, since LEDs do not contain lead or mercury. LED lighting
facilitates a full-spectrum light source with larger color gamut. LEDs
have an exceptionally long lifespan, which enables their use in
applications where long-term reliability is highly desirable, such as traffic
lights. Machine vision systems require a focused, bright, and
homogeneous light source—LEDs are a great match. LEDs, with their
simple-to-implement dynamic light-tuning, would also allow you to set
the light in your living room to green when you need to relax and to red
when it's time for bullfighting.
Drivers
LEDs are inherently current-driven devices; i.e., their brightness varies
with their forward current, IF. Depending on the color as well as the
forward current, the LEDs' forward voltage drop, VF, varies as well. Thus,
driving LEDs with a constant current is essential to achieve the desired
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color and brightness level. An LED driver scheme can be as simple as a
voltage source and a ballast resistor (Figure 2a). This solution works best
for narrow-input range, low-current applications in which the LED's
forward voltage drop is slightly below the input supply voltage. But
variations in the input supply voltage or the LED forward voltage drop
will increase the LED current and, therefore, the light intensity and the
color will shift.
Figure 2: Simplified LED driver schemes
Linear regulators can be used to provide tighter LED current control in
small step-down ratio applications (Figure 2b). In the case of low-current
step-up requirements, switching capacitor circuits can be utilized (Figure
3c).
For wide-input range, high-current applications, simple driver schemes
such as those mentioned above unfortunately yield high power dissipation
and poor efficiency. Consequently, more efficient and relatively more
complex solutions such as switching regulators are required (Figure 2d).
Switching regulators process power by interrupting the power flow and
controlling the conversion duty cycle, which results in pulsating current
and voltage. They can be configured in isolated and non-isolated
configurations to realize voltage or current step-down (buck), step-up
(boost) or both (buck-boost) functions.
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In general, designers select a switching-regulator topology based on a
tradeoff between cost and desired performance at a given power
conversion requirement. On the other hand, in order to properly drive
LEDs, the switching regulators should be configured as constant-current
sources. Which switching-regulator topology can be simply configured as
a current source while providing an optimal tradeoff between cost and
performance? Stay tuned for part two of this series.
About the authors
Sameh Sarhan is a staff applications engineer for the Medium
Voltage/High Voltage Power Management group in Santa Clara, CA. He
has been involved with power electronics in various forms since 1998,
having worked for FRC Corp. and Vicor Corp. His experience includes
the design of hard/soft switching power supplies from a few watts to 600
watts. Sameh received a bachelor's degree in electronics engineering in
1996 from Cairo University (Egypt).
Chris Richardson is an applications engineer in the Power Management
Products group, Medium and High Voltage Division. His responsibilities
are divided between lab work, bench evaluation of new ICs, written work
such as datasheets and applications notes, and training for field
engineers and seminars. Since joining National Semiconductor in 2001,
Chris has worked mainly on synchronous buck controllers and regulators.
In the last three years he has focused on products for the emerging high
brightness LED market in the automotive and industrial areas. Chris
holds a BSEE from the Virginia Polytechnic Institute and State University.