外文翻译--- -LED照明知识：PWM调光.docx

附录AA matter of light：PWM dimming By Sameh Sarhan and Chris Richardson, National Semiconductor Whether you drive LEDs with a buck, boost, buck-boost or linear regulator, the common thread is drive circuitry to control the light output. A few applications are as simple as ON and OFF, but the greater number of applications call for dimming the output between zero and 100 percent, often with fine resolution. The designer has two main choices: adjust the LED current linearly (analog dimming), or use switching circuitry that works at a frequency high enough for the eye to average the light output (digital dimming). Using pulse-width modulation (PWM) to set the period and duty cycle (Fig. 1) is perhaps the easiest way to accomplish digital dimming, and a buck regulator topology will often provide the best peRFormance. Figure 1: LED driver using PWM dimming, with waveforms.PWM dimming preferred Analog dimming is often simpler to implement. We vary the output of the LED driver in proportion to a control voltage. Analog dimming introduces no new frequencies as potential sources of EMC/EMI. However, PWM dimming is used in most designs, owing to a fundamental property of LEDs: the character of the light emitted shifts in proportion to the average drive current. For monochromatic LEDs, the dominant wavELength changes. For white LEDs, the correlated color temperature (CCT) changes. Its difficult for the human eye to detect a change of a few nanometers in a red, green, or blue LED, especially when the light intensity is also changing. A change in color temperature of white light, however, is easily detected. Most white LEDs consist of a die that emits photons in the blue spectrum, which strike a phosphor coating that in turn emits photons over a broad range of visible light. At low currents the phosphor dominates and the light tends to be more yellow. At high currents the blue emission of the LED dominates, giving the light a blue cast, leading to a higher CCT. In applications with more than one white LED, a difference in CCT between two adjacent LEDs can be both obvious and unpleasant. That concept extends to light sources that blend light from multiple monochromatic LEDs. When we have more than one light source, any difference between them jars the senses. LED manufacturers specify a certain drive current in the electrical characteristics tables of their products, and they guarantee the dominant wavelength or CCT only at those specified currents. Dimming with PWM ensures that the LEDs emit the color that the lighting designer needs, regardless of the intensity. Such precise control is particularly important in RGB applications where we blend light of different colors to produce white. From the driver IC perspective, analog dimming presents a serious challenge to the output current accuracy. Almost every LED driver uses a resistor of some type in series with the output to sense current. The current-sense voltage, VSNS, is selected as a compromise to maintain low Power dissipation while keeping a high signal-to-noise ratio (SNR). Tolerances, offsets, and delays in the driver introduce an error that remains relatively fixed. To reduce output current in a closed-loop system, VSNS, must be reduced. That in turn reduces the output current accuracy and ultimately the output current cannot be specified, controlled, or guaranteed. In general, dimming with PWM allows more accurate, linear control over the light output down to much lower levels than analog dimming. Dimming frequency vs. contrast ratio The LED drivers finite response time to a PWM dimming signal creates design issues. There are three main types of delay (Fig. 2). The longer these delays, the lower the achievable contrast ratio (a measure of control over lighting intensity). Figure 2: Dimming delays. As shown, tn represents the propagation delay from the time logic signal VDIM goes high to the time that the LED driver begins to increase the output current. In addition, tsu is the time needed for the output current to slew from zero to the target level, and tsn is the time needed for the output current to slew from the target level back down to zero. In general, the lower the dimming frequency, fDIM, the higher contrast ratio, as these fixed delays consume a smaller portion of the dimming period, TDIM.The lower limit for fDIM is approximately 120 Hz, below which the eye no longer blends the pulses into a perceived continuous light. The upper limit is determined by the minimum contrast ratio that is required. Contrast ratio is typically expressed as the inverse of the minimum on-time, i.e., CR = 1 / tON-MIN : 1 where tON-MIN = tD + tSU. Applications in machine vision and industrial inspection often require much higher PWM dimming frequencies because the high-speed cameras and sensors used respond much more quickly than the human eye. In such applications the goal of rapid turn-on and turn-off of the LED light source is not to reduce the average light output, but to synchronize the light output with the sensor or camera capture times. Dimming with a switching regulator Switching regulator-based LED drivers require special consideration in order to be shut off and turned on at hundreds or thousands of times per second. Regulators designed for standard power supplies often have an enable pin or shutdown pin to which a logic-level PWM signal can be applied, but the associated delay, tD, is often quite long. This is because the silicon design emphasizes low shutdown current over response time. Dedicated switching regulations for driving LEDs will do the opposite, keeping their internal control circuits active while the enable pin is logic low to minimize tD, while suffering a higher operating current while the LEDs are off. Optimizing light control with PWM requires minimum slew-up and slew-down delays not only for best contrast ratio, but to minimize the time that the LED spends between zero and the target level (where the dominant wavelength and CCT are not guaranteed). A standard switching regulator will have a soft-start and often a soft-shutdown, but dedicated LED drivers do everything within their control to reduce these slew rates. Reducing tSU and tSN involves both the silicon design and the topology of switching regulator that is used. Buck regulators are superior to all other switching topologies with respect to fast slew rates for two distinct reasons. First, the buck regulator is the only switching converter that delivers power to the output while the control switch is on. This makes the control loops of buck regulators with voltage-mode or current-mode PWM (not to be confused with the dimming via PWM) faster than the boost regulator or the various buck-boost topologies. Power delivery during the control switchs on-time also adapts easily to hysteretic control, which is even faster than the best voltage-mode or current-mode control loops. Second, the buck regulators inductor is connected to the output during the entire switching cycle. This ensures a continuous output current and means that the output capacitor can be eliminated. Without an output capacitor the buck regulator becomes a true, high impedance current source, capable of slewing the output voltage very quickly. Cuk and zeta converters can claim continuous output inductors, but fall behind when their slower control loops (and lower efficiency) are factored in. Faster than the enable pin Even a pure hysteretic buck regulator without an output capacitor will not be capable of meeting the requirements of some PWM dimming systems. These applications need high PWM dimming frequency and high contrast ratio, which in turn requires fast slew rates and short delay times. Along with machine vision and industrial inspection, examples of systems that need high performance include backlighting of LCD panels and video projection. In some cases the PWM dimming frequency must be pushed to beyond the audio band, to 25 kHz or more. With the total dimming period reduced to a matter of microseconds, total rise and fall times for the LED current, including propagation delays, must be reduced to the nanosecond range. Consider a fast buck regulator with no output capacitor. The delays in turning the output current on and off come from the ICs propagation delay and the physical properties of the output inductor. For truly high speed PWM dimming, both must be bypassed. The best way to accomplish this is by using a power switch in parallel with the LED chain (Fig. 3). To turn the LEDs off, the drive current is shunted through the switch, which is typically an n-MOSFET. The IC continues to operate and the inductor current continues to flow. The main disadvantage of this method is that power is wasted while the LEDs are off, even through the output voltage drops to equal the current sense voltage during this time. Figure 3: Shunt FET circuit, with waveforms. Dimming with a shunt FET causes rapid shifts in the output voltage, to which the ICs control loop must respond in an attempt to keep the output current constant. As with logic-pin dimming, the faster the control loop, the better the response, and buck regulators with hysteretic control provide the best response. Fast PWM with boost and buck-boost Neither the boost regulator nor any of the buck-boost topologies are well suited to PWM dimming. Thats because in the continuous conduction mode (CCM), each one exhibits a right-half plane zero, which makes it difficult to achieve the high control loop bandwidth needed in clocked regulators. The time-domain effects of the right-half plane zero also make it much more difficult to use hysteretic control for boost or buck-boost circuits. In addition, the boost regulator cannot tolerate an output voltage that falls below the input voltage. Such a condition causes a short circuit at the input, and makes dimming with a parallel FET impossible. Among the buck-boost topologies, parallel FET dimming is still impossible or at best impractical due to the requirement for an output capacitor (the SEPIC, buck-boost and flyback), or the uncontrolled input inductor current during output short circuits (Cuk and zeta). When true fast PWM dimming is required, the best solution is a two-stage system that uses a buck regulator as the second, LED driving stage. When space and cost do not permit this approach, the next best choice is a series switch (Fig. 4). Figure 4: Boost regulator with series DIM switch. LED current can be shut off immediately. On the other hand, special consideration must be given to the system response. Such an open circuit is in effect a fast, extreme unloading transient that also disconnects the feedback loop and will cause the regulators output voltage to rise without bound. Clamping circuits for the output and/or the error amplifier are required to prevent failure due to over-voltage. These clamps are difficult to realize with external circuitry, hence series FET dimming is practical only with dedicated boost/buck-boost LED driver ICs. In summary, proper control of LED lighting requires careful attention right from the start of the design process. The more sophisticated the light source, the more likely that PWM dimming will be used. This in turn requires the system designer to carefully consider the LED driver topology. Buck regulators offer many advantages for PWM dimming. If the dimming frequency must be high, or the slew rates must be fast, or both, then the buck regulator is the way to go. 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 bachelors 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. Source: National Semiconductor Corporation附录BLED照明知识：PWM调光不管你用Buck, Boost, Buck-Boost还是线性调节器来驱动LED，它们的共同思路都是用驱动电路来控制光的输出。一些应用只是简单地来实现“开”和“关”地功能，但是更多地应用需求是要从0到100%调节光的亮度，而且经常要有很高的精度。设计者主要有两个选择：线性调节LED电流（模拟调光），或者使用开关电路以相对于人眼识别力来说足够高的频率工作来改变光输出的平均值（数字调光）。使用脉冲宽度调制（PWM）来设置周期和占空度（图1）可能是最简单的实现数字调光的方法，并且Buck调节器拓扑往往能够提供一个最好的性能。 图1：使用PWM调光的LED驱动及其波形。推荐的PWM调光 模拟调光通常可以很简单的来实现。我们可以通过一个控制电压来成比例地改变LED驱动的输出。模拟调光不会引入潜在的电磁兼容/电磁干扰（EMC/EMI）频率。然而，在大多数设计中要使用PWM调光，这是由于LED的一个基本性质：发射光的特性要随着平均驱动电流而偏移。对于单色LED来说，其主波长会改变。对白光LED来说，其相关颜色温度（CCT）会改变。对于人眼来说，很难察觉到红、绿或蓝LED中几纳米波长的变化，特别是在光强也在变化的时候。但是白光的颜色温度变化是很容易检测的。 大多数LED包含一个发射蓝光谱光子的区域，它透过一个磷面提供一个宽幅可见光。低电流的时候，磷光占主导，光趋近于黄色。高电流的时候，LED蓝光占主导，光呈现蓝色，从而达到了一个高CCT。当使用一个以上的白光LED的时候，相邻LED的CCT的不同会很明显也是不希望发生的。同样延伸到光源应用里，混合多个单色LED也会存在同样的问题。当我们使用一个以上的光源的时候，LED中任何的差异都会被察觉到。 LED生产商在他们的产品电气特性表中特别制定了一个驱动电流，这样就能保证只以这些特定驱动电流来产生的光波长或CCT。用PWM调光保证了LED发出设计者需要的颜色，而光的强度另当别论。这种精细控制在RGB应用中特别重要，以混合不同颜色的光来产生白光。 从驱动IC的前景来看，模拟调光面临着一个严峻的挑战，这就是输出电流精度。几乎每个LED驱动都要用到某种串联电阻来辨别电流。电流辨别电压（VSNS）通过折衷低能耗损失和高信噪比来选定。驱动中的容差、偏移和延迟导致了一个相对固定的误差。要在一个闭环系统中降低输出电流就必须降低VSNS。这样就会反过来降低输出电流的精度，最终，输出电流无法指定、控制或保证。通常来说，相对于模拟调光，PWM调光可以提高精度，线性控制光输出到更低级。 调光频率VS对比度 LED驱动对PWM调光信号的不可忽视的回应时间产生了一个设计问题。这里主要有三种主要延迟（图2）。这些延迟越长，可以达到的对比度就越低（光强的控制尺度）。 图2：调光延迟。如图所示，tn表示从时间逻辑信号VDIM提升到足以使LED驱动开始提高输出电流的时候的过渡延迟。另外，tsu输出电流从零提升到目标级所需要的时间，相反，tsn是输出电流从目标级下降到零所需要的时间。一般来说，调光频率（fDIM）越低，对比度越高，这是因为这些固定延迟消耗了一小部分的调光周期（TDIM）。fDIM的下限大概是120Hz，低于这个下限，肉眼就不会再把脉冲混合成一个感觉起来持续的光。另外，上限是由达到最小对比度来确定的。 对比度通常由最小脉宽值的倒数来表示： CR = 1 / tON-MIN : 1 这里tON-MIN = tD + tSU。在机器视觉和工业检验应用中常常需要更高的PWM调光频率，因为高速相机和传感器需要远远快于人眼的反应时间。在这种应用中，LED光源的快速开通和关闭的目的不是为了降低输出光的平均强度，而是为了使输出光与传感器和相机时间同步。用开关调节器调光 基于开关调节器的LED驱动需要一些特别考虑，以便于每秒钟关掉和开启成百上千次。用于通常供电的调节器常常有一个开启或关掉针脚来供逻辑电平PWM信号连接，但是与此相关的延迟（tD）常常很久。这是因为硅设计强调回应时间中的低关断电流。而驱动LED的专用开关调节则相反，当开启针脚为逻辑低以最小化tD时，内部控制电路始终保持开启，然而当LED关断的时候，控制电流却很高。 用PWM来优化光源控制需要最小化上升和下降延迟，这不仅是为了达到最好的对比度，而且也为了最小化LED从零到目标电平的时间（这里主导光波长和CCT不能保证）。标准开关调节器常常会有一个缓开和缓关的过程，但是LED专用驱动可以做所有的事情，其中包括降低信号转换速率的控制。降低tSU 和 tSN要从硅设计和开关调节器拓扑两方面入手。 Bu