LED驱动需要不需要电容.docx

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Driving LEDs: To Cap or Not to Cap
Introduction
High-brightness LEDs are available today with forward currents more than 100 times greater than their predecessors. These new devices are not just high brightness, but are high power as well. Single die with dissipations of 5W and multi-die modules with power in excess of 25W are now available. The requirements of high efficiency and low dissipation dictate a switching power supply for this new generation of High-Brightness (HB), High-Power (HP) LEDs, as a voltage regulator and a current limiting resistor are no longer appropriate. High-brightness, high-power LEDs require a constant-current source to take full advantage of their ever-increasing luminous efficiency and vibrant, pure color. The topology of choice for this new breed of switching constant current sources is the basic buck converter. The most convincing argument for using a buck converter is the ease with which this simple DC-DC converter can be turned into a constant-current source. This article will explain the selection of, or possible exclusion of, an output capacitor when designing a buck regulator for constant-current drive of HB LEDs.

Controlled Current
The buck regulator is uniquely suited to be a constant current driver because the output inductor is in series with the load. Regardless of whether a buck regulator is used as a voltage source or a current source, selection of the inductor forms the cornerstone of the system design. With an inductor in series with the output, the average inductor current is always equal to the average output current, and the buck converter naturally maintains control of the AC-current ripple. By definition, the LED drive is a constant load system; hence a large amount of output capacitance is not necessary to maintain VOduring load transients.
No Output Cap Yields High Output Impedance
In theory, a perfect current source has infinite output impedance, allowing the voltage to slew infinitely fast in order to maintain a constant current. For switching regulator designers who have concentrated on voltage regulators, this concept may take a moment to sink in. Completely removing the output capacitor from a buck regulator forces the output impedance to depend on the inductor. Without any capacitance to oppose changes in VO, the output current (referred to as forward current, or IF) slew rate depends entirely upon the inductance, the input voltage, and the output voltage. (VO is equal to the combined forward voltage, VF, of each series-connected LED)
LED manufacturers generally recommend a ripple current, ΔIF, of ±5% to ±20% of the DC forward current. Over the typical switching regulator frequency range of 50 kHz to 2 MHz the ripple itself is not visible to the human eye. These limits come from increasing thermal losses at higher ripple current (a property of the LED semiconductor PN junction itself) and a practical limit to the inductance used. The percentages are similar to the recommended current ripple ratio in buck voltage regulators. Inductor selection for a fixed-frequency current regulator is therefore governed by the same equations as a voltage regulator:

One difference is that the inductance used for current regulators without output capacitors tends to be higher because the drive currents for the emerging standards of 1W, 3W, and 5W HB LEDs are 350 mA, 700 mA, and 1A respectively. Modern buck voltage regulators tend to use inductors in the range of 0.1 µH to 10 µH with saturation currents from 5A to 50A. Current drivers at similar switching frequencies tend to require inductors ranging from 10 µH to 1000 µH and saturation currents ranging from 0.5A to 5A.
The main goal of high output impedance is to create a system capable of responding to PWM dimming signals, the preferred method of controlling the light output of LEDs. The dimming signal might be applied to the enable pin of the regulator, in which case the output current can slew from zero to the target and back to zero without the delay of CO being charged and discharged. For even faster, higher resolution dimming, a shunt switch, usually a MOSFET, can be placed in parallel with the LED array, allowing the continuous flow of current at all times. Again, with no output capacitor to slow the slew rate, dimming frequencies into the 10’s of kHz are possible. This is a critical requirement in applications such as backlighting of flat-panel displays, and the creation of white light using an RGB array.

Using an Output Capacitor Reduces Size and Cost
Some amount of output capacitance can be useful as an AC current filter. Applications such as retrofitting of incandescent and halogen lights often require that the LED and driver be placed in a small space formerly occupied by a light bulb. Invariably the inductor is the largest, most expensive component after the LEDs themselves. For the sake of efficiency (especially important in cramped quarters), the designer generally chooses the lowest switching frequency that allows the solution (mostly the inductor) to fit. Allowing a large ripple current in the inductor and filtering the LED current results in a smaller, less expensive solution.For example, to drive a single white LED (VF≈3.5V) at 1A with a ripple current ΔiFof ±5% from an input of 12V at 500 kHz would require a 50 µH inductor with a current rating of 1.1A. A typical ferrite core device that fits this application might be 10 mm square and 4.5 mm in height. In contrast, if the inductor ripple current is allowed to increase to ±30% (typical for a low-current voltage regulator) then the inductance required is less than 10 µH, and an inductor measuring 6.0 mm square and only 2.8 mm in height size can be used. The output capacitance required is calculated based on the dynamic resistance, rD, of the LED, the sense resistance, RSNS , and the impedance of the capacitor at the switching frequency, using the following expressions:

Conclusion
The high brightness, high power LED represents the biggest change in lighting design since the introduction of fluorescent bulbs. Using LEDs requires a fundamental change in the complexity of electronics used for lighting systems. Currently a large portion of LED lighting design is retrofitting of incandescent, halogen, and fluorescent installations. Such systems rarely include sophisticated dimming control, and place a high value on small size. These are the applications where an output capacitor is a welcome addition to the driver circuit.
In the future, the higher cost of LEDs for general lighting will be balanced by new levels of control over brightness, tone, and color. Lighting in homes and businesses will require fast PWM dimming, requiring current drivers to minimize or eliminate their output capacitance. These systems will draw upon experience from today’s fast-dimming applications which have already shed the output capacitor to provide the best response time.

LED驱动需不需要电容
现如今高量发光二极管的正向电流越来越大，远远高过原先二极管的百倍以上。这些新材料的LED灯已经不单单只是高亮度了，而且功率越来越高。5W的单晶管损耗，甚至超过了25W的多晶片堆叠式的LED损耗现如今也是存在的。为满足现如今的极高的LED驱动效率和很低的损耗的要求，必须有新一代的高亮度的而且是高功率的开关电源驱动器去实现。而以以往的仅仅靠电压调节器和一个限流电阻的传统驱动器已经没法满足现在的需求了。要求的高亮度而且高效率的发光二极管驱动器需要一个稳定的恒流源去保证能够充分发挥光源的发光效率。同时能够使光源的亮度和纯度得到更充分的发挥。这种新型的自动切换恒流源的拓扑是工程设计中最基本的BUCK变换器。使用基本拓扑BUCK变换器的最优说服力的理由是这个DC-DC变换器很简单便可以转为恒流模式。这篇文章将解释，选择或者不使用一个输出电容对设计一个高亮的发光二极管驱动的BUCK恒流源将会产生的影响。

被控电流
BUCK变换器具有特有优点对于恒流的LED的驱动器，为什么？因为在BUCK变换器的基本结构中，我们可以看到，他的电感是和负载相串联的。先不管我们用BUCK变换器是作为恒流源使用还是作为电压源使用，选择具有电感串联的形式设计驱动系统是基础。和一个输出大电感串联，这样有一个优点，就是电感的电流总是和负载的输出电流相等，因此，BUCK变换器很容易控制输出的交流纹波电流。LED驱动器通过定义作为是一个恒定负载系统，当负载发生变化的时候，大容量的输出电容并不是维持稳定输出系统的必须条件。
没有输出电容产生高输出阻抗
在理论上，一个完美的电流源必须拥有无限大的输出电阻，可以保证极快的维持电流为一个常数。集中在研究电压调节器的开关电源工程师在实现这些理论可能需要一些时间。从BUCK变换器中完全移除输出电容，那么迫使完全靠电感来保证输出阻抗。没有用任何电容而要保证稳定不变的输出电压VO，输出电流（被成为正向电流或者IF）的变化率是完全取决于电感的、还有输入电压还有输出电压。（VO等同于连接起来的正向电压，即串联的LED压降VF）
LED制造商总是会推荐一个允许的纹波电流，∆IF，一般为5%~20%的直流电流。在典型的开关电源稳压器，50KHZ到2MHZ的频率范围纹波，人类的眼睛是看不见的。这些在高的纹波电流时会有更高的热损耗（LED半导体本质是PN结），这些条件限制了电感的使用。这个百分率类似于BUCK电压调节器中的纹波电流。电感器是按规定的频率选择的，因此，恒流器是和电压调节器具有相同的方程的：
L=VINVF=VIN-VF∆iL×fsw
一个不使用输出电容只使用电感器的恒流源的不同之处在于其电感量更高一些，因为驱动1W、3W、5W的 高亮的驱动电流分别是350毫安、750毫安、和1安。现在的技术BUCK变换器倾向于使用电感量在0.1μH到10μH的电感，其饱和电流在5A到50A。当前的电流源在现在的开关频率范围内倾向于使用的电感量在10μH到1000μH，同样其饱和电流的值是0.5A到5A。

要达到高输出阻抗的目标，创建一个系统应对PWM调光信号，实现控制LED是首先的方法。调光信号可能应用是作用于调节器的使能端，可以实现输出电流从零上升到目标电流，再从目标电流下降到零的目标，而避免了因为有输出电容在时的充放电而导致的延时。为了有更快更高效的调节方式，通常在LED组上并联一个MOSFET，可以保证电流总是连续的。同样，没有输出电容去减缓转换的速度，调光频率达到10K这么高是完全可能的。这个技术在平板显示器和其他的背光应用中是非常有利的，还比如RGB，和其他白光的调节。

使用一个输出电容可以降低电路面积和成本
一定数量的输出电容在交流电流滤波器是非常有用的。应用在白炽灯改造中的LED,我们通常都是将LED 和驱动器都一起放在一个狭小的空间里面，其中除了LED本身，发现电感总是最昂贵的也是最大体积的器件。为了达到高效率的目的（特别是在重要的节点），工程师总是用选择最低的开关频率达到提高效率的目的，而这其中电感的设计变得极为困难。在保证电感和输出调节器已经是最大的纹波电流的时候，依然可以正常工作，目前需要一个更加小体积和便宜的解决方案。例如驱动一个白光LED(VF≈3.5V)在1A的驱动电流，而5%输出电流的纹波电流作为12V的输出，即使用500K的开关频率，仍然需要一个50μH的电感，其纹波电流最大达1.1A。一个典型的铁氧体磁芯使用这个需求可能要达到10平方毫米和4.5毫米的高度。相反，如果允许的电感纹波电流从5%提升到30%（典型的电压调节器），那么所需要的电感量则为10μH，这是一个横截面积只有6平方毫米，高度只有2.8的铁氧体。所需要的输出电容计算是基于其动态电阻rD，还有在开关频率下的LED等效电阻RSNS,可以使用以下公式：
CO=12π×fsw×（ESR+ZC），ZC=∆iF∆iL-∆iF×rD
0.1μF和10μF 的陶瓷电容是非常适合在许多的电路中使用，通过添加一个很小的输出电容可以降低整块电路板的大小，同时又可以降低成本的解决方案。

结论
采用高亮度，高功率的LED照明是自采用荧光灯照明以来照明设计最大的变化。使用LED需要一个复杂的电子照明系统根本的变化。目前很大一部分的LED照明的设计是为了改造白炽灯和荧光灯等。这样的系统很少包括负载的调光系统，也很少有高功率密度的设计。这些应用中驱动电路使用输出电容是非常受到欢迎的。在未来，一般的LED成本会越来越高，因为照明将会在亮度、色调、颜色等方面出现新层次的控制。未来的照明，不管是在家里还是在企业中，都需要快速的PWM调光，要求当前的输出电容最小化或者直接消除他们的输出电容。这样的系统将会应用现在的经验，在保证低成本高效率时拥有最快的响应时间。