buck开关电源EMI抑制

Reducing Emissions in the Buck Converter SMPS

减少Buck变换器开关电源的EMI发射

Abstract

?Switched Mode Power Supply demands are increasing, as the electronics industry requires more DC-DC conversion. In the past, linear regulators

have been used to regulate power, but as the difference between supply voltages and desired output voltage increases, they become very inefficient.

The BUCK power supply is efficient in converting higher voltages to lower voltages, but unfortunately in the process, both change in current (dI/dt) and change in voltage (dV/dt) are experienced. These changing parameters can cause excessive emissions in the RF spectrum, in conducted and radiated forms. We will examine the modes under which these emissions are

allowed to propagate, and investigate techniques used to reduce them.

?开关电源的需求正在增加，如电子行业需要更多的DC -DC转换。在过去，常用线性稳压器，但随着电源电压和所需的输出电压之间的差额的增加，它们变得非常低效。BUCK降压型电源在将高电压变到低电压时有更高的转换效率，但不幸的是，在这个过程中，电流的变化（di/ dt）和电压的变化（dv/ dt）都是经常的。这些不断变化的参数可能会以传导和辐射的形式造成超标发射的RF频谱。我们将研究发射被传播的模式，调查减少它们可采用的技术。

Introduction

?One primary contributor to the low frequency emissions is the switching frequency of the converter, found typically in the 100’s of kHz range. Energy at the fundamental

frequency along with several of its harmonics can find its way out onto the wire harness and radiate effectively.

These emissions are derived from, among other things, the sudden changes in current flow (dI/dt) as a result of the regulator (SMPS IC in Figure 1) turning on and off

during its periodic cycle.

?

导言

低频EMI发射的一个主要贡献者是转换器的开关频率，典型的发现在几百kHz的频率范围。基频与它的几个谐波的能量可以在线缆上找到它的出路形成辐射。除其他外，这些发射是来自于调整器(SMPS IC in Figure 1)在它的周期循环中不停地开和关引起的电流突变(dI/dt)所造成的结果。

Figure 1. Typical BUCK SMPS Circuit Diagram.图1 典型的BUCK开关电源简图

?When the SMPS IC turns ON, the current flows through L1, SMPS IC, L2, and is delivered to the load (in parallel with C4). When the SMPS IC turns OFF, the current flowing through the SMPS IC stops. At this same moment, the energy stored in the inductor (L2) is released to the load, as the “free-wheeling”diode (CR1) begins

conducting. It is this switching that creates current flow discontinuities at the input to the power supply. These current spikes in turn can drive the wire harness, attached to the product (Position 1 in Figure 1), like a transmitting antenna. Equation 1 can be used to calculate resonant frequencies (Hertz) of the cabling; substitute the length of the attached cable (meters) for l, and the speed of light (3x108m/s) for C. Using 2, 4, and 20 times the length of the attached cable for l allows other resonant frequencies to be calculated. If any resonant frequencies of the cabling correspond with undesired RF frequencies coming from the Power Supply, the resonance can exaggerate the RF emissions problem.

?当SMPS IC打开，电流流经L1，SMPS IC，L2，并交付给负载（并联电容C4 ）。当SMPS IC关闭，电流到SMPS IC停止。在这同一时刻，储存在电感（L2）上的能量释放给负载，因为“free-wheeling”二极管（CR1 ）开始导通。正是这种开关转换，造成电源输入电流的不连续性。这些电流峰值反过来可以推动连到产品上的线束（图1位置

1 ），如发射天线。方程1可用于计算电缆的谐振频率（赫兹）; λ替代成附加电缆的长

度L（米），光速C为3x108米/秒,用2 ，4 ，和20倍的电缆长度，可计算其他谐振频率。如果有任何与电缆长度相关的共振频率，不受欢迎的RF频率从电源发出，共振可以恶化RF辐射问题。

λ= C / f(1)

Figure 2

Figure 2 shows discontinuities at the input to the power supply (measuring between VIN of the SMPS IC and ground) while the regulator is switching. Notice that the discontinuities at VIN correspond directly to sharp changes in the SMPS IC’s output voltage (measured between Figure 1 position 4, and ground).

图2示出在整流器转换时电源输入的不连续性（测量SMPS IC 的VIN和ground），。请注意，在VIN的不连续性直接对应SMPS IC的输出电压的急剧变化（测量图1的位置4和ground之间）。

?At the output of the regulator (position 4 in Figure 1), dur-ing switching states, two separate resonant RLC networks can be defined. These networks produce an under-damped response when “excited”

by a step function (ie. switching!) and allow high frequency ringing to oscillate for several cycles.

?在稳压器的输出（图1中的位置4），在转换状态，两个单独的共振RLC网络可以被界定。这些网络在开关阶梯激励下产生不完全衰减响应，并允许高频振铃振荡几个周期。

?Broadband RF emissions commonly seen anywhere from 40 –140 MHz are a direct result of this ringing. The R, L, and C components that make up the networks are defined by the path that the current flow takes during each switch state. RLC network #1 is formed when the SMPS IC output turns OFF. In this state the current flows from ground through CR1, L2, C4, and the LOAD.

?到处常见从40 -140兆赫的宽带射频发射，就是这个振铃的直接结果。组成网络的R ，L和C元件，由在每个开关状态电流流动的路径确定。SMPS IC 输出关闭时形成RLC network #1，在该状态电流流动从ground通过CR1, L2, C4，和负载。

?Each of these devices has impedance that is made up of R’s, L’s, and C’s (including the PCB layout traces and parasitics). When combined, these properties form the resonant network that gets

“excited”by the step response of the switching (ON -OFF, or OFF-ON).

?所有这些器件都有RLC形成的阻抗（包括PCB布局线迹和寄生）。组合后，这些参数形成谐振网络，获得由开关阶跃（ON -OFF, 或OFF-ON)）的“激励”。

?RLC network #2 is formed when the SMPS IC output turns ON. In this state, the current flows from the OUT pin of the SMPS IC (Figure 1), through L2, C4, and the LOAD.

?RLC network #2形成于SMPS IC输出打开。在此状态下，电流流动从SMPS IC的输出引脚（图1 ），通过L2，C4和负载。Each of these two paths has a unique frequency response, and is tuned

differently.

?这两个路径有着独特的频率响应，并调整不同。

?An RC snubber circuit (R1 & C3) can be added in parallel to the output of the regulator to create a more “dampened”response in the two unique RLC networks.

?一个RC缓冲电路(R1 & C3)可以并行增加到调节器的输出上，在这两个独特的RLC网络上以产生一个更加“dampened”的响应

?If values are chosen correctly, the snubber circuit will reduce the amplitude and number of cycles of the unwanted ringing. See waveforms in Figure 3 that illustrate this ringing. Note the frequency of the ringing is directly related to high frequency emissions seen during testing (see Figure 4 @ 60MHz).?如果值是选择正确的话，缓冲电路将减少一些不想要的振铃的振幅和持续周期数。看波形图3可说明此振铃。注意振铃的频率，与高频发射测试过程中看到的直接相关（见图4 @ 60MHz ）。

Figure 3. Leading edge, ringing waveforms.

Ringing measured on the output of the regulator (position 4 in Figure 1) also affects the current flow through the output inductor, and causes a corresponding change in the magnetic field surrounding the device. This changing magnetic field can couple onto neighboring traces or planes allowing the RF energy to proliferate throughout the board.在稳压器输出端测量到的振铃（在图1位置4 ）也影响到流经输出电感的电流，并引起了器件周围的磁场的相应的变化。这一变化的磁场可以耦合到旁边的线迹或平面使RF能量扩散到整个板。

Layout Considerations布局考虑

?

One of the most important layout considerations for buck converters is to minimize parasitic capacitance and inductance at the output of the regulator. Parasitics contribute significantly to the ringing and other distortion on the output waveform. Do not place a ground

plane on any layer of the board stack-up directly below inductor L2;

this creates the parasitic capacitance that should be avoided. An option to consider would be to place a power bus (+) beneath

inductor L2 if using a multi-layer board.

?

对降压转换器布局考虑其中最重要的是尽量减少稳压器的输出端的寄生电容和电感。寄生对输出波形的振铃和其他失真作出重大贡献。不要在电感L2直接下面任何层上安排一个接地面;这创造了寄生电容，应加以避免。如果使用多层电路板，一种可选考虑将是在电感L2下方放一个电源总线（+ ）。

Loop Areas环面积

?Loop areas need to be controlled and minimized (physical area) in the layout to reduce overall emissions. A loop formed between the output pin of the SMPS IC and the ground on the load

(connected across C4) contains large amplitude dI/dt waveforms and must be controlled to the fullest extent possible. Another loop is formed between position 1 and position 3 in Figure 1.

?

为降低整体的辐射，布局中对环面积需要控制和减少到最低限度（物理面积）。在SMPS IC的输出引脚与负载(connected across C4)的ground之间形成的环，包含大振幅的dI/dt波形，必须予以尽可能充分的控制，图1上位置1和3之间形成另一回路。

?Finally, a loop area formed between the input to the supply (position 1) (usually at the main connector) and the ground to the SMPC IC, contains small discontinuities (dV/dt and dI/dt).

?最后，在电源输入(position 1，通常是主要的连接器）和SMPC IC的ground之间形成一个环面积，包含小的不连续（dV/dt and dI/dt) 。

?Loop areas are kept small through proper floor planning and routing of traces.

?通过适当的布局规划和布线保持小的环面积

?Initially our output stage (CR1, L2, C4) was not properly designed and the loop area formed there was approximately 3 in2.

?最初我们的输出级（CR1, L2, C4）未妥善设计，形成的环面积有大约三平方英寸。

?Redesigning the output stage allowed us to reduce the area to 1.5 in2.

?重新设计输出级，使我们能够减少该面积到一点五平方英寸。

?This change brought improvements in the amount of cycles and amplitude of the unwanted ringing on the output (Figure 1 position 4).

?Although this was not the only change to the design, reducing this loop area had a direct correlation to reducing broadband emissions in the frequency range of 40MHz –140MHz (Figures 4 & 5).

?这种变化带来了对输出上（图1位置4 ）不必要的振铃的持续周期数和振幅的改进。虽然这不是设计上唯一的改变，但减少该环路面积与减少40MHz -140MHz频率范围宽带发射直接相关（图4和5 ）。

Feedback Trace反馈线

?The feedback trace is used by the SMPS IC to sense the status of its output (connected between C4 and the feedback pin of the SMPS IC). Therefore, it is critical to route it away from any noisy circuitry, in particular, the output inductor L2. If using a multi-layer PCB, the

feedback trace of the regulator can be embedded in a layer below the top layer (further down the board stack-up) and shielded by

ground from the layers above and below. A ferrite may also be

placed in series with this trace to reduce RF energy that can enter the SMPS IC and can adversely influence the output waveform.

?

SMPS IC使用反馈线感知其输出状态（C4和SMPS IC反馈引脚之间连接的）。因此，关键是要让它的布线远离噪音电路，尤其是输出电感L2 。如果使用多层电路板，调节器的反馈线可以嵌入在最上层以下的一个层，从在上面和下面用地面层屏蔽。铁氧体也可串入在这一反馈线上，以减少能进入SMPS IC并对输出波形产生不利影响的射频能量。

Component Considerations

组件思考

?Whenever possible, use surface mount devices to minimize lead inductance. L2 should be a closed core inductor. This type of

component keeps most of the magnetic field confined within the core during the switching of the regulator. Industry suppliers have various options to choose from. The best approach is to ask vendors for

samples, and try each type during testing. Choosing the right device is a key factor in reducing emissions, since the field surrounding the inductor is constantly changing (i.e. ringing, switching current) and can couple to neighboring components and traces, etc.

?只要有可能，使用表面贴装器件，以减少引线电感。L2应该是一个封闭磁芯电感。这种类型的组件在调节器开关动作时可保持大多数磁场局限于磁芯。工业供应商的各种选择方案可供选择。最好的办法是要求供应商提供样品，并尝试测试每种类型。选择正确的器件是一个减少发射的关键因素。因为电感周围的场是不断变化的（即振铃，开关电流），并可以耦合到邻近的元件和迹线等

Snubber Circuit缓冲电路

? A snubber circuit can be used in parallel with the output of the SMPS IC (Figure 1 position 4 to ground) to reduce distortion and ringing on the output waveform. Typically this circuit is located between the “free-wheeling diode CR1,”and the output inductor L2.

?This circuit works like a high frequency shunt to allow RF energy to return to ground (i.e. capacitor to ground).

?One important factor to consider when choosing to use a snubber circuit is that you must sacrifice some efficiency in the power conversion.

?Some of the power will be dissipated in the snubber circuit itself

?The series resistor is used to limit the amount of RF current taking this path to ground and the capacitor is used to “tune”the frequency.

?The switching waveform at the location of the snubber circuit is primarily a square wave, and Fourier theory states that it contains high frequency content that is dependent upon the rise time of the waveform.

?Consequently, without a series resistor in the snubber circuit, there will be significant power dissipation in the shunt capacitor. The snubber circuit can be effective at reducing broadband RF emissions typically seen between 40MHz and 140MHz. Typical values are R = 20 ohms, and C = .01uf. Also, low parasitic inductance components should be used to avoid forming resonant tank circuits. Therefore, avoid using wire-wound resistors, or leaded capacitors.

?缓冲电路可用于并联到SMPS IC输出上（图1中位置4到地），以减少输出波形的失真和振铃。通常这个电路位于free-wheeling diode CR1和输出电感L2之间的。这个电路具有高频分流作用，使RF能量返回地面（即电容器到地）。

?当你选择使用一个缓冲电路时需要考虑的一个重要因素是这样做必须牺牲电源转换的一些效率。

?部分功率将消耗在缓冲电路本身。

?该串联电阻是用来限制这条途径到地的射频电流，电容是用来“调”频率。

?缓冲电路位置的开关波形主要是一个方波，按傅立叶理论，它包含的高频成分取决于波形的上升时间。?因此，在缓冲电路如果没有一个串联电阻，将有显著的功耗消耗在并联电容器上。该缓冲电路可以有效地减少通常可见的40MHz到140MHz之间宽带射频发射量。典型值为R = 20欧姆，和c = 0.01 uf。此外，应使用低寄生电感元件，以避免形成谐振电路。因此，避免使用线绕电阻或引线电容。

Diode

二极管

?The diode (CR1) should be placed on the same side of the PCB where the other power supply circuitry is placed. This is done in order to minimize inductance (by avoiding the use of vias) while the diode is in “conduction”mode and current is flowing through the diode (CR1) and out to the LOAD (C4). This proved to be very beneficial for reducing the emissions near 70 MHz. Diodes that are available in the industry can have various switching characteristics such as: soft start, ultra slow start and fast start. These terms refer to how fast or slow the diode switches from the reverse diode block mode (when the SMPS IC output is ON), to the forward conducting mode (when the SMPS IC is OFF). Much of this parameter has to do with the forward voltage required to “setup”the p-n junction. Tradeoffs must be considered when selecting the freewheeling diode and the output inductor. The longer the inductor is in a non-steady state mode, the more heat it will be required to dissipate.

?二极管（CR1 ）应放在印刷电路板上的其他供电电路同一面。这样做是为了尽量减少电感（避免使用过孔），而二极管是在“导通”模式和电流是流经二极管（CR1 ）和向负载（C4）。这被证明对减少接近70兆赫的发射是非常有益的。工业中能得到的二极管，可以有不同的开关特性，如：软启动，超慢启动，快速启动。这些条款是指当从二极管的反向阻塞模式（SMPS IC output开），转换成正向导通模式（SMPS IC关闭）转换速度有多快或多慢。需要正向电压“setup”PN结。必须权衡考虑选择freewheeling二极管和输出电感。时间越长，电感是一种非稳态模式，更多的热量将需要消散。

When selecting diodes from vendors, look for the symbol “Vf”that indicates the forward voltage required to turn the diode on. A smaller voltage rating means that the diode will turn on faster.

Each diode has its own characteristic impedance that can affect the nature of the high frequency emission (40MHz –140MHz). The process of selecting the right diode can be trial and error. This is due to the parasitic inductance and capacitance inherent and unique to each individual layout. If you have test equipment available, one of the best ways to approach this is to obtain samples from vendors and place each diode on the PCB while observing the RF emissions performance.?在选择二极管供应商，寻找有符号“Vf”，表明需要正向电压打开二极管。一个较小的额定电压是指二极管就会更快。每个二极管都有自己的特性阻抗，可以影响高频发射（40MHz -140MHz ）的性质，。选择合适的二极管的过程中可以反复试验和除错。这是由于每一个独特的布局，寄生电感和电容也是固有的、独特的。如果你得到测试设备，最佳途径之一，从供应商获取样品，每个二极管安装到PCB上，同时验证RF发射性能。

Front End Filtration前端过滤

?

Clean input power is critical for “quiet”SMPS IC operation. In order to reduce the RF emissions within the low frequency range (i.e.

switching frequency up to several Megahertz), front-end filtration must be carefully selected. Given that there are two types of

discontinuities seen at the input to the SMPS IC (i.e. voltage and current), two types of filtering need to be addressed.

?为使SMPS IC“安静”运行，清洁输入电源是至关重要的。为了减少低频范围内（即从开关频率至高达数兆赫）RF发射，前端滤波必须仔细挑选。鉴于看到SMPS IC输入端有两种类型的不连续性（即电压和电流），两种类型的滤波需要加以解决。

Series Inductance

串联电感

?The series inductor (L1) stores energy and releases it as necessary to reduce current spikes. Caution should be used when selecting this inductor. First, the device should be properly rated for steady state current flow. Second, a balance should be considered between having enough inductance to smooth larger current spikes, but also keeping a low series DC resistance. Large voltage drops can occur depending upon current draw, subsequently reducing the overall voltage available to other circuitry including the SMPS IC.

?

串联电感（L1）存贮能量，在需要时分发，以减少电流峰值。注意选择此电感。第一，该器件应适当限定稳态电流。第二，应在有足够大的电感来平滑大的电流峰值，而且还要保持低串联直流电阻之间平衡考虑。大电压跌落，可能会发生取决于拉电流，随后降低总体电压提供给其他电路包括开关电源集成电路。

Bulk Capacitance

散装电容

?Bulk parallel capacitance is required at the input of the SMPS IC to remove voltage discontinuities, and should be placed as closely as possible to the input pins of the SMPS IC. However, if the bulk capacitor is too large, the charging period will be long and may cause large current spikes (i.e. higher emissions). If using Electrolytic

capacitors, it is best to choose the lowest ESR (Equivalent Series Resistance)

possible. This ensures that stored charge is delivered, with the lowest impedance, to the SMPS IC. In some cases it may be necessary to add a small ceramic capacitor in parallel with the input between the bulk capacitor and the SMPS IC. Ceramic

capacitors have very low ESR and can a provide charge at faster rates to reduce unwanted swings in the voltage.

?

需要在开关电源集成电路输入并联散装电容，消除电压不连贯性，并应放在尽可能靠近开关电源集成电路的输入引脚。但是，如果大容量电容过大，充电将长的，可能会造成大电流峰值（即高发射）。如果使用电解电容，最好是尽可能选择最低的ESR （等效串联电阻）。这可以确保存贮充电，对开关电源集成电路最低阻抗。在某些情况下可能有必要加一个小的陶瓷电容并联到输入，在大容量电容和开关电源集成电路之间。陶瓷电容器具有非常低ESR和能提供更快的充电率，以减少不必要的电压波动。

Common Mode Choke共模扼流圈

?Common mode chokes are effective input filters. From our experience, ferrite core CMC’s were effective at

reducing emissions in the ringing frequency range (40 MHz –140 MHz) but did not help at the switching

frequency (260 kHz). Using an iron core transformer at the input did the opposite where it adequately reduced switching frequency noise but did not reduce the ringing frequency emissions.

?

共模扼流圈是有效的输入滤波器。根据我们的经验，铁氧体磁芯CMC在减少振铃频率范围（40兆赫-140兆赫）的发射方面是有效的，但对开关频率（260千赫）没有帮助。在输入端使用铁芯变压器充分降低开关频率的噪音，但没有减少振铃频率发射。

Design Methodology

设计方法

?One of the best ways to approach a power supply design is to focus only on the power supply section initially. Remove any circuitry from the PCB that is not related to the power supply.

?

电源设计最佳途径之一是开始只注重电源部分。再移走印刷电路板上与电源不相关的任何电路。?This allows the designer to implement a layout that is as close to ideal as possible, given the shape and size of the PCB.

?这使得设计人员能够采用的布局是尽可能接近理想，规定电路板的形状和大小。

?An artificial load can be made to draw the same expected current, as the load will draw on the board in the power supply’s final application. The artificial load can be connected during initial testing to obtain results. We recommend using a conducted emissions test as a way to

benchmark and monitor improvements as the design is optimized. One primary advantage to this method is that efforts are focused on the power supply design. Not having the other application specific circuitry placed on the PCB makes it easier to manipulate the power supply design. It also requires less time to make changes and release another prototype revision. After the optimal power supply design has been determined, the remaining circuitry can be added. Changing as little of the power supply section as possible will maintain the improved EMC performance that has been achieved.

?人工负载可以得出相同的预期电流，和板上最终应用在电源上的负载一样。人工负荷可连接在初步测试以取得结果。我们建议您使用传导发射测试作为建立标杆和监测设计优化的改进。一个主要优势是，这种方法的努力主要集中在电源设计。没有其他的具体应用电路放在PCB板更容易操纵电源设计。这还需要更少的时间进行更改，并释放另一个原型修订。在优化电源设计已经确定，剩下的电路可以添加。电源部分尽可能少的改变，以保持EMC性能方面已取得的改善。