热门搜索:

  • /?36
  • 下载费用:10 金币 ?

板级EMC设计.pdf

关?键?词:
板级 EMC 设计
资源描述:
? Freescale Semiconductor, Inc., 2005. All rights reserved.Freescale Semiconductor Application Note AN2321 Rev. 1, 10/2005 Designing for Board Level Electromagnetic Compatibility by: T.C. Lun Applications Engineering Microcontroller Division This application note discusses board level electromagnetic compatibility (EMC), from component selection, circuit design, to printed circuit board layout. The text is divided into the following parts: ? PART 1: An overview of EMC  PART 2: Component selection and circuit design techniques  PART 3: Printed circuit board layout techniques  APPENDIX A: Glossary of EMC terms  APPENDIX B: Immunity measurement standardsDesigning for Board Level Electromagnetic Compatibility, Rev. 1 2 Freescale SemiconductorPART 1: AN OVERVIEW OF ELECTROMAGNETIC INTERFERENCE AND COMPATIBILITY PART 1: AN OVERVIEW OF ELECTROMAGNETIC INTERFERENCE AND COMPATIBILITY Electromagnetic interference (EMI) is a major problem in modern electronic circuits. To overcome the interference, the designer has to either remove the source of the interference, or protect the circuit being affected. The ultimate goal is to have the circuit board operating as intended — to achieve electromagnetic compatibility (EMC). Achieving board level EMC may not be enough. Although the circuit may be working at the board level, but it may be radiating noise to other parts of the system, causing problems at the system level. Furthermore, EMC at the system or equipment level may have to satisfy certain emission standards, so that the equipment does not affect other equipment or appliances. Many developed countries have strict EMC standards on electrical equipment and appliances; to meet these, the designer will have to think about EMI suppression — starting from the board level. Elements of the Electromagnetic Environment A simple EMI model consists of three elements:  EMI source  Coupling path  Receptor This is shown graphically in Figure 1. Figure 1. EMI Elements EMI source EMI sources include microprocessors, microcontrollers, electrostatic discharges, transmitters, transient power components such as electromechanical relays, switching power supplies, and lightning. Within a microcontroller system, the clock circuitry is usually the biggest generator of wide-band noise, which is noise that is distributed throughout the frequency spectrum. With the increase of faster semiconductors, with faster edge rates, these circuits can produce harmonic disturbances up to 300MHz. EMI source Coupling path Receptor Control emissions (Reduce noise source level) (Reduce propagation efficiency) Control susceptibility (Reduce propagation efficiency) (Increase receptor immunity)PART 1: AN OVERVIEW OF ELECTROMAGNETIC INTERFERENCE AND COMPATIBILITY Designing for Board Level Electromagnetic Compatibility, Rev. 1 Freescale Semiconductor 3Coupling path The simplest way noise can be coupled into a circuit is through conductors. If a wire runs through a noisy environment, the wire will pick up the noise inductively and pass it into the rest of the circuit. An example of this type of coupling is found when noise enters a system through the power supply leads. Noise carried on the power supply lines are conducted to all circuits. Coupling can also occur in circuits that share common impedances. For instance, two circuits that share the conductor carrying the supply voltage and the conductor carrying the return path to ground. If one circuit creates a sudden demand in current, the other circuit’s voltage supply will drop due to the common impedance both circuits share between the supply lines and the source impedance. This coupling effect can be reduced by decreasing the common impedance. Unfortunately, source impedance coupling is inherent to the power supply and cannot be reduced. The same effect occurs in the return-to-ground conductor. Digital return currents that flow in one circuit create ground bounce in the other circuit’s return path. An unstable ground will severely degrade the performance of low-level analog circuits, such as operational amplifiers, analog-to-digital converters, and sensors. Coupling also can occur with radiated electric and magnetic fields which are common to all electrical circuits. Whenever current changes, electromagnetic waves are generated. These waves can couple over to nearby conductors and interfere with other signals within the circuit. Receptor All electronic circuits are receptive to EMI transmissions. Most EMI are received from conductive transients, although some are received from direct radio frequency (RF) transmissions. In digital circuits, the most critical signals are usually the most vulnerable to EMI. These include reset, interrupt, and control line signals. Analog low-level amplifiers, control circuits, and power regulators also are susceptible to noise interference. To design for EMC and to meet EMC standards, the designer should minimize emissions (RF energy exiting from products), and increase susceptibility or immunity from emissions (RF energy entering into the products). Both emission and immunity can be classified by radiated and conductive coupling, as shown in Figure 1. The radiated coupling path will be more efficient in the higher frequencies while a conducted coupling path will be more efficient in the lower frequencies. Cost of EMC The most cost-effective way to design for EMC is to consider the EMC requirement at the early stages of the design (see Figure 2).Designing for Board Level Electromagnetic Compatibility, Rev. 1 4 Freescale SemiconductorPART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES Figure 2. Cost of EMC Measures It is unlikely that EMC will be the primary concern when the designer first chooses the components, designs the circuit, and designs the PCB layout. But if the suggestions in this application note are kept in mind, the possibility of poor component choice, poor circuit design, and poor PCB layout can be reduced. PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES Component selection and circuit design are major factors that will affect board level EMC performance. Each type of electronic components has its own characteristics, and therefore requires careful design considerations. The following sections will discuss some common electronic components and circuit design techniques for reducing or suppressing EMI. Component Packages There are basically two types of packages for all electronic components: leaded and leadless. Leaded components have parasitic effects, especially at high frequencies. The lead forms a low value inductor, about 1nH/mm per lead. The end terminations can also produce a small capacitive effect, in the region of 4pF. Therefore, it is usually the lead length that should be reduced as much as possible. Leadless and surface mount components have less parasitics compared with leaded components. Typically, 0.5nH of parasitic inductance with a small end termination capacitance of about 0.3pF. From an EMC viewpoint, surface mount components is preferred, followed by radial leaded, and then axial leaded. Product Definition Circuit Design PCB Layout Prototype Functional and Compliance Test Product Launch Mass Cost of EMC Measures ProductionPART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES Designing for Board Level Electromagnetic Compatibility, Rev. 1 Freescale Semiconductor 5Resistors Surface mount resistors are always preferred over leaded types because of their low parasitic elements. For the leaded type, the carbon film type is the preferred choice, followed by the metal film, then the wire wound. The metal film resistor, with its dominant parasitic elements at relatively low frequencies (in the MHz), is therefore suitable for high power density or high accuracy circuits. The wire wound resistor is highly inductive, therefore it should be avoided in frequency sensitive applications. It is best for high power handling circuits. In amplifier designs, the resistor choice is very important. At high frequencies, the impedance will increase by the effect of the inductance in the resistor. Therefore, the placement of the gain setting resistors should be as close as possible to the amplifier circuit to minimize the board inductance. In pull-up/pull-down resistor circuits, the fast switching from the transistors or IC circuits create ringing. To minimize this effect, all biasing resistors must be placed as close as possible to the active device and its local power and ground to minimize the inductance from the PCB trace. In regulator or reference circuits, the DC bias resistor must be placed as close as possible to the active device to minimize decoupling effect (i.e. improve transient response time). In RC filter networks the inductive effect from the resistor must be considered because the parasitic inductance of the wire wound resistor can easily cause local oscillation. Capacitors Selecting the right capacitor is not easy due to their many types and behaviors. Nonetheless, the capacitor is one component that can solve many EMC problems. The following sections describe the most common types, their characteristics and uses. Aluminium electrolytic capacitors are usually constructed by winding metal foils spirally between a thin layer of dielectric, which gives high capacitance per unit volume but increases internal inductance of the part. Tantalum capacitors are made from a block of the dielectric with direct plate and pin connections, which gives a lower internal inductance than aluminium electrolytic capacitors. Ceramic capacitors are constructed of multiple parallel metal plates within a ceramic dielectric. The dominant parasitic is the inductance of the plate structure and this usually dominates the impedance for most types in the lower MHz region. The difference in frequency response of different dielectric materials mean a type of capacitor is more suited to one application than another. Aluminium and tantalum electrolytic types dominate at the low frequency end, mainly in reservoir and low frequency filtering applications. In the mid-frequency range (from kHz to MHz) the ceramic capacitor dominates, for decoupling and higher frequency filters. Special low-loss (usually higher cost) ceramic and mica capacitors are available for very high frequency applications and microwave circuits.Designing for Board Level Electromagnetic Compatibility, Rev. 1 6 Freescale SemiconductorPART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES For best EMC performance, it is important to have a low ESR (equivalent series resistance) value as this provides a higher attenuation to signals, especially frequencies close to the self-resonant frequency of the capacitor in use. Bypass capacitors The main function of the bypass capacitor is to create an AC shunt to remove undesirable energy from entering susceptible areas. The bypass capacitor is acting as a high frequency bypass source to reduce the transient circuit demand on the power supply unit. Usually, the aluminium or tantalum capacitor is a good choice for bypass capacitors, its value depends on the transient current demand on the PCB, but it is usually in the range of 10 to 470 μF. Larger values are required on PCBs with a large number of integrated circuits, fast switching circuits, and PSUs having long leads to the PCB. Decoupling capacitors During active device switching, the high frequency switching noise created is distributed along the power supply lines. The main function of the decoupling capacitor is to provide a localized source of DC power for the active devices, thus reducing the switching noise propagating across the board and decoupling the noise to ground. Ideally, the bypass and decoupling should be placed as close as possible to the power supply inlet to help filter high frequency noise. The value of the decoupling capacitor is approximately 1/100 to 1/1000 of the bypass capacitor. For better EMC performance, decoupling capacitors should placed as close as possible to each IC, because track impedance will reduce the effectiveness of the decoupling function. Ceramic capacitors are usually selected for decoupling; choosing a value depends on the rise and fall times of the fastest signal. For example, with a 33MHz clock frequency, use 4.7nF to 100nF; with a 100MHz clock frequency, use 10nF. Apart from the capacitive value when choosing the decoupling capacitor, the low ESR of the capacitor also affects its decoupling capabilities. For decoupling, it is preferable to choose capacitors with a ESR value below 1 ?. Capacitor self-resonance The following briefly discusses how to choose the value of bypass and decoupling capacitors based on their self-resonant frequency. In Figure 3, the capacitor remains capacitive up to its self-resonant frequency. After that, the capacitor turns inductive, due to its lead length and trace inductance. Table 1 lists the self-resonant frequency for two types of ceramic capacitors, one with standard 0.25 inch leads with interconnect inductance of 3.75nH and the other surface mount with interconnect inductance of 1nH. We see that the self-resonant frequency of the surface mount type is double that of the through- hole type.PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES Designing for Board Level Electromagnetic Compatibility, Rev. 1 Freescale Semiconductor 7Figure 3. Impedance and Different Dielectric Materials Another factor that affect the effectiveness of the decoupling capacitor is the dielectric material of the capacitor. Two common materials are used in the manufacture of decoupling capacitors: barium titanate ceramic (Z5U) and strontium titanate (NPO). Z5U has a larger dielectric constant, with a self-resonant frequency from 1MHz to 20MHz. NPO has a lower dielectric constant, but a higher self-resonant frequency (greater than 10MHz). Therefore, Z5U is more suitable for low frequency decoupling, while NPO is good for decoupling at over 50MHz. A common practice is to use two decoupling capacitors in parallel. This configuration can provide a wider spectral distribution to reduce the switching noise induced by the power supply networks. Multiple decoupling capacitors connected in parallel can provide 6dB improvement to suppress RF currents generated by active device switching. The multiple decoupling capacitors not only provide wider spectral distribution, but also provide greater trace width such that to reduce lead inductance. Therefore, it will significantly improves the effectiveness of decoupling. The value of the two capacitors should differ by two orders of magnitude to provide effective decoupling (e.g. 0.1 μF + 0.001 μF connected in parallel). A point should be noted for digital circuit decoupling. A low ESR value is more important than the self- resonant frequency because a low ESR value can provide a lower impedance path to ground such tha
? 汽车智库所有资源均是用户自行上传分享,仅供网友学习交流,未经上传用户书面授权,请勿作他用。
0条评论

还可以输入200字符

暂无评论,赶快抢占沙发吧。

关于本文
本文标题:板级EMC设计.pdf
链接地址:http://www.autoekb.com/p-1593.html
关于我们 - 网站声明 - 网站地图 - 资源地图 - 友情链接 - 网站客服客服 - 联系我们

copyright@ 2008-2018 mywenku网站版权所有
经营许可证编号:京ICP备12026657号-3?

收起
展开