Why is EMC rectification difficult? Do you understand?

What is EMC? What is its role? EMC is a difficult point in the industry. The article introduces the three laws of EMC, the three elements of EMC problems, the characteristics of electromagnetic disturbance, and the five-level EMC design method; it provides suggestions for companies to deal with EMC. ; The author believes that EMC improvements should be treated symptomatically just like diagnosing and treating diseases; the author advocates adhering to EMC rules and considering and solving EMC problems as early as possible - conducting EMC design.

EMC is an important part of product certification. With my country's accession to the WTO, whether it is Chinese products going to the international market or foreign products flooding into the Chinese market, almost all kinds of product certifications must be carried out. Product certification, from the perspective of international trade, is essentially a technical trade barrier. Only by continuously improving product quality and breaking through technical barriers can we develop overseas markets and promote the development of foreign trade. The new domestic 3C certification has replaced the original CCIB and CCEE certification. "CCC" is the English abbreviation of China Compulsory Certification, my country's compulsory product certification mark. Only products that have obtained 3C certification can enter the domestic market. 3C certification has detailed regulations on the safety performance, EMC and other aspects of electromechanical and electrical products.

EMC is a technical difficulty for most enterprises. It is our established policy to vigorously develop high-tech industries with the electronics and information industries as the main body. Today is an era where even technology is catching up with the trend. Mechanical and electrical products are changing with each passing day. "Thin, light, short, small and multi-functional" has become fashionable. Products with high technical content such as digital products, mechatronic products, information appliances, multimedia equipment and so on are emerging in endlessly. Updates The speed of replacement is accelerating day by day, the level of chip integration and product working speed are constantly improving, and the accompanying electromagnetic disturbance problems are becoming increasingly complex. At present, my country's overall EMC research started late. Many companies do not have enough awareness of EMC, lack EMC experience and testing equipment, and have varying degrees of confusion in product design, production processes and component selection. In the end, EMC often fails to meet standard requirements. EMC issues are currently a technical difficulty for most enterprises! Most enterprises engaged in the manufacturing of electromechanical products share the same sentiment. Solving EMC issues is much more difficult than solving product safety issues. This further intensifies the phenomenon that some companies prefer to reduce costs and sacrifice EMC requirements in the face of brand competition and price competition.

The indicators for 3C certification of mechanical and electrical products involve two aspects: product safety and EMC. From the perspective of certification testing, the main reason why products fail to meet the "3C" certification requirements is that EMC cannot pass the test. In fact, whether in daily inspection or various product certifications, it is common for EMC tests to fail. With the smooth development of 3C certification, some companies are worried about EMC issues. It is our honorable duty to help companies. This manuscript does not have complex theoretical analysis and derivation, but simply introduces the three laws of EMC,

The three elements of EMC problems, the characteristics of electromagnetic disturbance, and the five-level EMC design method; on the one hand, we strive to be practical, and on the other hand, we intend to inspire others. Understand and utilize three important laws in the field of EMC. The author believes that EMC cannot be completely solved by theory alone. EMC is a practical project. If you are engaged in the EMC industry and can deeply understand the following three important laws in the EMC field and insist on applying them in practice, you will definitely get twice the result with half the effort.

1) Rule 1. EMC cost-effectiveness ratio relationship rule:

The earlier EMC issues are considered and resolved, the lower the cost and the better the effect. Carrying out EMC design during the R&D stage of new products, rather than waiting until the product fails EMC testing before making improvements, can greatly save costs and improve efficiency. On the contrary, efficiency will be greatly reduced and costs will be greatly increased. Experience tells us that it is the most time-saving and most cost-effective to carry out EMC design at the same time as functional design, and then pass EMC testing when the prototype and prototype are completed. On the contrary, EMC is not considered during the product development stage, and improvements are made only after the EMC is found to be unqualified after being put into production. Not only does it bring great technical difficulties, but rework will inevitably bring a great waste of cost and time, and even involves structural design and PCB design. Defects, inability to implement improvement measures, resulting in products not being put on the market.

2) Rule 2: The larger the high-frequency current loop area S, the more severe the EMI radiation.

High-frequency signal current flows through the smallest path of the inductor. When the frequency is high, the wiring reactance is generally greater than the resistance. The wiring acts as an inductance to high-frequency signals, and the series inductance causes radiation. Electromagnetic radiation is mostly generated by high-frequency current loops on the EUT equipment under test. The worst case is an open-circuit antenna. The corresponding treatment method is to reduce or shorten the wiring, reduce the high-frequency current loop area, and try to eliminate any abnormal antennas required for normal operation, such as discontinuous wiring or overly long pins of components with antenna effects. One of the most important tasks to reduce radiation harassment or improve radio frequency radiation anti-interference capability is to find ways to reduce the high-frequency current loop area S.

3) Rule 3: The higher the loop current frequency f, the more severe the EMI radiation caused. The electromagnetic radiation field intensity increases in proportion to the square of the current frequency f.

The second most important way to reduce radiation harassment or improve the anti-interference ability of radio frequency radiation is to find ways to reduce the frequency f of the high-frequency current of the harassment source, that is, to reduce the frequency f of the harassment electromagnetic wave. The following content of this article uses the above three laws to advocate early consideration of EMC issues and introduces EMC design and EMC issue improvements.

Improving EMC problems is like diagnosing and treating diseases. If the product fails the EMC test, we can only know from the measurement results which frequency points are "exceeding the standard", and where the electromagnetic disturbance at these frequencies comes from is often the hardest problem for engineers to find and solve. . Product EMC issues are difficult to say and easy to say. Just like treating a patient, the key is to see whether your disease is curable or incurable, and whether treatable diseases can be treated based on syndrome differentiation. Traditional Chinese medicine pays attention to inspection, smell, touch, diagnosis and questioning; modern medicine pays more attention to blood pressure measurement, BT, CT, laboratory tests, etc. The purpose is to find out the focus and cause of the disease, and to treat the disease accordingly.

Similarly, improving EMC problems as a way to diagnose and treat diseases is a combination of science and fun. First of all, based on the path and mechanism of EMI generation, that is, the factors that cause EMC problems, make some judgments based on the circuit principle of the EUT (sample under test, the same below), such as IT equipment and AV audio and video equipment causing EMC problems. First, make inferences about the causes or internal sources of harassment, and then use the test chart of the test items to see the essence through the phenomenon, and analyze the reasons for the out-of-tolerance - clarify the sources of harassment, and thoroughly explore the channels of harassment, so that you can be targeted. To analyze the causes of out-of-tolerance, you can use a high-frequency oscilloscope or spectrum analyzer plus a field probe to verify the analysis results. From the frequency domain to the time domain, and then from the frequency domain to the time domain, analyze and find the corresponding circuits and devices that cause EMC problems.

Three Elements of EMC Issues Switching power supplies and digital equipment generate strong radiation because pulse currents and voltages are rich in high-frequency harmonics. Electromagnetic interference includes radiation (high frequency) EMI and conduction (low frequency) EMI. That is, there are two main ways to generate EMC problems: one is in the form of space electromagnetic wave interference; the other is through conduction. In other words, EMC is generated The three elements of the problem are: electromagnetic interference sources, coupling pathways, and sensitive equipment. Radiated interference mainly pollutes the space electromagnetic environment in the form of electromagnetic waves through the housing and connecting wires; conducted interference harasses the public power grid through power lines or affects connected equipment through other terminals (such as radio frequency terminals, input terminals).

Conducted, radiated, and disturbed sources -------(path)------ Possible sources of disturbance from sensitive receptor near-field coupling IT and AV equipment

A) FM receivers and TV receivers oscillate locally, and the fundamental wave and harmonics are generated by the tuner and the local oscillation circuit;

B) The switching pulses and high-frequency harmonics of the switching power supply, the square wave and high-frequency harmonics of the synchronization signal, the horizontal and field signals and high-frequency harmonics generated by the line scanning imaging circuit;

C) Various clock signals and high-frequency harmonics required for digital circuit operation, and their combinations, various clocks such as CPU chip working clock, MPEG decoder working clock, video synchronization clock (27MHz, 16.9344MHz, 40.5MHz), etc. ;

D) Unwanted signals and spurious signals caused by digital signal square waves and high-frequency harmonics, high-order harmonics generated by crystal oscillators, nonlinear circuit phenomena (nonlinear distortion, intermodulation, saturation distortion, cutoff distortion), etc.;

E) 非正弦波波形，波形毛剌、过冲、振铃，电路设计存在的寄生频率点。

F) 对于敏感受体通过耦合途径接受的外部骚扰包括浪涌、快速脉冲群、静电、电压跌落、电压变化和各种电磁场。

电磁骚扰的特性

① 单位脉冲的频谱最宽;

② 频谱中低频含量取决于脉冲的面积，高频分量取决于脉冲前后沿的陡度;

③ 晶体振荡电平必须满足一定幅度, 数字电路才能按一定的时序工作，使晶振产生的骚扰呈现覆盖带宽、骚扰电平高的特点;

④ 收发天线极化、方向特性相同时，EMI辐射和接受最严重;收发天线面积越大, EMI危害逾大;

⑤ 骚扰途径：辐射，传导，耦合和辐射、传导、耦合的组合。

⑥ 电源线传导骚扰主要由共模电流产生;

⑦ 辐射骚扰主要由差模电流形成的环路产生。EMC设计前面已经提过，EMC的根本问题，解决 EMC问题的根本办法，无论从市场经济的原则出发，还是从其它方面考虑，都是趁早进行EMC设计。从设计立项的一开始，就把EMC要求纳入设计任务书，作为设计的输入之一。

EMC设计，简单地说，就是仔细预测可能发生的各种EMC问题，进行方案和电路的优化选型，寻找一种优化电路、机械结构和PCB的设计解决方案，提高产品的设计质量，确保达到功能和性能指标的情况下，兼顾成本效益，避免EMC问题。为抑制和消除骚扰源，减小高频信号频率、减小高频电流回路面积、减小共阻抗耦合或感应耦合，选用低速、低辐射器件，选用屏蔽机箱、屏蔽电缆和I/O滤波器都是常用的措施。

一般来说，EMC设计可分五个层次。以下为五个层次EMC设计要点：

1) 方案选择、主要部件、集成电路的选型、电路和机械结构设计;对于产品的成功与否，第一层次设计是最基本、最重要的，任何错误都意味着该产品项目彻底失败。这一层主要EMC考虑体现在：

a)方案选择、主要部件、集成电路的选型主要考虑减少辐射骚扰或提高射频辐射抗干扰能力，尽量选用本身发射小的芯片，如翻转时间长、工作速率低的器件，多地线脚的芯片(芯片实质就是集成度较高的电路模块，封装时多装地线脚，可以减小高速差模电流环面积S，相应地减小芯片的发射);避免使用大功率、高损耗器件，它们往往是大的辐射源;

b)保证所选器件不工作在非线性区，以免产生谐波分量成为干扰源。

c)电路和机械结构设计除考虑减少辐射骚扰或提高射频辐射抗干扰能力外，主要考虑电源电路防外部骚扰包括浪涌、快速脉冲群、静电、电压跌落、电压变化等;

d)电路设计或方案应不使数字信号波形产生过冲，应使无用的谐波振荡幅度最小，使无用的高次谐波成分最少，避免引发强烈的电磁骚扰;

e)对集总参数电路，增加阻尼、减小Q值，防止振荡;

2)PCB的EMC设计;

对于产品的成功与否， PCB的EMC设计是重要的一环。PCB设计不合理，会产生无法补救的后果;PCB良好的EMC设计，有事半功倍的效果。PCB 的EMC设计应遵循以下内容：

a) 尽量减小所有的高速信号及时钟信号线构成的环路面积，连接线要尽可能短，并使信号线紧邻地回路;

b) 使用小型化器件和多层线路板，多层印制板可紧缩布线空间，高频特性好，容易实现EMC;

c) 印制板层数选择考虑关键信号的屏蔽和隔离要求，先确定所需信号层数，然后考虑成本的前提下，增加地平面和电源层是PCB EMC设计最好的措施之一;

d) 印制板分层原理与布置印刷电路、布置排线的原理一样，元件面下面为地平面，关键电源平面与其对应的地平面相邻，相邻层的关键信号不跨区，所有的信号层特别是高速信号、时钟信号与地平面相邻，尽量避免两信号层相邻;

e) 个别电源层、地层不能作为一个连续的平面时，采用多网孔连接形成地格蜂窝网，有效减小电流环路面积，减小公共阻抗R，加大信号与地层分布电容;

f) 线路板布线设计时顺序考虑：电源和地/时钟线/信号线，布线应该短、直、粗、匀，不要直角和突变， 应有“之”字形，用圆角代替尖锐走线，尽可能加宽电源和地的布线，电源和地层的分割，尽量符合微带线和带状线要求;

g) 走线尽可能远离骚扰源，布线考虑铁氧体材料的使用，预留磁珠和贴片滤波器的位置，以备按需加减;

3)电与接地、高速信号线路及内部线缆的EMC设计;PCB的EMC设计中也提到供电与接地、高速信号线路的EMC设计，此外，还应遵循以下内容：

a)芯片间使用低阻抗地连接(地平面)，不同芯片供电脚间阻抗尽量小，芯片供电脚(意思是离芯片供电 脚很近的供电线上)与地间接高频旁路电容，供电布线预留磁珠和贴片滤波器的位置，以备按需加减;

b) 布线、I/O排线的核心原则就是减小电流环面积S,布置排线的原理与印制板分层原理一样，关键电源线与其对应的地线相邻，所有的信号层特别是高速信号、时钟信号线与地线相邻，尽量避免两信号线相邻;

c) 为避免接地线长度过长(接近λ/4)，可采用多点就近接地，接地线高频阻抗要小;

d) 减小电缆的天线效应及减小偶极子天线效应，跨线、I/O排线采用屏蔽性能好的线缆，内导线采用多股双绞线，使空间场互抵，屏蔽层可作为回线;

e) 机内采用屏蔽线防止感应噪声;

f) 波器的输入输出线应拉开距离，忌并行走线，以免影响滤波效果;

h) I/O接口注意高速电路阻抗匹配，减小、消除反射;

4) 屏蔽设计;屏蔽好的要求有三：完整的电连续体;滤波措施;良好的接地。

对于信息技术IT类设备,当主板及配置选定的情况下, 提高整机的屏蔽效果和各个部分的隔离效果非常重要，尤其个人计算机和液晶显示器。这里只说屏蔽设计:

a) 计算机机壳内骚扰场强较大，机壳塑料部分未涂导 电材料或所涂导电材料不佳，机箱有孔洞、缝隙，不是一个完整电连续体，进出线滤波不好，最终都可导致辐射骚扰超出限值。机箱为了更好屏蔽电磁辐射，既能照顾到机箱的散热需求，又能有效地防止 磁波的衍射，开孔尺寸一般不超过4mm;

b) 根据产品实际进行屏蔽设计，端口、通风孔、孔洞、连接缝隙的屏蔽性都是值得考虑的因素;

c) 液晶显示器为了更好屏蔽电磁辐射可以采用喷涂导电材料的外壳(接缝处要喷涂导电材料);

d) 为了将辐射减到最小，尽量使用通过了CQC(EMC方面)自愿认证的机箱;

e) 为保证机箱的密封性，要使用精密模具冲压成型，设计适当的弹点和卷边;

f) 变压器加静电屏蔽及接地等

5)输入/输出的滤波设计电源线滤波和信号线滤波的重要性并不亚于机箱屏蔽，滤波关键是针对EMC 要求，兼顾达标和经济的原则。在I/O接口部位，一般采用高频滤波效果好、安装简单的滤波连接器。在电缆上缠绕或套用铁氧体磁环也能起到一定的滤波吸波作用。设计或使用信号线滤波器时，滤波器的截止频率须高于电缆上要传输的信号频率。

a) 传导骚扰问题处理的方法主要是低通滤波。在1MHz以上时，传导发射问题通常是由辐射发射的耦合而引起的，须综合运用抑制传导发射和辐射发射的技术措施，如屏蔽、去耦和滤波。

b) The attenuation performance of the filter circuit is closely related to the impedance of the source and load. The greater the mismatch, the better the filter is at attenuating electromagnetic disturbance. In most cases, the power line exhibits low impedance, so the input to the filter should be high impedance. On the other hand, a device can be either high or low impedance. For high impedance linear supplies, to obtain impedance mismatch, the load side should be designed with low impedance. For low-impedance devices such as switching power supplies and synchronous motors, the load side is designed to be high-impedance.

c) Subtract common mode and differential mode capacitance, add and subtract common mode and differential mode coils, adjust capacitance parameters and the number of coil turns, and the curves of common mode and differential mode insertion loss versus frequency can be changed. The leakage current of the filter refers to the current flowing between the phase line and neutral line and the shell ground. It mainly depends on the common mode capacitance connected between the phase line and ground and the neutral line and ground. The larger the capacity of the common mode capacitor, the smaller the common mode impedance, and the better the common mode harassment suppression effect. However, safety standards stipulate that the leakage current cannot be too large.

d) The power filter should be installed close to the entrance of the power cord. It would be better if it can be integrated with the interface. For metal shielded chassis, choose an independent power supply shielding filter, install it at the entrance of the power cord, and ensure that the filter shell is in good electrical contact with the equipment chassis (ground). This has the best effect. The filter ground is usually fixed to a common ground metal member at the cable outlet.

Advice for businesses

1. Understand the three elements of EMC problems, the characteristics of electromagnetic disturbance, electromagnetic disturbance sources and transmission channels, master the five levels of EMC design rules, insist on using EMC rules, and consider and solve EMC problems as early as possible; when encountering PCB, it must be redesigned or the structure must be redesigned. During the design, everyone only regretted that EMC was considered too late. Treating a disease is worse than preventing it. Treatment must be targeted sooner rather than later. The same principle applies to solving EMC problems.

2. When the EMC of a product does not meet the requirements and needs to be rectified, the source of electromagnetic disturbance and the coupling path must be diagnosed first like treating a disease, and then the method mentioned in the key points of EMC design should be used to target the problem and comprehensively use shielding and filtering to absorb waves. , grounding and other measures to implement improvements. During the improvement process, if the test fails again, first review whether the problem judgment is correct? Are the countermeasures wrong? Do the device parameters need to be adjusted? Don't change your original intention all at once, and take your time. Special attention should be paid during rectification. After correctly diagnosing the electromagnetic disturbance source and coupling path, when using EMC suppression devices, not only must the appropriate selection be made, but also the devices used must be genuine, so that the treatment will not last long.

3. The factory should identify key production processes, and operators of key processes should be trained and formulate corresponding process instructions or standard samples (you can take photos and give pictures) to keep the production process under control. Take the most simplified prototype with a certain EMC margin as a standard part to check the production and assembly processes. During inspection, focus on inspection/verification of key EMC components and materials, as well as assembly process consistency inspection.

4. In order to verify that products continue to meet standard requirements, factories should conduct confirmation inspections of products at appropriate stages (if they do not have the testing conditions themselves, samples will be sent to competent institutions for inspection) to ensure that products continue to meet EMC requirements, in case of changes. can be discovered in time.

5. When the EMC key components of the product need to be changed or adjusted, use new components to replace the original components and re-manufacture several prototypes for testing to confirm the impact of changes and adjustments on the EMC key components of the entire machine.

summary

Understand the three elements of EMC problems, the characteristics of electromagnetic disturbance, electromagnetic disturbance sources and transmission channels, and master the five-level EMC design method. EMC will become rule-based. Adhere to EMC rules, consider and solve EMC problems as early as possible, and you can save money. Save time and effort, get twice the result with half the effort. In terms of EMC work, patients should be treated according to the symptoms according to the way traditional Chinese medicine doctors treat patients, and be consciously humanized, which can increase their interest in solving problems. 90% of EMC is designed, and very little has been corrected. It is better to prevent the disease than to treat it. To treat the disease, we must start with the right symptoms, sooner rather than later. The reason is simple, but it needs more attention. The above is some analysis of EMC rectification, I hope it can help you.