Recognize and utilize three important laws in the EMC field

EMC is a difficult point in the industry; the article describes EMC's three laws, the three elements of the EMC problem, the characteristics of electromagnetic harassment, and the five-level EMC design method; provides companies with advice on EMC; the author believes that EMC improvement should be treated like a disease. Treat symptomatically; the author advocates adhering to the EMC law, and early consideration and resolution of EMC issues - EMC design.

EMC is an important part of product certification. With China's accession to the WTO, whether it is a Chinese product that moves into an international market or a foreign product that has flocked to the Chinese market, almost all kinds of product certifications are required. Product certification, from the perspective of international trade, is essentially a technical barrier to trade. We can only open up overseas markets and promote the development of foreign trade by continuously improving product quality and breaking through technical barriers. The new domestic 3C certification replaces the original CCIB and CCEE certification. “CCC” is the English abbreviation for China Compulsory Certification, a mandatory product certification mark in China. Only products that have achieved 3C certification can enter the domestic market. 3C certification made detailed provisions on the safety performance and EMC of electromechanical and electrical products.

EMC is the technical difficulty of most companies. We are vigorously developing high-tech industries with electronic and information industries as the mainstay. This is our established policy. Today is an era in which even technology is catching up with trends. Electromechanical products are changing with each passing day, and “thin and small and multifunctional” has become fashionable. Products with high technical content such as digital products, mechatronics, information appliances, and multimedia devices are emerging in an endless stream. The speed of replacement is accelerating, and the level of chip integration and product work continue to increase. The accompanying electromagnetic harassment problem is increasingly complex. At present, China's overall EMC research started relatively 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. Afterwards, EMC cannot meet the standard requirements. The EMC problem is the technical difficulty of most companies today! The majority of companies engaged in the manufacturing of electromechanical products share the same feeling. Solving EMC problems is much more difficult than solving product safety problems. This has further exacerbated the fact that some companies are facing brand competition and price competition and are biased towards reducing costs and sacrificing EMC requirements.

The indicators of 3C certification for electromechanical products involve product safety and EMC. From the point of view of certification testing, the main reason why products fail to meet the "3C" certification requirements is that the EMC side cannot pass. In fact, EMC testing is not a common practice in daily inspections or in various product certifications. With the smooth development of 3C certification, some companies are frowning over EMC issues and helping companies is our glorious responsibility. There is no complicated theoretical analysis and derivation of this draft. It is only a brief introduction to the three laws of EMC.

The three elements of the EMC problem, the characteristics of electromagnetic harassment, and the five-level EMC design method, on the one hand, strive to be practical, and on the other hand, the intention is to initiate. Recognizing and utilizing three important laws in the field of EMC: The author believes that EMC can not be solved completely by theory, and EMC is a practical project. Engaged in the EMC industry, if you can deeply understand the following three important areas of the EMC field, and persist in using it in practice, you will surely receive a multiplier effect.

1) Law 1, EMC cost-effectiveness relation law:

The sooner the EMC problem is considered and the sooner it is resolved, the lower the cost and the better the effect. In the new product development stage, EMC design is conducted. Compared with the EMC test failure of the product, the cost can be greatly saved and the efficiency can be greatly improved. Conversely, the efficiency will be greatly reduced, and the cost will be greatly increased. Experience tells us that EMC design at the same time as the functional design, through the EMC test after the prototype and prototype are completed, is the most time-saving and most economical. On the contrary, EMC is not considered during the product development stage. After EMC is found to be unqualified after being put into production, not only is it technically difficult, but also rework will inevitably lead to a huge waste of cost and time, even as it relates to structural design and PCB design. Defects, unable to implement improvement measures, resulting in the product can not be listed.

2) Law 2. The higher the S area of ​​the high-frequency current loop, the more severe the EMI radiation.

The high frequency signal current flows through the inductor minimum path. When the frequency is high, generally the line reactance is greater than the resistance, the connection line is the inductance to the high frequency signal, the series inductance causes the radiation. Electromagnetic radiation is mostly generated by the high-frequency current loop on the EUT device under test. The worst case is the open-circuit antenna. The corresponding processing method is to reduce, shorten the connection, reduce the high-frequency current loop area, try to eliminate any antenna that is not needed for normal operation, such as discontinuous wiring or prolonged pins with antenna elements. One of the most important tasks to reduce radiated disturbances or improve the ability of radio frequency radiation to resist interference is to find ways to reduce the high frequency current loop area S.

3) Law 3: The higher the loop current frequency f is, the more severe the EMI radiation is, and the electromagnetic radiation field strength increases proportional to the square of the current frequency f.

The second most important way to reduce the radiation disturbance or improve the anti-interference ability of radio frequency radiation is to find ways to reduce the source high-frequency current frequency f, that is, to reduce the frequency f of the harassing electromagnetic wave. The following content of this article is to use the above three laws to promote early consideration of EMC issues, and to introduce EMC design and improvement of EMC issues.

Improve EMC issues like treating diseases. If the product does not pass the EMC test, we can only know from the measurement results which frequency points “exceed”, and where the electromagnetic disturbance from these frequencies comes from, it is often the most difficult to find and most difficult to solve. . The product EMC problem is difficult to say. It is easy to say. Just as if you are treating a patient, the key is to see whether the disease is curable or incurable, and whether it can be treated with symptoms. Chinese medicine doctors pay attention to seeing patients; medical professionals pay more attention to blood pressure, BT, CT, and laboratory tests. The purpose is to find out the lesions and causes, so that they can be treated due to illness.

In the same way, to improve the EMC problem and see it as a diagnosis and treatment of disease is a combination of scientific and interesting. First of all, according to the EMI generation approach and mechanism, that is, the elements of the EMC problem, first make some judgments on the circuit principle of the EUT (tested sample, the same below), such as IT problems caused by IT equipment and AV audio and video equipment. What is the reason or source of internal harassment? First, inference, and then combined with the test project test chart to see the essence through the phenomenon, analysis of the reasons for the poor - to clarify the source of harassment, thoroughly understand the harassment route, in order to target. To analyze the cause of the overshoot, use high-frequency oscilloscopes or spectrum analyzers and field probes 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 produce EMC problems.

The three elements of the EMC problem are switching power supplies and digital devices. Because pulsed currents and voltages are rich in high-frequency harmonics, they generate strong radiation. Electromagnetic interference includes radiation type (high frequency) EMI, conduction type (low frequency) EMI, that is, the problem of EMC is mainly caused by two ways: one is the form of space electromagnetic interference; the other is through conduction, in other words, the generation of EMC The three elements of the problem are: sources of electromagnetic interference, coupled pathways, and sensitive equipment. Radiated interference mainly contaminates the space electromagnetic environment in the form of electromagnetic waves through the shell and the connecting wire; Conducted interference is the equipment connected to the public power grid through the power cord or through other terminals (such as: RF terminals, input terminals).

Conduction, radiation, and harassment sources ------- (pathway) ------ Sensitive receptors Near-field coupling Possible sources of IT, AV equipment harassment

A) FM receiver, TV receiver local oscillation, fundamental wave and harmonics generated by the high-frequency head, the local oscillation circuit;

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

C) Various clock signals and high-frequency harmonics and their combinations required for digital circuit operation, various clocks such as CPU chip operation clock, MPEG decoder operation clock, video synchronization clock (27MHz, 16.9444MHz, 40.5MHz), etc. ;

D) Digital signal square wave and high-frequency harmonics, higher harmonics generated by the crystal oscillator, unwanted signals and spurious signals caused by nonlinear circuit phenomena (non-linear distortion, intermodulation, saturation distortion, cut-off distortion);

E) Non-sinusoidal waveforms, waveform fringing, overshoot, ringing, spurious frequency points in the circuit design.

F) External disturbances received by sensitive receptors through the coupling pathway include surges, rapid bursts, static electricity, voltage drops, voltage changes, and various electromagnetic fields.

Electromagnetic disturbance characteristics

1 The spectrum of the unit pulse is the widest;

2 The low frequency content in the spectrum depends on the area of ​​the pulse, and the high frequency component depends on the steepness of the pulse front and back edges;

3 The crystal oscillation level must meet a certain degree, the digital circuit can work according to a certain time sequence, so that the disturbance generated by the crystal oscillator has the characteristics of high coverage bandwidth and disturbance level;

4 When the polarization and directional characteristics of the transceiving antenna are the same, the EMI radiation and reception are the most serious; the larger the transponder antenna area, the greater the EMI hazard.

5 Harassment routes: radiation, conduction, coupling and combinations of radiation, conduction, and coupling.

6 Mains conduction harassment is mainly caused by common mode currents;

7 Radiated disturbances are mainly caused by loops formed by differential-mode currents. EMC design has mentioned in advance that the fundamental problem of EMC, the fundamental approach to solve EMC problems, starting from the principles of market economy, or from other aspects, are all EMC design as soon as possible. From the very beginning of the design project, EMC requirements were incorporated into the design mission as one of the inputs to the design.

EMC design, simply put, is to carefully predict various EMC problems that may occur, to optimize the selection of solutions and circuits, to find a design solution that optimizes the circuit, mechanical structure, and PCB, and to improve product design quality and ensure that In the case of functional and performance indicators, taking into account cost-effectiveness and avoiding EMC problems. To suppress and eliminate sources of disturbance, reduce the frequency of high-frequency signals, reduce the area of ​​high-frequency current loops, reduce common impedance coupling or inductive coupling, select low-speed, low-radiation devices, use shielded chassis, shielded cables, and I/O filters Are commonly used measures.

In general, EMC design can be divided into five levels. The following are five levels of EMC design points:

1) Plan selection, main components, selection of integrated circuits, circuit and mechanical structure design; for the success of the product, the first level design is the most basic and most important, any mistake means that the product project has failed completely. This layer of major EMC considerations is reflected in:

a) The selection of options, main components, and the selection of integrated circuits are mainly to reduce the radiation disturbance or increase the anti-jamming capability of RF radiation. Try to use a small chip that emits itself, such as a device with a long flip time and a low operating rate. The chip (the essence of the chip is a circuit module with a higher degree of integration. When the package is mounted with multiple ground pins, the high speed differential mode current loop area S can be reduced, and the chip emission can be reduced accordingly); high power and high loss devices can be avoided. They are often large sources of radiation;

b) Ensure that the selected device does not operate in the non-linear region, so as to avoid harmonic components becoming interference sources.

c) Circuit and mechanical structure design In addition to considering reducing radiation disturbance or improving the radio frequency radiation anti-jamming capability, the main consideration is to protect the power circuit against external disturbances including surges, rapid bursts, static electricity, voltage drops, voltage changes, etc.;

d) The circuit design or scheme should not overshoot the digital signal waveform. It should minimize the useless harmonic oscillation amplitude, minimize the useless high-order harmonic components and avoid strong electromagnetic disturbance.

e) For lumped parameter circuits, increase damping, decrease Q, and prevent oscillation;

2) EMC design of the PCB;

For the success of the product, PCB's EMC design is an important part. Unreasonable PCB design can have irreparable consequences;

PCB good EMC design, have a multiplier effect. The PCB's EMC design should follow the following:

a) Minimize the loop area formed by all the high-speed signals and clock signal lines, the connection lines should be as short as possible, and the signal lines should be close to the ground loop;

b) Using miniaturized devices and multi-layer circuit boards, multi-layer printed boards can shrink wiring space, high frequency characteristics, easy to achieve EMC;

c) Choosing the number of printed circuit boards to consider the shielding and isolation requirements of key signals, first determining the number of required signal layers, and then considering the cost, increasing the ground plane and power plane is one of the best measures for PCB EMC design;

d) The principle of layering the printed board is the same as the principle of layout of the printed circuit and layout of the cable. The bottom of the component plane is the ground plane. The key power plane and its corresponding ground plane are adjacent. The key signals of adjacent layers do not span the area. The signal layer, especially the high-speed signal and the clock signal, are adjacent to the ground plane, and the two signal layers are avoided as much as possible.

e) When individual power layers and strata cannot be used as a continuous plane, a multicell mesh connection is used to form a cellular cellular network, effectively reducing the area of ​​the current loop, reducing the common impedance R, and increasing the distributed capacitance between the signal and the stratum;

f) The layout of the circuit board layout is considered in order: power supply and ground/clock/signal lines. The wiring should be short, straight, thick, uniform, not right angles and abrupt changes. It should have a zigzag shape, use fillets instead of sharp traces, Widen the wiring of the power supply and ground as much as possible, divide the power supply and the ground, and try to meet the microstrip and stripline requirements as much as possible;

g) Keep traces as far away as possible from sources of harassment. Cabling considers the use of ferrite materials, and reserves the location of magnetic beads and patch filters in preparation for addition and subtraction on demand;

3) EMC design of electricity and grounding, high-speed signal lines and internal cables; EMC design of PCBs also mentions EMC design of power supply and grounding and high-speed signal lines. In addition, the following should also be observed:

a) Use low-impedance connections between the chips (ground plane). The impedance between the pins of different chip supply pins is as small as possible. The chip supply pin (meaning the supply line close to the chip supply pin) and the ground indirect high-frequency bypass capacitor provide power supply. The position of the magnetic beads and the patch filter are reserved for the addition and subtraction as needed;

b) The core principle of cabling and I/O cabling is to reduce the current loop area S. The principle of layout cabling is the same as the principle of PCB lamination. The key power cord is adjacent to its corresponding ground and all signal layers are special. High-speed signal, clock signal line and ground line adjacent, try to avoid the two signal lines adjacent;

c) In order to avoid that the length of the grounding wire is too long (close to λ/4), multiple points can be grounded nearby, and the high frequency impedance of the grounding wire should be small;

d) Reduce the antenna effect of the cable and reduce the effect of the dipole antenna. The cable with good shielding performance is used for the over-the-line and I/O cables. The multi-strand twisted-pair cable is used for the inner conductor to make the space field reach each other and the shielding layer. Can be used as a return line;

e) The use of shielded wires inside the machine to prevent induced noise;

f) The input and output lines of the wave device should be separated by a distance. Avoid directing lines and avoid affecting the filtering effect.

h) I/O interface attention to high-speed circuit impedance matching, reduce, eliminate reflection;

4) Shield design; there are three requirements for shielding: complete electrical continuum; filtering measures; good grounding.

For information technology IT equipment, when the motherboard and configuration are selected, it is very important to improve the shielding effect of the whole machine and the isolation effect of each part, especially personal computers and liquid crystal displays. Here only shielding design:

a) The computer chassis has a large disturbance field strength. The plastic part of the housing is not coated with conductive material or the conductive material is poor. The chassis has holes and gaps. It is not a complete electrical continuum, and the inlet and outlet lines are not well filtered. Finally, they can be filtered. Causes radiation disturbance beyond the limit. In order to better shield electromagnetic radiation, the chassis can not only take care of the heat dissipation requirements of the chassis, but also effectively prevent the diffraction of magnetic waves. The size of the openings is generally not more than 4mm.

b) According to the actual shielding design of the product, the shielding of ports, ventilation holes, holes, and connection gaps are all factors worth considering;

c) The liquid crystal display may be coated with a shell of a conductive material for the purpose of better shielding electromagnetic radiation (electrically conductive material shall be sprayed on the joints);

d) In order to minimize radiation, use a chassis that has passed the voluntary certification of CQC (EMC);

e) In order to ensure the sealing of the chassis, precision die stamping and molding shall be used to design appropriate bullets and curls;

f) Transformer plus electrostatic shielding and grounding

5) Input/output filtering design The importance of power line filtering and signal line filtering is no less important than chassis shielding. The key to filtering is to meet EMC requirements, taking into account compliance and economic principles. In the I/O interface part, a filter connector with high frequency filtering effect and simple installation is generally used. Wrap or apply ferrite ring on the cable can also play a certain role of filtering and absorbing wave. When designing or using a signal line filter, the cutoff frequency of the filter must be higher than the frequency of the signal to be transmitted on the cable.

a) The method of dealing with conduction disturbances is mainly low-pass filtering. Above 1 MHz, conducted emission problems are usually caused by the coupling of radiated emissions. Technical measures must be taken to suppress conducted emissions and radiated emissions, such as shielding, decoupling, and filtering.

b) The attenuation performance of the filter circuit is strongly related to the impedance of the source and the load. The greater the mismatch, the better the filter attenuates the electromagnetic disturbance. In most cases, the power line exhibits low impedance, and the input of the filter should be high impedance. On the other hand, the device may be either high impedance or low impedance. For high impedance of the linear power supply, the load side should be designed for low impedance in order to obtain impedance mismatch. For low impedance devices such as switching power supplies and synchronous motors, the load side is designed for high impedance.

c) Reduce the common-mode and differential-mode capacitance, add and subtract common-mode and differential-mode coils, adjust the capacitance parameters and the number of turns of the coil, and the common-mode and differential-mode insertion loss vs. frequency curve can be changed. The leakage current of the filter refers to the current flowing between the phase line and the neutral line and the housing ground. It mainly depends on the common-mode capacitance connected between the phase line and the ground and the neutral line and ground. The greater the capacity of the common-mode capacitor, the smaller the common-mode impedance, and the better the suppression effect of the common-mode disturbance, but the safety standard specifies that the leakage current should not be too large.

d) The location of the power filter should be close to the entrance of the power cord, if it can be integrated with the interface better. For metal shielded enclosures, use an independent power-shielding filter installed at the entrance of the power cord and ensure that the filter housing is in good electrical contact with the equipment chassis (ground). This is best. The filter ground is usually fixed to the common ground metal member at the outlet of the cable.

Advice to the company

First, understand the three elements of the EMC problem, the characteristics of electromagnetic harassment, sources of electromagnetic harassment, and transmission methods. Grasp five levels of EMC design rules, adhere to the use of EMC laws, and consider and solve EMC problems as soon as possible; encounter PCB must be redesigned or structure must be renewed When designing, everyone only regrets EMC's late consideration. Treatment is not as good as preventing disease. Treatment must be symptomatic. It should be sooner rather than later. Solving the EMC problem is the same principle.

Second, when the product's EMC does not meet the requirements of the need for rectification, the first must be the same as medical treatment, diagnosis of electromagnetic disturbance sources, coupling pathways, and then use the EMC mentioned in the design points, symptomatic start, comprehensive use of shielding, filtering and absorbing wave , grounding and other measures to implement improvements. On the way to improvement, if the test does not pass again, will the review of the problem be correct? Is the countermeasure wrong? Does the use of device parameters need to be adjusted? Do not change your mind immediately. You should not be bothered. When rectifying, special attention should be paid to the correct diagnosis of electromagnetic disturbance sources and coupling paths. When EMC suppression devices are used, not only should they be selected properly, but the devices used must be genuine, and they will not be permanently cured.

Third, the factory should identify the key production processes, key process operators should be trained to develop the corresponding process work instructions or standard samples (can take pictures to give pictures), so that the production process controlled. Take the prototype that is the most simplified and EMC has a certain amount of margin as a standard part, check the production and assembly process, and during the inspection, focus on the inspection/verification of key EMC components and materials, and the consistency check of the assembly process.

Fourth, in order to verify that the product continues to meet the standard requirements, the factory shall carry out a confirmation inspection of the product at an appropriate stage (when it does not have the detection conditions, the sample shall be sent to a competent organization for inspection) to ensure that the product continues to meet the EMC requirements in case of a change. Can be found in time.

Fifth, when the key parts of the EMC product are to be changed or adjusted, new devices are used to replace the original devices to re-create several prototypes for testing, and to confirm the impact of changes and adjustments of EMC critical components on the overall EMC.

summary

Understand the three elements of the EMC problem, the characteristics of electromagnetic harassment, sources of electromagnetic disturbances, and the means of transmission. Master the five levels of EMC design method. EMC will become a rule, adhere to EMC laws, and consider and solve EMC problems as soon as possible. Save time and effort and do more with less. In terms of EMC's work, according to the methods used by old Chinese medicine doctors, they are treated according to the disease, and they are conscious of humanity, which can increase their interest in solving problems.

EMC90% is designed. There are very few and very few rectifications. Treatment is not as good as preventing disease. Treatment must be symptomatic and should be started sooner rather than later. The reason is simple and needs more attention.

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