This article describes the three major laws EMC, EMC problems three elements, characteristics of electromagnetic interference, as well as five-level EMC design. Help engineers design the beginning that paying attention to solve EMC problems, product certification to ensure clearance.
EMC is an important product certification, whether it is the international market of Chinese products, the Chinese market is still flock to foreign products, almost every carry out a wide range of product certification. Product certification, from the perspective of international trade, in essence, technical barriers to trade. We only continue to improve product quality, technical barriers to break through, to open up overseas markets to promote the development of foreign trade. "CCC" is our mandatory product certification mark - China Compulsory Certification of the initials only get 3C certification of products to enter the domestic market. 3C certification detailed provisions for electrical and mechanical safety performance of electrical products, EMC and so on.
EMC problem is the current technical difficulties most enterprises! In mechanical and electrical products manufactured in the majority of enterprises Jie feel the same. In particular, some companies face brand competition and price competition, tend to reduce costs, the expense of the phenomenon EMC requirements, making the problem even more prominent.
Machinery and electronic products 3C certification indicators relate to product safety, EMC aspects. Certification testing from the point of view, the main products reach the "3C" certification requirements of EMC aspects of Calabasas, Calif. In fact the case, regardless of the daily inspection or certification in a variety of products, EMC test pass more common, some companies frown on EMC problems, therefore, from the beginning of design is starting to focus on enterprise-class EMC engineer urgent attention to the problem .
EMC Three Laws
A law
EMC cost-effective law relationship: EMC consider the issue sooner, the sooner resolved, the smaller the cost, the better.
In the new product development stage design for EMC, EMC test failure than to wait until the product was improved, costs can be significant savings, efficiency can be greatly improved; on the contrary, efficiency will be greatly reduced, the cost will be greatly increased.
Experience tells us that, in the functional design, while EMC design, to prototype, test the prototype is completed through EMC, is the most save time and most cost-effective. Instead, the product development stage without considering EMC, EMC found that production will only failed to improve, not only to bring technically very difficult, a lot of waste and rework inevitably bring cost and time, even as it relates to the structural design, PCB design defects, unable to implement improvements, resulting in the product not be listed.
Law two
larger high-frequency current loop area S, the more severe EMI radiation.
A high-frequency signal current flows through the inductor smallest path. When the frequency is higher, generally greater than the resistance wiring reactance, inductance of the connection is high-frequency signal, the series inductance caused by radiation. Most of the high-frequency electromagnetic radiation is measured EUT current loop generated on the device, the worst case is that the antenna forms an open circuit. Corresponding to the processing method is to reduce, shorten wiring, reducing high frequency current loop area, try to eliminate any non-normal operation of the antenna required, such as discrete wires or pins of the components of the antenna effect long.
Radiation disturbance or reduction of the most important task to improve the anti-interference ability of the RF radiation, is to find ways to reduce the high-frequency current loop area S.
Law three
higher frequency f of the loop current, the more serious due to EMI radiation, electromagnetic radiation field strength proportional to the square of the frequency f increases with the current.
The most important way to reduce the radiation disturbance or interference rejection capability to improve two RF radiation, is to find ways to reduce the high-frequency current disturbance source frequency F, i.e. the frequency of the electromagnetic wave decreases harassment f.
The following article is the use of the above three laws, advocate the issue as early as possible considering EMC, EMC introduced EMC design problems and improve.
Improved EMC problems, as diagnosis and treatment of disease. If the product does not pass the EMC test, from the measurement results can only know which frequencies "excessive", while the frequency of these electromagnetic interference from where it is often the most difficult to discover engineer door, the most difficult problem .
EMC product issues, hardly difficult to say, but also easy to say and easy. Improved EMC problems, first of all, depending on the route and mechanism of EMI generated, that is, elements of EMC problems generated for the EUT (sample to be tested, the same below) of circuit theory, to make some judgments, such as IT and AV audio and video equipment category class What equipment is causing EMC problems or cause internal disturbance source is to be inferred, combined with test items look beyond test charts, analysis tolerance reasons - to figure out the source of the harassment, the harassment thorough way to touch so targeted.
Analysis tolerance reasons, a high frequency may be used together with the oscilloscope or spectrum analyzer verification field probe analysis results from the frequency domain to the time domain and from the frequency domain to the time domain analysis, find the corresponding generating circuits and devices EMC problems.
Three elements of EMC problems
Since the switching power supply device and a digital pulse having a current and voltage harmonic very rich, and therefore will have a strong radiation. Including electromagnetic interference emission type (high frequency) the EMI, conduction (low frequency) the EMI, EMC problems which produce mainly via two pathways: a space in the form of electromagnetic interference; the other is in the form of conductive, in other words, generating EMC problems are three elements: a source of electromagnetic interference, coupling path, sensitive equipment. Interference of radiation through the housing and connected to the main line in the form of electromagnetic pollution electromagnetic environment space; harassment conducted interference public power grid or by other terminals (such as: a radio terminal, an input terminal) is connected via power supply lines of Effect devices.
Conduction, radiation, ------ disturbance source (route) ----- sensitive receptors near field coupling IT, AV source device may harassment
a) FM receiver, the TV receiver local oscillation, harmonic and fundamental wave, the local oscillation circuit generates a high-frequency head;
b) switching pulse switch and power supply higher harmonic, the harmonic sweep synchronization signal side, the row scanning line generated by the imaging circuit, and a higher harmonic field signal;
c) the various clock signals and the high frequency harmonics required digital circuit, combinations thereof, and the various clock chip such as CPU operation clock, MPEG decoder operation clock, a video synchronization clock (27MHz, 16.9344MHz, 40.5MHz), etc. ;
d) a digital signal spread party harmonic, the harmonic generation crystal, nonlinear circuit phenomenon (non-linear distortion, intermodulation, distortion saturation, cutoff distortion) and other unwanted signals, caused by spurious signals;
e) non-sinusoidal waveform, a waveform burrs, overshoot, ringing, the presence of parasitic circuit design frequency.
f) sensitive receptors for receiving an external coupling path comprises a surge harassment, rapid burst, static electricity, voltage drop, and various changes in the voltage field.
Characteristics of electromagnetic interference
① unit pulse spectrum widest;
Area ② spectrum of low-frequency content depends on the pulse, the high frequency component depending on the steepness of the front and rear edge of the pulse;
③ crystal oscillator must meet certain amplitude level, digital circuitry can work according to a certain timing, so that the crystal produced exhibits cover harassment bandwidth harassment high level characteristic;
④ transceiver antenna polarization, the same directional characteristic, EMI radiation and receive the most severe; larger area receiving antenna, EMI harm than large;
⑤ harassment ways: radiation, conduction, and radiation coupling, conductive, coupling combinations.
⑥ power line conducted disturbances produced primarily by the common mode currents;
⑦ radiation disturbance primary loop formed by the differential mode currents.
About EMC's three laws and EMC issues three elements will make EMC problems become rule-based, adhere to the laws of EMC EMC makes the time and effort to solve the problem of the province more efficiently.
EMC design, simply put, is to carefully predict various EMC problems that may occur, to optimize the selection of programs and circuits, to find an optimized circuit, the mechanical structure and PCB design solutions, designed to improve the quality of the product, to ensure reach the case of function and performance, taking into account cost-effectiveness, avoid EMC problems. To suppress and eliminate the disturbance source, reducing high frequency signal frequency, high-frequency current loop area is reduced, reducing the total impedance or inductive coupling, selection of low-speed, low radiation device, use shielded enclosure, shielded cables and I / O filter They are commonly used measures.
EMC design five levels
In general, EMC design can be divided into five levels. The following is a five-level EMC design features:
A) program selection, the main components, integrated circuits selection, circuit design and mechanical design belong to the first level, is the most basic, most important, any error means that the product project a complete failure. This layer is mainly reflected in the EMC considerations:
1) Select embodiment, the main member, the main consideration of selection of the integrated circuit to reduce radiation disturbance or interference ability to improve the RF radiation, emitted try to use a small chip itself, such as long turn around time, low operating rate of the device, a multi-foot ground chip (chip essence higher integration of a circuit module, a multi-loaded ground pin when the package can be reduced high-speed differential-mode current loop area S, a correspondingly reduced transmit chip); avoid the use of high power, high loss of the device, they are often a large source of radiation;
2) ensure that the selected device is not working in the linear region, in order to avoid harmonic components becomes an interference source.
3) mechanical and circuit design considerations in addition to reducing or increasing the RF radiation radiated disturbances anti-jamming capability, the main consideration external power supply circuit includes a surge prevention harassment, rapid burst, static electricity, voltage drop, the voltage change;
4) the circuit design or embodiment of the digital signal waveform should not overshoot should minimize unwanted harmonic oscillation amplitude of the unwanted harmonic component at least, to avoid triggering a strong electromagnetic interference;
5) of the lumped parameter circuit, increase the damping, the Q value decreases, preventing oscillations;
B) PCB design is unreasonable, will have irreparable consequences, and good EMC design, there is a multiplier effect. PCB EMC design should follow the following:
1) to minimize the loop area of all high-speed signal and the clock signal lines, the connection lines as short as possible, and in close proximity to the signal line loop;
2) the use of miniaturized devices and multilayer wiring boards, multilayer printed wiring space may be tight, good high frequency characteristics, easy to achieve the EMC;
3) PCB layers take the key selection signal and shield isolation requirements, first determine the number of signal layers, and then consider their costs, increased power and ground plane layer PCB EMC design is one of the best measures;
4) PCB hierarchical principle and arrangement of the printed circuit, the cable is arranged as the principle element below the surface of the ground plane, power plane corresponding to the key ground plane adjacent to the key signals of adjacent layers do not cross, all signal layers as high-speed signal, a clock signal and the ground plane adjacent to avoid two adjacent signal layers;
5) individual power supply layer, not as a continuous ground plane, using a multi-cellular mesh grid connection is formed, the effective area of the current loop is reduced, reducing the common impedance R, increase the distributed capacitance signal with the formation;
6) the order of the wiring circuit board design considerations: Power and ground / clock lines / signal lines, the wiring should be short, straight, thick, uniform, straight and not mutated, should have "" shape, with rounded corners instead of sharp traces, split widened as cabling, power supply and ground and the ground, as far as possible in line with the microstrip and stripline requirements;
7) traces as far away from the disturbance source, consider the use of ferrite material of the wiring, and a patch reservation bead filter position, needed to prepare subtraction;
C) EMC design of the PCB and the ground power supply also mentioned, the EMC design of high-speed signal lines, in addition, should follow the following:
1) the inter-chip connection using a low impedance ground (ground plane), different impedance as small as possible supply pin chips, chip supply pin (pin means from the power the chip close to the power supply line) and indirectly frequency bypass capacitor, power supply placeholder wiring beads and patch filter to prepare demand subtraction;
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The core wire, I / O cable principle is to reduce the current loop area S, and the cable arrangement principle PCB hierarchical principle, the key corresponding to the power ground line adjacent thereto, all of the signal layers as high-speed signal, a clock signal line and ground adjacent to avoid two adjacent signal lines;
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In order to avoid too long a ground line (near λ / 4), multi-point can be close to ground, the high frequency impedance ground wire is smaller;
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Reducing the antenna effect and reduces the effects of a dipole antenna cable, across the line, I / O cable using the cable a good shielding performance, the use of multiple twisted pair wire, so that the space field netting, the shielding layer can be used as loop;
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Shielded cable to prevent noise in the induction machine;
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Input-output line should wave distance, and avoid running line, so as not to affect the filtering effect;
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I / O interface Note that high-speed circuit impedance matching, reduced, to eliminate reflections;
D) shielding design, good shielding three requirements: complete an electrical continuum; filter measures; good grounding. For IT IT class device, in the case where the selected configuration and the motherboard and to raise the barrier effect and shielding effect of the various parts is important, in particular, personal computers and liquid crystal displays. Here only said shielding design:
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The computer housing large disturbance field strength, the plastic housing part of electrically conductive material coated or not coated with the electrically conductive material is poor, the chassis has holes, gaps, not a complete electrical continuum, good filter inlet and outlet, eventually can lead to radiation harassment beyond the limit. For a better electromagnetic radiation shielding enclosure, not only take care of the cooling needs of the chassis, and can effectively prevent diffracted wave opening size of not more than 4mm;
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According to shield the actual product design, ports, vents, holes, gaps shielding connection is worth considerations;
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The liquid crystal display in order to better electromagnetic radiation shielding shell spraying the conductive material may be employed (the conductive material to be sprayed seams);
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In order to minimize radiation, adopted to make use of CQC (EMC aspect) voluntary certification chassis;
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To ensure the tightness of the enclosure, to the use of precision molding and design appropriate projectile point and the bead;
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Electrostatic shielding and grounding transformer plus the like;
E) The importance of filtering and signal line filtering and no less chassis shielding, filtering key is against EMC requirements, taking into account the principles of compliance and the economy.
1) In the I / O interface portion, the general good high frequency filtering effect, easy installation of filter connectors. Or applied on the cable winding ferrite core can also play a role in absorbing filter. When designing or using the signal line filters, filter cutoff frequency higher than the frequency of the signal to be transmitted to the cable.
2) A method of conducting harassment process mainly low-pass filtering. When the above 1MHz, the problem is usually conduction emission radiation emitted by the coupling caused shall Suppression integrated conduction and radiation emission emitted technical measures, such as shielding, filtering and decoupling.
3) a lot better attenuation properties of the filter circuit impedance relationship with the source and load, the greater the mismatch, electromagnetic interference filter attenuation effect. In most cases, the performance of a low impedance power supply line, the input of the filter should be high impedance. On the other hand, the device can be both a high-impedance, low impedance could. For high linear power impedance, in order to obtain the impedance mismatch, the load should be designed as a low impedance end. For such low-impedance power supply switching devices and synchronous motor, designed for high end load impedance.
4) reducing common mode and differential mode capacitor, addition and subtraction of common mode and differential mode chokes, and the parameters adjusting the capacitance of coil turns, the common mode and differential mode insertion loss curve versus frequency can be changed. Filter leakage current is current flowing between the line and neutral to the housing. It mainly depends on the common-mode capacitor connected between the phase and neutral lines and ground and ground. The larger the capacity of the common-mode capacitance, the smaller the common mode impedance, common mode disturbance suppressing the better, but the leakage current safety standards must not be too large.
5) the filter mounting position should be close to the power supply line at the entrance, as better integration of the interface can be made. For the metal shield case, the choice of filter mask independent power supply, the inlet of the power cables, and ensure that the filter housing and the chassis (ground) good electrical contact, the effect is the best. Filter grounding normally fixed to a metal member in common at the outlet of the cable.
Suggest
First, understand the characteristics of the three elements of EMC problems, electromagnetic interference and electromagnetic interference sources and routes of transmission, grasp the five-level EMC design rules, stick with EMC law, considered as early as possible and solve EMC problems, or encounter PCB must be redesigned or structure must redesign, we consider only regret EMC was late.
Second, when the product does not meet the EMC requirements need rectification, the first diagnostic sources of electromagnetic interference coupling path, and then using the method mentioned EMC design features, the integrated use of shielding, absorbing filtering, and other measures to improve the ground. Improved the way, do not pass the test, first determine whether the right to review the problem? Measures whether mistakes? Use device parameters need adjustment? Do not suddenly change their mind, should calmly. Pay special attention to the rectification, the correct diagnosis electromagnetic interference sources, the coupled channels, when using EMC suppression devices, not only to select the appropriate device to be genuine and used, will solve the problem.
Third, the Factory shall identify critical production processes, the key processes operating personnel should be trained to develop the appropriate process work instructions or standard sample (pictures can be taken given the way the picture), the production processes under control. Take the simplest and EMC has a certain margin of prototype as standard, check production, assembly process, when the inspection emphatically EMC critical components and material inspection / validation, as well as the assembly process consistency check.
Fourth, in order to verify that products consistently meet standard requirements, Production shall be inspected at appropriate stages to confirm (the time itself does not have the test conditions, the ability of institutions to send samples to test), in order to ensure continued compliance with EMC requirements, also in case of change timely discovery.
Fifth, when the product EMC key components to change, adjust, apply the new devices replace the original device re-create several prototypes were tested and found to affect EMC key components changes and adjustments to the whole of EMC.
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