Wednesday, December 29, 2010

【 Weak current College 】 power electronic equipment electromagnetic compatibility 】

Summary: this article discusses the fully electronic equipment electromagnetic compatibility issues. A more detailed analysis of the sources of interference, the delivery way, interference and describes effective suppression and prevent interference of the various measures and principles.

Keywords: electronic equipment electromagnetic compatibility effective suppression of interference sources

1 Introduction

With the rapid development of electronic technology, modern electronic equipment is widely applied in various spheres of human life. Currently, the electronic device is in a period of rapid development, and the development process is to continue growing speed. Electronic equipment is widely applied and development will inevitably lead to them in their surrounding space generated electromagnetic field levels continue to increase. In other words, electronic device inevitably in the electromagnetic environment (EME). Therefore, you must resolve the electronic equipment in the electromagnetic environment of suit. Electromagnetic compatibility (EMC) is about electromagnetic interference (EMI) impact science. At present, the world-wide, electromagnetic compatibility issues has formed a new discipline. Electromagnetic compatibility of central subject is the study of control and eliminate electromagnetic interference, making the electronic devices or systems and other devices linked to work, does not lead to devices or systems of any part of the work performance of deterioration or lower. A design ideal electronic device or system should neither radiation any unwanted energy, and should not be subject to any undesired effects of energy.

2 classification of electromagnetic interference sources

Various forms of electromagnetic interference is electronic equipment electromagnetic compatibility of the main factors, therefore, it is the electromagnetic compatibility design need to study.

2-1 internal interference

Internal interference means electronic devices internal components of the mutual interference between, including the following.

(1) the power over the wire capacitance and insulation resistance resulting leakage caused by interference, and (with frequency)

(2) of the signal through the ground, power and transmission line impedance coupled with each other, or traverse the mutual inductance between the cause of the interference;

(3) device or a certain components within the system, affecting the symbol itself fever or other component stability caused by the interference;

(4) high power and high voltage parts resulting magnetic field, electric field effect through the coupling of the other part.

2-2 external interference

External interference is an electronic device or system outside factors on line, equipment or system, including the following.

(1) external high-voltage, power through insulated electric leakage noise electronic circuits, equipment or system;

(2) external power equipment in the space to produce a very strong magnetic field, through the mutual induction coupling interference with electronic circuits, equipment or system;

(3) space electromagnetic wave on the electronic circuit or system interference;

(4) operating environment temperature instability caused by the electronic circuit, device, or system internal components parameter changes caused by the interference;

(5) consists of industrial equipment and power supply from power grid voltage through the power transformer of interference.

3 ways of transmission interference

When interference sources of higher frequency and wavelength of the signal, noise and interference than is small dimensions of the object structure, or interference with interference between the distance r >> λ/2 π, the interference signal can be considered a radiation field, it forms a plane electromagnetic wave to deputy shoots electromagnetic energy into the channel object by interference.

(2) interfere with the signal to leakage and coupled forms, such as through the insulation bearing structures (including air) as a medium, coupled with public impedance is interference lines, equipment or systems.

If the interference sources less frequently, the interference signal wavelength λ is tampering with the structure of a dimension object, or the source of interference and noise object distance r << λ/2 π, interference may be considered quasi-static field, enter the induction field tampering object paths.

(3) interfere with the signal transmission through a direct line, introducing equipment or systems.

4 electromagnetic compatibility design rationale

4-1 grounding

Grounding is electronic equipment of a very important issue. Grounding purposes there are three:

(1) on the ground that the entire circuit system all cell circuit has a public reference zero potential, ensure the circuit system can do for a stable way.

(2) prevent the interference of external electromagnetic field. Chassis grounding can make because of static induction and accumulation in the chassis of a large number of charge-discharge through the Earth, otherwise the charge of high voltage may cause device internal spark discharges causing interference. In addition, the circuit's shield, to select appropriate grounding, or you can obtain good shielding effect.

(3) to ensure that safe work. When a direct lightning Electromagnetic induction, you can avoid the destruction of electronic devices; when low frequency AC power input voltage for insulated adverse or otherwise communicates directly with the enclosure, you can avoid the operations staff of electric shock accident. In addition, many medical devices are connected directly to the patient's body, when the enclosure with 110V or 220V voltage, a fatal danger.

Therefore, the Earth is suppressing noise preventing interference with the main method. Grounding can be understood as an equipotential surface point, or potential is the circuit or system benchmark potential, but not necessarily to earth potential. In order to prevent possible damage caused by lightning and the personal safety of staff, the electronic equipment of the enclosure and the engine room of the metal components, etc., must be connected with the Earth, and the grounding resistance is usually very small, cannot exceed the specified value.

Circuit of grounding basically there are three types of single-point grounding, multi-point grounding andMixed grounding. Single-point grounding is a line that only one physical point is defined as ground reference points. Other points of the various needs of ground directly received on this point. Multi-point grounding refers to a system where each received directly received from its recent ground plane, so that the ground wire length minimum. Ground plane, can be the equipment of the plate, can also be run through the entire system to wire in larger systems, can also be a device structure frame and so on. Mixed grounding is to simply pick up locations, using high-frequency bypass capacitor and connect the ground plane. But as far as possible prevent bypass capacitance and inductance of the leader of resonance.

4-2 screen

Shielding is on two space area of isolation between the metal to control electric, magnetic and electromagnetic waves from one zone to another zone of induction and radiation. Specifically, it is a shield to components, circuits, components, cable, or an entire system of sources of interference, avoid interference surrounded by electromagnetic field to diffusion; Shield will receive circuits, equipment or system surrounded by external electromagnetic fields prevent them.

Because the shield from the wires, cables, components, circuit or system, and other external interference electromagnetic waves and internal electromagnetic waves are absorbed energy play (Eddy current loss), reflective energy (electromagnetic wave shield interface reflection on) and offset the energy (Electromagnetic induction in the shield on the reverse of electromagnetic field, electromagnetic interference may be offset by some), so the shield with reduced interference function.

Shield material choice principle is:

(1) when the interference of the higher frequency electromagnetic field, the use of low resistivity (high conductivity) of metal materials produced by Eddy current (P = I2R, lower resistivity (conductivity), the greater the consumption of power), the formation of external electromagnetic effects of cancellation, thus shielding effect.

(2) when the interference electromagnetic waves of frequencies lower, to adopt high permeability rate of material, so that the lines limited to shield itself, prevent the spread to the shield of space.

(3) in some instances, if the requirements for high-frequency and low frequency electromagnetic fields has a good shielding effect, often using different metallic materials composition of multi-layered shield.

4-3 other interference method

(1) filter

Filtering is suppressed and prevent interference of important measures. Filter can significantly reduce conduction interference level, frequency spectrum components because of interference is not equal to the useful signal frequency, filter for these and useful signal frequency different ingredients good suppression capabilities to other interference suppression difficult to play a role. Therefore, the use of filtering network whether interference sources and elimination of interfere with coupling, or enhanced reception equipment of anti-interference ability, strong measures. Use resistance capacity and inductance and capacitance decoupling network able to isolate the circuits and power, the coupling between the Elimination of circuit, and to avoid interference with the signal into the circuit. For high frequency circuit can be one of the two capacitors and inductors (HF choke) C L C M π type filter. There are many kinds of filters, select the appropriate filter to eliminate unwanted coupling.

(2) the correct selection of passive components

Utility of passive components and not "ideal", its characteristics and the ideal characteristics are different. Practical component itself may be a source of interference, so the correct selection of passive components is important. Sometimes you can use a component has properties to suppress and prevent interference.

(3) circuit technology

Sometimes using shielding still does not meet the reduction and prevention of interference may be combined with the shielded, balanced measures and circuit technology. Balance circuit is a double line circuit two wires connected to the two conductors of all circuits, or for other conductors have the same impedance. Its purpose is to enable the two conductors are picked to interference signal equal. When the noise is a total State of the signal, you can load disappear. In addition, you can use other circuit technologies, such as the contact network, shaping circuit, Integrator and gated circuit and so on. In short, the use of circuit technology is suppressed and prevent interference of important measures.

5 electromagnetic compatibility issues specifications and standards

Interference (CISPR) and the Special Committee, the main research radio interference noise in the system of measurement. In 1976, CISPR began to develop standards for EMI electromagnetic interference. 1900 in October on the basis of several revised standards, subsequently published reprint the Committee also agreed with the international radio communication, the Commission considered together into electronic products of electromagnetic compatibility testing to develop data requirements and specific methods. Developed by information technology equipment noise as object "industrial, scientific and medical radio disturbance characteristics of electrical equipment and its measurement method allowed values" (Standard No. 11); "vehicles, boats and spark ignition start driving appliances radio disturbance characteristics of measurement methods and allowed values" (criterion 12); the "radio and television receivers of radio interference characteristics of measurement methods and allowed values" (standard 13). Until the mid-1992, the international standard is the ultimate perfection of the EMI. CISPR recommended tolerances have to many countries in the world, and adopted as the basis for their national regulations.

Radio transmitter power levels is the impact on surrounding radio electronic equipment, produce interference level. Therefore the radio transmitter power levels should be restricted. For example, according to radio communications Advisory Committee 357-1, recommendations, in satellite communication systems and Terrestrial microwave relay communication lines in common use (5800 ~ 8100MHz) band, when given to the antenna on the power not exceeding 13dBW, you should limit the microwave relay communication line transmitter effective radiated power (i.e. the transmitter power and antenna gain product) values for 55dBW. Recommendations at the limits of the Earth station of satellite communications and the communications satellite radiation powerPower flux density. Many other radio services, such as amateur radio, mobile communication systems of transmitter power of maximum value should be restricted.

Frequency planning in the country and the world has been widely used to increase the utilization of radio frequency resources in a way ensuring radio electronic equipment electromagnetic compatibility is one of the important measures. So it should be strictly in accordance with international agreements (radio frequency allocation table) and the national file, the implementation of national, regional division of the band and theband allocation between business. According to the frequency-space allocation principles for wireless channel assignment. Frequency planning must ensure that each radio electronic equipment interference level minimum, or elimination of interference by the State radio regulatory Commission in charge of coordination.

In recent years, many departments are carrying out electromagnetic compatibility test and the relevant technical standards work, has developed a series of standards and specifications. For example, the national standards for industrial GB3907-83 basic methods of measurement of radio interference; GB4824.1-84 for industrial, scientific and medical radio frequency device radio interference permitted values; GB6279-86 for vehicles, boats and spark-ignition engine-driven equipment radio characteristics measurement method and allowed values, and so on. State radio regulatory Commission on work, family, medical, and other electronic equipment, usage frequency, bandwidth and maximum radiated made specific provisions. This will ensure that electronic equipment to function and normal life of the people and the promotion of modern science and technology more rapidly, plays an important role.

6 some typical electromagnetic compatibility issues

As a result of electronic technology in the widely used in all industries, in the space of human activity everywhere full of electromagnetic wave, electronic device does not solve the problem of electromagnetic interference, the work cannot be compatible. In practice, people in the study of anti-jamming technology also accumulated a great deal of experience and continuous research in many practical ways to eliminate electromagnetic interference.

Experiments find car work, electromagnetic interference is quite prominent, serious damage electronic components. Therefore, car electronic equipment to the worst of the electromagnetic environment, automotive electronic equipment electromagnetic compatibility issues are also special attention. Auto ignition produces high-frequency radiation. Japan and the United States and other advanced countries, environmental sector in order to prevent the auto electrical noise pollution on the environment, regulations can only be used with damping (as carbon core) shielding line as a point of FireWire, practice shows that this is a very effective measure.

In order to address the micro-electro-technology, especially computer in automotive application and extension, according to the needs and practical requirements, you can design the effect good filter circuit, where you can make the most of the prior level due to conduction and enters the system noise elimination at the entrance to the circuit system; you can set the isolation circuit, such as transformer isolation and photoelectric isolation and resolution through the power lines and signal lines and ground into the circuit of conducted EMI, at the same time prevents public impedance, long-line transmission interference resulting from; you can also set the energy absorbing circuit, thereby reducing the circuit, the noise energy absorption devices; or by selecting components and reasonable arrangements for the circuit system, so that the interference is reduced.

Microcomputer software anti-interference equipment mainly stable memory data and guarantee programs. Microcomputer is a programmable control device, the software can support and enhance the hardware anti-interference ability. If the computer system of random memory RAM is mainly used for measurement and control of the data in temporary storage, the memory space is smaller, on hold, if some of the collection to group data averaging as sampling results, you can avoid acquisition due to interference and destroyed the data authenticity; if kept in the randomised in-memory data loss due to interference or data changes, you can set the random memory area; in order to reduce testing on random memory area, in the random access memory chip write signal cable with trigger, only write data until the CPU. Software anti-jamming measure also many, such as digital filtering program, narrow pulse delay program, logical state of false identification, etc. Sometimes, you must use software and hardware combination approach to suppress interference, a common approach is to set a timer to protect program up and running.

In recent years, electronic instrument towards "light, thin, short, small" and multipurpose, high performance and low cost. Plastic enclosure, plastics or Panel is widely used in electronic devices, and external electromagnetic wave is very easy to penetrate through the shell or on the instrument panel, normally produce harmful interference and electromagnetic waves generated by the instrument, also very vulnerable to radiation to the surrounding space, other electronic devices work correctly. To make this kind of electronic devices to meet the requirements for electromagnetic compatibility, in practice, a plastic metal processing technology, such as sputtering plating, vacuum plating (AL), electroplating and chemical plating copper, paste metal foil (Cu or AL) and coated with conductive coatings, etc. After metal processing, enabling fully insulated plastic surface or plastic itself (conductive plastic) with metal reflection (such as mobile phones). Absorption, transmission and attenuation characteristics of electromagnetic waves, which plays the role of electromagnetic interference shielding. Practical applications, using conductive coatings for shielding coating, excellent performance and affordable. Where the need to shield, resulting in a closed and grounded conductive shell, and two different electromagnetic isolation. Practice shows that if the shielding material can reach (30 ~ 40) dB attenuation above shielding effect, it is practical and feasible.

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