Electromagnetic compatibility of AC stabilized power supply-EEWORLD

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At present, with the development of science and technology, electrical and electronic products with high technical content and complex internal structure are increasingly widely used. Electromagnetic interference causes the performance of electrical and electronic products to decline, and the phenomenon of failure to work often occurs. Severe cases can cause quality accidents and equipment damage and other losses. And this development process continues at an increasing rate. The widespread application and development of AC stabilized power supplies will inevitably lead to the continuous increase of the electromagnetic field level generated by them in the surrounding space. In other words, AC stabilized power supplies inevitably work in the electromagnetic environment (EME).
Therefore, the adaptability of AC stabilized power supplies in the electromagnetic environment must be solved. Electromagnetic compatibility (EMC) is a science about resisting the influence of electromagnetic interference (EMI). At present, the problem of electromagnetic compatibility has become a new discipline worldwide. The central topic of electromagnetic compatibility is to study the control and elimination of electromagnetic interference, so that when the AC stabilized power supply system is connected with other equipment to work, it will not cause the deterioration or reduction of the working performance of any part of the equipment or system. An ideally designed electronic device or system should neither radiate any unwanted energy nor be affected by any unwanted energy.
1 Electromagnetic Interference Source
Electromagnetic interference is the main factor affecting the electromagnetic compatibility of AC stabilized power supply, and it is the main content that needs to be studied in electromagnetic compatibility design.
1.1 Internal Interference
Internal interference refers to the mutual interference between the components inside the AC stabilized power supply. First of all, the circuits inside the AC stabilized power supply equipment should not interfere with each other. Due to the high operating frequency, there are more and more occasions for the mixed design of digital circuits and analog circuits. The magnetic field and electric field generated by high-power and high-voltage components affect other components through coupling and cause interference; some components inside the equipment or system heat up, affecting the stability of the components themselves or other components and causing interference. The electromagnetic interference intensity generated by the AC stabilized power supply is lower than a specific limit value, which is very important for computer equipment. The pulse current in computer equipment will interfere with surrounding equipment, especially radio equipment. If effective radiation suppression measures are not taken, the computer will seriously affect the normal operation of the system. In order to make the equipment have a certain resistance to external electromagnetic interference, it is necessary to improve the ability of the equipment to withstand the surrounding electromagnetic interference in the environment and have good electromagnetic compatibility performance stability.
1.2 External Interference
External interference refers to the interference of external factors of the AC stabilized power supply on lines and equipment.
External high voltage and power supply interfere with electronic circuits, equipment or systems through insulation leakage; external high-power equipment generates a strong magnetic field in space, which interferes with electronic circuits, equipment or systems through mutual inductance coupling; space electromagnetic waves interfere with electronic circuits or systems; unstable working environment temperature causes interference caused by changes in the parameters of components inside electronic circuits, equipment or systems; equipment powered by industrial power grid and interference generated by grid voltage through power transformer.
2 Electromagnetic compatibility research
2.1 Grounding
There are three purposes of grounding:
(1) Grounding enables all unit circuits in the entire circuit system to have a common reference zero potential to ensure that the circuit system can work stably.
(2) Prevent interference from external electromagnetic fields. Casing grounding can allow a large amount of charge accumulated on the casing due to electrostatic induction to be discharged through the earth. Otherwise, the high voltage formed by these charges may cause spark discharge inside the equipment and cause interference. In addition, for the circuit shield, if a suitable grounding is selected, a good shielding effect can also be obtained.
(3) Ensure safe operation.

When electromagnetic induction of direct lightning occurs, the destruction of electronic equipment can be avoided; when the input voltage of the industrial frequency AC power supply is directly connected to the casing due to poor insulation or other reasons, electric shock accidents of operators can be avoided. In addition, many medical devices are directly connected to the patient's body. When the casing carries a voltage of 110V or 220V, fatal danger will occur.
Therefore, grounding is the main method to suppress noise and prevent interference. Grounding can be understood as an equipotential point or equipotential surface, which is the reference potential of the circuit or system, but not necessarily the earth potential. In order to prevent possible damage caused by lightning strikes and personal safety of workers, the casing of electronic equipment and metal components of the computer room must be connected to the earth, and the grounding resistance is generally very small and cannot exceed the specified value.
Solution to grounding problems: Reasonable "grounding" is an important measure to reduce "ground" noise interference and must be paid special attention to. Usually, no matter who supplies the power, the ground wire is gathered to a common point, and then connected to the common end of the system. All loads of power supply 1 return to the common end of power supply 1, and all loads of power supply 2 return to the common end of power supply 2. Finally, a thick wire is used to connect the common ends together. In a multi-power system, it may be necessary to conduct a judgment test to determine the ground connection method to achieve the best solution.
2.2 Shielding
Shielding is to isolate two spatial regions with metal to control the induction and radiation of electric fields, magnetic fields and electromagnetic waves from one region to another. Specifically, it is to use a shield to surround the interference source of components, circuits, assemblies, cables or the entire system to prevent the interference electromagnetic field from spreading outward; use a shield to surround the receiving circuit, equipment or system to prevent them from being affected by the external electromagnetic field.
Because the shield absorbs energy (eddy current loss), reflects energy (interface reflection of electromagnetic waves on the shield) and offsets energy (electromagnetic induction generates a reverse electromagnetic field on the shielding layer, which can offset part of the interference electromagnetic waves) from external interference electromagnetic waves such as wires, cables, components, circuits or systems and internal electromagnetic waves, the shield has the function of reducing interference.
3 Other interference suppression methods
(1) Filtering
Filtering is an important measure to suppress and prevent interference. Filters can significantly reduce the level of conducted interference, because the interference spectrum components are different from the frequency of the useful signal. Filters have good suppression capabilities for these components with different frequencies from the useful signal, thus playing a role that other interference suppression cannot play. Therefore, the use of filter networks is a powerful measure whether it is to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving equipment. The use of resistor-capacitor and inductor-capacitor decoupling networks can isolate the circuit from the power supply, eliminate the coupling between circuits, and prevent interference signals from entering the circuit. For high-frequency circuits, a CLCMπ filter consisting of two capacitors and an inductor (high-frequency choke) can be used. There are many types of filters, and choosing the right filter can eliminate unwanted coupling.
(2) Correct selection of passive components
Practical passive components are not "ideal", and their characteristics are different from ideal characteristics. Practical components themselves may be a source of interference, so it is very important to select passive components correctly. Sometimes the characteristics of the components can also be used to suppress and prevent interference.
(3) Circuit technology
Sometimes, shielding still cannot meet the requirements of suppressing and preventing interference. It can be combined with shielding and circuit technologies such as balancing measures.

A balanced circuit refers to a two-wire circuit in which the two conductors and all circuits connected to the two conductors have the same impedance to the ground or other conductors. The purpose is to make the interference signals picked up by the two conductors equal. The interference noise at this time is a common-mode signal that can disappear on the load. In addition, other circuit technologies can also be used, such as contact networks, shaping circuits, integration circuits and gating circuits. In short, the use of circuit technology is also an important measure to suppress and prevent interference.
(4) Plastic metallization
In recent years, AC regulated power supplies and electronic instruments have developed in the direction of "light, thin, short, small" and multifunctional, high-performance and low-cost. Plastic chassis, plastic parts or panels are widely used in AC regulated power supplies, so external electromagnetic waves can easily penetrate the casing or panel, causing harmful interference to the normal operation of the instrument, and the electromagnetic waves generated by the instrument can also easily radiate to the surrounding space, affecting the normal operation of other electronic instruments. In order to make this electronic instrument meet the requirements of electromagnetic compatibility, people have studied the process of plastic metallization in practice, such as sputtering galvanizing, vacuum plating (Al), electroplating or chemical copper plating, pasting metal foil (Cu or Al) and coating conductive paint.
After metallization, the completely insulated plastic surface or the plastic itself (conductive plastic) has the characteristics of reflecting, absorbing, conducting and attenuating electromagnetic waves like metal, thus playing a role in shielding electromagnetic wave interference. In practical applications, conductive paint is used as a shielding coating, which has excellent performance and is affordable. Where shielding is required, a closed conductive shell is made and grounded to isolate the two different electromagnetic waves inside and outside. Practice shows that if the shielding material can achieve a shielding effect of 30-40dB or more attenuation, it is practical and feasible.
(5) Other suppression measures
Since electronic technology is widely used and the radiation of various interference equipment is very complex, it is impossible to completely eliminate electromagnetic interference. However, according to the principle of electromagnetic compatibility, many technical measures can be taken to reduce electromagnetic interference and control it within a certain range, so as to ensure the compatibility of the system or equipment. For example, when the communication system is initially designed, on-site radio wave testing should be strictly carried out, and the frequency and polarization mode should be selected in a targeted manner to avoid interference from radar, mobile communication and other clutter; when selecting the path of the high-voltage line, the radio station (station) should be avoided as much as possible or the terrain and ground objects of the receiving area should be fully utilized for shielding; the receiving equipment and the industrial interference source equipment should be properly configured to keep the receiving equipment away from various industrial interference sources; in the design of microwave communication circuits, in order to reduce interference, the reflection point of the microwave circuit can be adjusted by the antenna high and low station method, and the reflected wave can be blocked by the mountain top so that it cannot interfere with the direct wave. In addition, the microwave tower is an independent tall building, and perfect grounding, shielding and other lightning protection measures should be adopted.
4 Conclusion
Ensuring the electromagnetic compatibility of equipment is a complex technical task, and there is no universal solution to this problem. Electromagnetic compatibility technology covers a wide range, and the field of electromagnetic compatibility is also developing. It is important to master the basic principles of electromagnetic compatibility, analyze and test carefully, and then choose the appropriate solution to the problem.

Reference address：Electromagnetic compatibility of AC stabilized power supply

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