Electromagnetic compatibility of power supply electronic equipment

        The widespread application and development of electronic devices have led to an increase in the electromagnetic field levels they generate in the space around them. This shows that electronic devices inevitably work in an electromagnetic environment (EME). Therefore, it is necessary and important to solve the adaptability of electronic devices in an electromagnetic environment.

　　1 Electromagnetic compatibility (EMC) refers to the ability of a device (or system) to operate in compliance with requirements in its electromagnetic environment and not to cause intolerable electromagnetic interference to any device in its environment.

　　There are two aspects to EMC requirements: one is that the electromagnetic interference generated by the equipment to the environment during normal operation cannot exceed a certain limit (not interfering with other electronic products); the other is that it has a certain degree of immunity, that is, electromagnetic sensitivity (not being interfered with by other electronic products). Electromagnetic compatibility (EMC) is a science about resisting the influence of electromagnetic interference (EMI). Controlling and eliminating electromagnetic interference so that when electronic equipment or systems work with other equipment, it does not cause deterioration or reduction of the working function of any department is the central topic of electromagnetic compatibility research.

　　2 Three basic elements of electromagnetic interference

　　Electromagnetic interference (EMI) is electronic noise that interferes with cable signals and degrades signal integrity.

　　There are three necessary prerequisites for any electromagnetic interference to occur: first, there must be an interference source; second, there must be a path and channel to propagate the interference energy; and third, there must be a response from the interfered object. In the electromagnetic compatibility theory, interfered objects are collectively referred to as sensitive devices. Therefore, the interference source, interference propagation path (or transmission channel) and sensitive equipment are called the three basic elements of electromagnetic interference.

　　3 Classification of electromagnetic interference sources

　　Electromagnetic interference is the main factor affecting the electromagnetic compatibility of electronic equipment. There are many ways to classify it, as follows:

　　Generally speaking, electromagnetic interference sources can be divided into two categories: natural interference sources and man-made interference sources.

　　Natural interference sources mainly refer to the atmospheric noise and cosmic noise in the outer space of the earth. They are the basic components of the earth's electromagnetic environment and the interference sources that cause interference to radio communications and space technology.

　　Artificial interference sources refer to electromagnetic energy interference generated by mechanical or other artificial devices, among which devices specifically used to emit electromagnetic energy are called intentional interference sources, such as radio equipment such as broadcasting, television, communication, radar and navigation. The other part is the emission of electromagnetic energy while completing its own functions, which is called unintentional interference sources, such as traffic vehicles, power lines, lighting appliances, electric machinery, household appliances, industrial and medical radio frequency equipment, etc.

　　Based on the properties of electromagnetic interference, it can be divided into functional interference sources and non-functional interference sources.

　　Functional interference sources refer to direct interference to other devices caused by the equipment in the process of realizing its functions; non-functional interference sources refer to side effects that are produced or additionally produced by electrical devices while realizing their own functions, such as arc discharge interference caused by opening, closing or blocking.

　　According to the frequency range of the interference signal, the interference sources can be divided into power frequency and audio interference sources (50Hz and its harmonics), very low frequency interference sources (below 30Hz), carrier frequency interference sources (10kHz~300kHz), radio frequency and video interference sources (300kHz), and microwave interference sources (300MHz~100GHz).

　　Based on the internal and external factors of electronic equipment, it can be divided into internal interference and external interference.

　　Internal interference refers to the mutual interference between components inside electronic equipment, including the following:

　　Interference caused by leakage of the working power supply through the distributed capacitance and insulation resistance of the line (related to the working frequency); interference caused by mutual coupling of signals through the impedance of the ground wire, power supply and transmission wire, or mutual inductance between wires; interference caused by heating of certain components inside the equipment or system, affecting the stability of the component itself or other components; interference caused by magnetic fields and electric fields generated by high-power and high-voltage components affecting other components through coupling.

　　External interference refers to the interference of factors outside the electronic equipment or system to the line, equipment or system, including the following:

　　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, interfering with electronic circuits, equipment or systems through mutual inductance coupling; interference caused by electromagnetic waves in space on electronic circuits or systems; interference caused by changes in the parameters of internal components of electronic circuits, equipment or systems due to unstable working environment temperature; equipment powered by the industrial power grid and interference caused by the grid voltage through the power transformer.

　　4 Interference transmission pathways

　　When the frequency of the interference source is high, the wavelength of the interference signal is smaller than the structural size of the interfered object, or the interval between the interference source and the interfered object is r>>λ/2π, the interference signal can be considered as a radiation field, which radiates electromagnetic field energy outward in the form of a plane electromagnetic wave and enters the path of the interfered object.

　　The interference signal enters the interfered line, equipment or system in the form of leakage and coupling through insulating supports (including air) and coupling through common impedance.

　　Interference signals can be introduced into lines, equipment or systems through direct conduction.

　　5 Basic principles of electromagnetic compatibility design

　　5.1 Grounding

　　Grounding has three functions:

　　Grounding enables all unit circuits in the entire circuit system to have a common reference zero potential, ensuring that the circuit system can operate smoothly.

　　Prevent interference from external electromagnetic fields. Grounding the chassis allows a large amount of charge accumulated on the chassis due to electrostatic induction to be discharged through the ground. Otherwise, the high voltage formed by these charges may cause spark discharge inside the equipment and cause interference.

　　Ensure safe operation. When electromagnetic induction of direct lightning occurs, it can prevent damage to electronic equipment; when the input voltage of the industrial frequency AC power supply is directly connected to the casing due to poor insulation or other reasons, it can prevent electric shock accidents to operators.

　　Therefore, grounding is the main method to suppress noise and prevent interference. In order to prevent personal safety and equipment damage caused by lightning strikes, the casing of electronic equipment must be protected by grounding.

　　5.2 Shielding

　　The shield has the function of reducing interference.

　　The principles for selecting shielding materials are as follows:

　　When the frequency of the interfering electromagnetic field is high, the eddy currents generated in the low-resistivity metal material are used to offset the external electromagnetic waves, thereby achieving a shielding effect.

　　When the frequency of the interfering electromagnetic wave is low, materials with high magnetic permeability should be used to confine the magnetic lines of force inside the shielding body and prevent them from spreading to the shielding space.

　　In some cases, if good shielding effect is required for both high-frequency and low-frequency electromagnetic fields, different metal materials are often used to form a multi-layer shielding body.

　　5.3 Other interference suppression methods

　　Filtering. Filtering is an important measure to suppress and prevent interference. The filter can significantly reduce the level of conducted interference and has a good ability to suppress interference signals, thus playing a role that other interference suppression methods cannot play.

　　Select passive components accurately. The component itself may be a source of interference, so it is very important to select passive components accurately. Sometimes you can also use the characteristics of the component to suppress and prevent interference.

　　In short, ensuring the electromagnetic compatibility of equipment is a complex technical task, and effectively solving electromagnetic compatibility problems is of great significance in daily production and life. If we want to make breakthroughs and developments in this regard, we must grasp the basic principles of electromagnetic compatibility, analyze carefully, and dare to innovate, and we will definitely find a more reliable solution to the problem.