1 Introduction With the rapid development of science and technology, the number and types of electrical and electronic equipment or systems are constantly increasing, making the electromagnetic environment increasingly complex. In a complex electromagnetic environment, whether various equipment or systems can work normally has become an urgent problem to be solved. As an important part of various equipment or systems, the switching power supply is both a source of interference and a victim of interference. High-power switching power supplies are often sources of interference. When various switching power supplies are working, they often generate some useful or useless electromagnetic energy, which can affect the normal operation of other equipment or systems. This is electromagnetic interference. Electromagnetic interference may reduce the performance of the switching power supply, or even shorten its service life, or make it unable to work normally at all. It can be seen that electromagnetic compatibility design is very important and indispensable in switching power supplies. In switching power supplies, the purpose of electromagnetic compatibility design is to enable the switching power supply to achieve electromagnetic compatibility in the expected electromagnetic environment. The requirements are that the switching power supply meets the relevant EMC standards and has the following two capabilities: (1) It can work normally in the expected electromagnetic environment without performance degradation or failure; (2) It does not pollute the electromagnetic environment. 2. Several Important Concepts Regarding Electromagnetic Compatibility 2.1 Electromagnetic Environment The electromagnetic environment refers to the distribution of power and time of radiated or conducted electromagnetic emission levels at different frequency ranges that a device or system may encounter during normal operation. The electromagnetic environment can sometimes be represented by field strength. Suppose there are N electromagnetic interference sources in the electromagnetic environment. At frequency f1, at the location of the sensitive device or system, the field strength value is: Ei(f1): The field strength value of the electromagnetic interference source numbered i at frequency f1, at the location of the sensitive device or system. 2.2 Electromagnetic Disturbance Electromagnetic disturbance refers to any electromagnetic phenomenon that may cause a degradation in the performance of a device, equipment, or system, or damage to living or inanimate objects. Electromagnetic disturbance is an objectively existing physical phenomenon, and its causes may be external factors or changes within the device itself. Electromagnetic disturbance can be divided into two main categories based on its source: natural disturbance and man-made disturbance. Natural disturbance is characterized by its uncontrollable source, such as electronic noise, atmospheric noise, extraterrestrial noise, and static electricity deposition. Man-made disturbances are characterized by their known and controllable sources. For example: high-frequency and microwave equipment, high-voltage equipment, switching equipment, spark equipment, nuclear electromagnetic pulse, etc. 2.3 Electromagnetic Interference Electromagnetic interference (EMI) refers to the degradation of the performance of equipment, transmission channels or systems caused by electromagnetic disturbances. Electromagnetic interference is the consequence of electromagnetic disturbances. The cause of electromagnetic interference may be a useful electromagnetic signal from another equipment or system, or it may be some kind of electromagnetic noise. 2.4 Electromagnetic Compatibility Electromagnetic compatibility (EMC) refers to the ability of a device, equipment or system to operate normally in its electromagnetic environment without generating any unacceptable electromagnetic disturbances to its environment. According to the above definition, electromagnetic compatibility includes the following two aspects: (1) The equipment or system should have the ability to resist a given electromagnetic disturbance. That is, it should not be affected by electromagnetic disturbances emitted by other equipment or systems in the same electromagnetic environment, resulting in unacceptable degradation of its working performance. (2) The equipment or system does not generate electromagnetic disturbances exceeding the specified limits. That is, it will not generate electromagnetic interference that causes other devices or systems in the same electromagnetic environment to experience performance degradation exceeding specified limits. 3 General Paths of Electromagnetic Interference Propagation There are two ways to transfer electromagnetic energy from the interference source to the interference target: conduction and radiation. From the receiver's perspective, coupling can be divided into two categories: conduction coupling and radiation coupling. Conduction coupling is further divided into direct conduction coupling, common impedance coupling, and transferred impedance coupling, while radiation coupling is further divided into field (antenna) to antenna coupling, field to cable coupling, and cable to cable coupling. As shown in Figure 1. Figure 1 Schematic diagram of electromagnetic interference propagation paths In practice, conduction coupling and radiation coupling are not entirely different; they can be transformed into each other. For example, when the current conducted in a metal conductor is large, radiation will also be severe. 4 Electromagnetic Compatibility Design of Switching Power Supplies Based on the propagation paths of electromagnetic interference, the electromagnetic compatibility design of switching power supplies includes: improving circuit design, grounding design, filtering design, and shielding design. 4.1 Improving Circuit Design The selected switching power supply circuit topology should not generate excessively high voltage and excessively large current to avoid high-voltage electric field interference and large-current magnetic field interference. Under the premise of meeting the requirements, the frequency band of the amplifier should be as narrow as possible to make it less susceptible to interference. Add appropriate buffer circuits. The following points should be noted when designing printed circuit boards: (1) When high, medium and low speed logic circuits are used at the same time, the high speed should be designed at the entrance of the circuit board; (2) Add RC decoupling filter to the signal entrance to eliminate long-line transmission interference; (3) The current loop in the circuit should be kept to a minimum; (4) The signal line and the return line should be as close as possible; (5) Use a larger ground plane to reduce the ground impedance; (6) The power line and the ground line should be close to each other; (7) In multilayer circuit boards, the power plane and the ground plane should be separated; (8) Use arc wiring to avoid abrupt changes; (9) Shorten the connection as much as possible; (10) Separate analog circuits and digital circuits, and separate power circuits and control circuits. 4.2 Grounding Design Grounding is an important method for suppressing electromagnetic noise in switching power supply equipment. The function of grounding: (1) Improve the stability of the system operation. If it is not connected to the earth, it is easily affected by the ground capacitance. (2) Discharge the static electricity induced on the chassis to avoid high voltage discharge. (3) Operation safety. Without considering safety grounding, only from the perspective of circuit reference point, grounding can be divided into floating ground, single-point grounding, multi-point grounding and mixed grounding. 1) Floating ground is shown in Figure 2. It isolates the reference ground in the switching power supply from the chassis, which can prevent the interference current in the chassis from being directly coupled to the power supply circuit. When the floating ground system is close to the high voltage, static charge may accumulate, causing damage, or cause electrostatic discharge, forming interference current. In the lightning environment, an electric arc is generated between the chassis and the unit circuit. Therefore, floating ground should not be used in the switching power supply. Figure 2 Floating ground 2) Single-point grounding is shown in Figure 3. Single-point grounding is divided into single-point series grounding and single-point parallel grounding. The advantage of single-point series grounding is that it is relatively simple. Its disadvantage is that each circuit will affect each other through the grounding wire. When using this grounding method, it is important to place the highest level circuit at point A, which is closest to the grounding point, to minimize the potential rise at points B and C. Compared to single-point series grounding, single-point parallel grounding has no common ground impedance interference, but it has a large number of ground wires and is less effective at high frequencies (above MHz). (a) Single-point series grounding (b) Single-point parallel grounding Map 3 Single-point grounding 3) Multi-point grounding As shown in Figure 4. Each grounding point is grounded nearby. Its advantages are: simple wiring, short leads, and a significant reduction in high-frequency standing wave phenomena. Its disadvantage is: the grounding impedance increases with frequency. Figure 4 Multi-point grounding 4) Hybrid grounding As shown in Figure 5. The actual situation is more complex and it is difficult to solve the problem with just one simple grounding method. Instead, a hybrid grounding is often formed by combining single-point grounding and multi-point grounding. Figure 5 Hybrid grounding 4.3 Filtering Design Filtering is a commonly used measure to eliminate interference. The following issues should be considered when designing and selecting filters: (1) Clearly define the operating frequency and the interference frequency to be suppressed. If the two are very close, a filter with a very steep frequency response is required to separate the two frequencies; (2) Ensure that the filter can work reliably under high voltage conditions; (3) When the filter is continuously supplied with the maximum rated current, its temperature rise should be low to ensure that the working performance of the components in the filter is not damaged when the rated current is continuously applied; (4) In order to make the frequency response of the filter during operation conform to the design value, the impedance values ​​of the signal source and load connected to it should be equal to the specified values ​​in the design; (5) The filter must have a shielded structure, and the shielded box cover and the body should have good electrical contact. The capacitor leads of the filter should be as short as possible, and it is best to use feedthrough capacitors with low lead length and short inductance; (6) It should have high working reliability because the faults of filters used for electromagnetic interference protection are often more difficult to find than the faults of other components. The following points should be noted when installing filters: 1) Power line filters should be installed as close as possible to the power inlet of the equipment, and power lines that have not passed through the filter should not meander within the equipment frame; 2) The capacitor leads in the filter should be as short as possible to avoid resonance at lower frequencies due to lead inductance and capacitive reactance; 3) A large short-circuit current flows through the grounding wire of the filter, which will cause additional electromagnetic radiation. Therefore, the filter components themselves should be well shielded and grounded; 4) The input and output lines of the filter should not cross, otherwise crosstalk will be caused by the input-output capacitive coupling path of the filter, thereby reducing the filtering characteristics. The usual method is to add a partition or shielding layer between the input and output terminals. 4.4 Shielding Design Shielding has two purposes: first, to limit the leakage of electromagnetic energy radiated from the internal area; and second, to prevent external radiation interference from entering the internal area. The working principle of electromagnetic shielding is to utilize the reflection, absorption, and guidance of electromagnetic energy flow by the shielding body. These effects are closely related to the charges, currents, and polarization phenomena induced on the surface of the shielding structure and within the shielding body. Design principles of shielding: (1) First, determine the electromagnetic environment, including the type of electromagnetic field, the field strength, frequency, and the distance from the shielding body to the source; (2) Determine the sensitivity of the receiver and the shielding requirements of the shielding body; (3) According to the requirements of electromagnetic shielding and the properties of the electromagnetic field, appropriately select the conductivity, permeability, and thickness of the material; (4) After determining the shielding material, design the shielding structure. For electric field shielding, mainly select high conductivity materials (such as copper). For magnetic field shielding, especially low frequency magnetic field shielding, mainly select iron or other high permeability materials. If the requirements are not met, the thickness can be increased under permissible conditions; (5) If a single layer of shielding cannot meet the shielding requirements, double or more layers of shielding can be used to obtain better shielding effect; (6) When the shielding room needs to be transparent, metal mesh shielding can be used. The effectiveness of metal mesh shielding is obviously not as good as that of solid metal shielding, so double-layer shielding is generally used; (7) For ventilation holes, detector openings, shielding shells, cable inlet and outlet connectors, etc., all openings are designed according to special requirements. 5 Conclusion This article briefly describes the important role of electromagnetic compatibility design in switching power supplies. There are three basic elements required for electromagnetic interference to occur: (1) electromagnetic interference source, (2) coupling path, and (3) sensitive equipment. In order to achieve electromagnetic compatibility, we must start from the above three basic elements and take appropriate measures. This article starts from these three basic elements and introduces several basic methods of electromagnetic compatibility design for switching power supplies for reference by engineering and technical personnel engaged in switching power supply design. References [1] Bai Tongyun, Lü Xiaode, Electromagnetic Compatibility Design, Beijing University of Posts and Telecommunications Press, 2001. [2] Wang Dingfa, Zhao Jiasheng, Electromagnetic Compatibility Principles and Design, University of Electronic Science and Technology of China Press, 1995. [3] Liu Pengcheng, Qiu Yang, Electromagnetic Compatibility Principles and Technology, Higher Education Press, 1993. [4] Lai Zuwu, Electromagnetic Interference Protection and Electromagnetic Compatibility, Atomic Energy Press, 1993. [5] Ron Lenk, Practical Design of Power Supplies, IEEE PRESS 1997. [6] Ye Huizhen, Yang Xingzhou, Switching Regulated Power Supplies, National Defense Industry Press, 1990. [7] Yao Fu'an, Electronic Circuit Design and Practice, Shandong Science and Technology Press, 2001.