Abstract: Interference generated by the switching power supply itself directly endangers the normal operation of electronic equipment. Suppressing the electromagnetic noise of the switching power supply itself is an important issue in the development and design of switching power supplies. This paper briefly introduces the mechanism of electromagnetic interference generation and propagation in switching power supplies and summarizes several main methods for suppressing the generation and propagation of electromagnetic interference in switching power supplies. Keywords: Switching power supply; Electromagnetic interference; Suppression 0 Introduction Switching power supplies, as power supply devices for electronic equipment, have advantages such as small size, light weight, and high efficiency, and are widely used in digital circuits. However, due to their operation in high-frequency switching states, they are strong interference sources, and the interference they generate directly endangers the normal operation of electronic equipment. Therefore, suppressing the electromagnetic noise of the switching power supply itself and improving its immunity to electromagnetic interference to ensure the long-term safe and reliable operation of electronic equipment is an important issue in the development and design of switching power supplies. 1 Generation of Interference in Switching Power Supplies Interference in switching power supplies is generally divided into two categories: one is the interference generated by the internal components of the switching power supply; the other is the interference generated by the switching power supply due to external factors. Both involve human factors and natural factors. 1.1 Internal Interference of Switching Power Supplies The EMI generated by switching power supplies mainly consists of high-order harmonic current interference generated by the basic rectifier and peak voltage interference generated by the power conversion circuit. 1.1.1 Basic Rectifier The rectification process of the basic rectifier is the most common cause of EMI. This is because the power frequency AC sine wave, after rectification, is no longer a single-frequency current, but becomes a DC component and a series of harmonic components of different frequencies. Harmonics (especially high-order harmonics) will generate conducted and radiated interference along the transmission line, causing distortion of the front-end current. On the one hand, this distorts the current waveform connected to the front-end power line; on the other hand, it generates radio frequency interference through the power line. 1.2 Power Conversion Circuit The power conversion circuit is the core of the switching power supply. It has a relatively wide bandwidth and rich harmonics. The main components generating this pulse interference are: 1) Switching transistors: There is distributed capacitance between the switching transistors and their heat sinks and the leads inside the casing and power supply. When a large pulse current (roughly a rectangular wave) flows through the switching transistor, the waveform contains many high-frequency components. At the same time, the device parameters used in the power supply, such as the storage time of the switching power transistor, the large current of the output stage, and the reverse recovery time of the switching rectifier diode, can cause a momentary short circuit in the circuit, generating a large short-circuit current. In addition, the load of the switching transistor is a high-frequency transformer or energy storage inductor. At the moment the switching transistor is turned on, a large inrush current occurs in the primary winding of the transformer, causing spike noise. 2) High-frequency transformers: The transformer in the switching power supply is used for isolation and transformation, but due to leakage inductance, it will generate electromagnetic induction noise. At the same time, under high-frequency conditions, the distributed capacitance between the transformer layers will transmit the high-order harmonic noise of the primary side to the secondary side. The distributed capacitance of the transformer to the casing forms another high-frequency path, making the electromagnetic field generated around the transformer more likely to couple to other leads and form noise. 3) When a rectifier diode is used for high-frequency rectification, due to the reverse recovery time, the charge accumulated by the forward current often cannot be immediately eliminated when a reverse voltage is applied (due to the presence of charge carriers, current still flows). If the slope of this reverse current recovery is too large, the inductance flowing through the coil will generate a spike voltage, which, under the influence of transformer leakage inductance and other distributed parameters, will generate strong high-frequency interference, with frequencies reaching tens of MHz. 4) Capacitors, inductors, and wires: Switching power supplies operate at high frequencies, which can cause changes in the characteristics of low-frequency components, thereby generating noise. 1.2 External Interference of Switching Power Supplies External interference of switching power supplies can exist in "common-mode" or "differential-mode" modes. The types of interference can vary from very short-duration spike interference to complete power loss. These include voltage changes, frequency changes, waveform distortion, continuous noise or clutter, and transients. The types of power supply interference are shown in Table 1. Among the various types of interference listed in Table 1, the main ones that can be transmitted through the power supply and cause damage to or affect the operation of equipment are electrical fast transient bursts and surge shock waves. Electrostatic discharge and other interferences will not cause any impact on the equipment as long as the power supply itself does not experience oscillation interruptions or output voltage drops. 2. Interference Coupling Paths of Switching Power Supplies There are two types of interference coupling paths in switching power supplies: conductive coupling and radiative coupling. 2.1 Conductive Coupling Conductive coupling is one of the main coupling paths between the interference source and the sensitive equipment. Conductive coupling requires a complete circuit connection between the interference source and the sensitive equipment. Electromagnetic interference is transmitted from the interference source to the sensitive equipment along this connection circuit, generating electromagnetic interference. According to its coupling method, it can be divided into circuit coupling, capacitive coupling, and inductive coupling. In switching power supplies, these three coupling methods coexist and are interconnected. 2.1.1 Circuit Coupling Circuit coupling is the most common and simplest conductive coupling method. It has the following types: 1) Direct Conductive Coupling When a conductor passes through an environment with interference, it picks up the interference energy and conducts it along the conductor to the circuit, causing interference to the circuit. 2) Common Impedance Coupling: When two or more circuits share a common impedance, the voltage formed by the current in one circuit across that common impedance will affect the other circuit. This is common impedance coupling. Common impedance coupling interference can be caused by factors such as power supply output impedance and the common impedance of grounding wires. 2.1.2 Capacitive Coupling: Capacitive coupling, also known as electrical coupling, occurs when the voltage spikes generated between two circuits are narrow pulses with relatively large amplitudes. Parasitic capacitance exists between their frequencies, allowing the charge in one circuit to affect the other branch through this parasitic capacitance. 2.1.3 Inductive Coupling: Inductive coupling, also known as magnetic coupling, occurs when there is mutual inductance between two circuits. When the interference source is a power source, the magnetic field generated by this current interferes with nearby signals through mutual inductance coupling. 2.2 Radiative Coupling: Interference coupling caused by radiation is called radiative coupling. Radiative coupling transmits electromagnetic energy from the interference source to the receiver in the form of an electromagnetic field. There are typically four main coupling paths: antenna coupling, wire induction coupling, closed-loop coupling, and slot coupling. 2.2.1 Antenna-to-Antenna Radiative Coupling: In practical engineering, there is a significant amount of antenna electromagnetic coupling. For example, long signal lines, control lines, input and output leads in switching power supplies exhibit antenna effects, capable of receiving electromagnetic interference and forming antenna radiative coupling. 2.2.2 Inductive Coupling of Electromagnetic Fields to Conductors: The cables of switching power supplies are generally composed of signal loop connection lines, power stage loop power supply lines, and ground wires. Each conductor forms a loop with its input impedance, output impedance, and return conductor. Therefore, the cable is the part of the internal circuitry exposed outside the chassis, making it most susceptible to coupling from the radiated field of the interference source, inducing interference voltage or current, which then enters the equipment along the conductor, forming radiated interference. 2.2.3 Electromagnetic Field Coupling to Closed Loops: Electromagnetic field coupling to closed loops refers to situations where the length of the most induced part of the loop is less than 1/4 of the wavelength. When the frequency of the radiated interference electromagnetic field is relatively low, electromagnetic coupling between the radiated interference electromagnetic field and the closed loop occurs. 2.2.4 Electromagnetic Field Coupling Through Openings and Seams Electromagnetic field coupling through openings and seams refers to electromagnetic interference radiated by electromagnetic fields penetrating the interior of non-metallic equipment casings, metal equipment casings, and braided metal shielding of cables. 3. Some Measures to Suppress Interference The three elements that form electromagnetic interference are the interference source, the propagation path, and the affected equipment. Therefore, suppressing electromagnetic interference should also start from these three aspects and take appropriate measures. First, the interference source should be suppressed to directly eliminate the cause of interference; second, the coupling and radiation between the interference source and the affected equipment should be eliminated to cut off the propagation path of electromagnetic interference; third, the immunity of the affected equipment should be improved to reduce its sensitivity to noise. Currently, most measures to suppress interference involve cutting off the coupling path between the electromagnetic interference source and the affected equipment. Commonly used methods include shielding, grounding, and filtering. 1) Shielding technology can effectively suppress electromagnetic radiation interference from switching power supplies, that is, using materials with good conductivity to shield the electric field and materials with high magnetic permeability to shield the magnetic field. Shielding has two purposes: one is to limit the leakage of internally radiated electromagnetic energy, and the other is to prevent external radiation interference from entering the internal area. The principle is to utilize the reflection, absorption, and guidance of electromagnetic energy by the shielding body. To suppress the radiation and electromagnetic interference generated by the switching power supply and its impact on other electronic equipment, the shielding cover can be manufactured entirely according to the method of magnetic field shielding. Then, the entire shielding cover is connected to the system chassis and ground as a whole, which can effectively shield the electromagnetic field. 2) Grounding is the establishment of a conductive path between two points so that electronic equipment or components can be connected to a reference point called "ground". Grounding is an important method for switching power supply equipment to suppress electromagnetic interference. Connecting certain parts of the power supply to the earth can play a role in suppressing interference. In circuit system design, the principle of "single-point grounding" should be followed. If multiple grounding is formed, a closed grounding loop will appear. When magnetic lines of force pass through the loop, magnetic induction noise will be generated. In practice, it is difficult to achieve "single-point grounding". Therefore, in order to reduce grounding impedance and eliminate the influence of distributed capacitance, planar or multi-point grounding is adopted, using a conductive plane as a reference ground. Each part that needs to be grounded is connected to the nearest reference ground. To further reduce the voltage drop of the grounding loop, a bypass capacitor can be used to reduce the amplitude of the return current. In circuit systems where low and high frequencies coexist, the ground wires of the low-frequency circuit, high-frequency circuit, and power circuit should be connected separately before being connected to a common reference point. 3) Filtering is an effective method for suppressing conducted interference and plays a crucial role in the electromagnetic compatibility design of equipment or systems. EMI filters, as important units for suppressing conducted interference on power lines, can suppress interference from the power grid that damages the power supply itself, and can also suppress interference generated by the switching power supply and fed back to the power grid. Many specialized filtering components, such as feedthrough capacitors, three-terminal capacitors, and ferrite cores, are also used in filter circuits to improve the filtering characteristics of the circuit. Properly designing or selecting filters, and correctly installing and using them, are important components of anti-interference technology. When selecting a filter, pay attention to the following points: (1) Determine 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 operated; (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, 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 choose short lead low inductance feedthrough capacitors; (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 capacitance; (3) A large short-circuit current passes 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 coupling path of the input and output capacitors 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 Conclusion There are many factors that cause electromagnetic interference in switching power supplies, and a lot of work still needs to be done to suppress electromagnetic interference. Comprehensive suppression of various noises in switching power supplies will make the switching power supply operate more safely and reliably. (end)