Abstract: This paper describes the composition of the main circuit of a high-frequency switching power supply. Based on the operating characteristics of high-frequency switching power supplies, it discusses methods for electromagnetic interference suppression, such as EMC design and shielding, for the switching power supply circuit and printed circuit board. By implementing these methods, the high-frequency switching power supply is ensured to meet the requirements of electromagnetic compatibility standards. Keywords: Electromagnetic compatibility; Electromagnetic interference; Suppression measures; High-frequency switching power supply; Printed circuit board 0 Introduction Currently, high-frequency switching power supplies are widely used in computer and peripheral equipment, communications, automatic control, and home appliances. However, a prominent drawback of high-frequency switching power supplies is their ability to generate strong electromagnetic interference (EMI). Because the primary rectifier bridge of a high-frequency switching power supply is a nonlinear device, the current it generates is a severely distorted sinusoidal half-wave containing abundant high-order harmonics, forming a series of continuous, intermittent, and transient interferences. Therefore, electromagnetic compatibility (EMC) design must be considered in the design of high-frequency switching power supplies. The power grid, situated entirely in a natural environment, connects various electronic and electrical devices and involves complex electromagnetic conversion processes, potentially causing several problems: external noise can cause malfunctions in the control circuits of high-frequency switching power supplies; communication equipment may malfunction due to noise from high-frequency switching power supplies; high-frequency switching power supplies can cause noise pollution to the power grid; and high-frequency switching power supplies can emit noise into space. Based on these considerations, and addressing the shortcomings of high-frequency switching power supplies, an electromagnetic compatibility (EMC) design study was conducted on its circuitry and printed circuit board (PCB). 1. EMC Design of High-Frequency Switching Power Supply 1.1 Composition of the Main Circuit of the High-Frequency Switching Power Supply The block diagram of the main circuit of the high-frequency switching power supply is shown in Figure 1. It consists of an input filter circuit, a high-frequency inverter circuit, an output rectifier circuit, and an output DC filter circuit. 1.2 EMC Design of the Input Filter Circuit The EMC design of the input filter circuit is shown in Figure 2. VD2 is a transient voltage suppressor diode, and RV1 is a varistor. Both have strong transient surge absorption capabilities, effectively protecting downstream components or circuits from surge voltage damage. Z1 is a DC electromagnetic interference (EMI) filter, which must be properly grounded, and the grounding wire must be short. L1 and C1 form a low-pass filter circuit. When the inductance of L1 is large, VD1 and R1 must be added to form a freewheeling loop to absorb the electric field energy released when L1 is disconnected; otherwise, the resulting voltage spikes will generate EMI. The magnetic core of L1 uses a closed magnetic core to avoid the leakage magnetic field of an open-loop core from generating EMI. C1 uses a large-capacity capacitor to reduce ripple voltage on the input line and weaken the electromagnetic field formed around the input conductor. 1.3 EMC Design of High-Frequency Inverter Circuit The EMC design of the high-frequency inverter circuit is shown in Figure 3. C2, C3, VT2, and VT3 form a half-bridge inverter circuit, with VT2 and VT3 being IGBTs or MOSFETs as switching transistors. R4 and C4 form an EMI absorption circuit. C5 and C6 are connected in parallel across VT2 and VT3. Due to the short switching time of VT2 and VT3 during turn-on and turn-off, as well as the presence of lead inductance and transformer leakage inductance, the circuit will generate high di/dt and du/dt, thus forming EMI. C4, C5, and C6 are low-inductance capacitors, and their capacitance is obtained by the formula LI2/2 = C△U2/2, where C is the circuit inductance, I is the circuit current, and △U is the overshoot voltage. 1.4 EMC Design of Output Rectifier Circuit The EMC design of the output rectifier circuit is shown in Figure 4. VD6 is a rectifier diode, and VD7 is a freewheeling diode. Since VD6 and VD7 operate in a high-frequency switching state, they are the main source of EMI. Connecting R5, C12 and R6, C13 respectively forms absorption circuits for VD6 and VD7 to absorb voltage spikes generated during switching. Reducing the number of rectifier diodes reduces EMI energy; therefore, under the same conditions, half-wave rectification generates less EMI than full-wave rectification and full-bridge rectification. To reduce diode EMI, diodes with soft recovery characteristics, low reverse recovery current, and short recovery time are selected. 1.5 EMC Design of Output DC Filter Circuit The two-port network model of the DC EMI filter is shown in Figure 5. Its hybrid parameter equation is: Where: g11 is the input admittance; g22 is the output impedance; g12 is the reverse current gain; g21 is the forward voltage gain. The equivalent schematic diagram shown in Figure 6 can be derived from equation (1). The design of the DC EMI filter must meet the following requirements: (1) Ensure that the filter does not affect the load-carrying capacity of the power supply while filtering; (2) For the input DC component, the filter should not cause attenuation as much as possible; (3) For harmonic components, the filter should have a good filtering effect. Combining the hybrid parameter equation and the equivalent schematic diagram, according to the first requirement, the input admittance and output impedance of the filter should be as small as possible, that is, g11 = g22 = 0. According to the second requirement, at low frequencies, the design values ​​of the reverse current gain g12 and the forward voltage gain g21 should be as close to 1 as possible, while the input admittance and output impedance should be as small as possible, i.e., g12 = g21 = 1, g11 = g22 = 0. According to the third requirement, at high frequencies, g11, g12, g21, and g22 should all be as small as possible. Based on the above conditions, the EMC design circuit of the output DC filter circuit is shown in Figure 7. L2, C17, and C18 form an LC filter circuit to reduce the magnitude of output voltage and current ripple, thereby reducing EMI propagated through radiation. Filter capacitors C17 and C18 should ideally be multiple capacitors connected in parallel to reduce the equivalent series resistance, thereby reducing ripple voltage. The output inductor L2 should be as large as possible to reduce output ripple current. C19 is used to filter common-mode interference on the conductors; a low-inductance capacitor should be selected, and the connection should be short. C20, C21, C22, and C23 are used to filter differential-mode interference on the output line; low-inductance three-terminal capacitors should be selected. Z2 is a DC filter; the input and output lines of the filter should be shielded and isolated. 1.6 EMC Design of Printed Circuit Board for Switching Power Supply The printed circuit board is the last step in the design of a high-frequency switching power supply. If the printed circuit board is not designed properly, the presence of small signal control lines, high-voltage busbars, high-frequency power switches, and magnetic components on the PCB will directly affect the anti-interference capability of each component and the reliability of the circuit operation, resulting in unstable power supply operation. The characteristic impedance of a single conductor consists of DC resistance R and self-inductance L, and its calculation formula is shown in the following two equations. Z=R+jwL (2) L=2lIn(2l/b+1/2) (3) Where: l is the length of the conductor; b is the width of the conductor. Obviously, the shorter the printed line, the smaller the DC resistance R. At the same time, increasing the width and thickness of the printed line can also reduce the DC resistance R. From equation (3), it can be seen that the shorter the length l of the printed line, the smaller the self-inductance L. Moreover, increasing the width b of the printed line can also reduce the self-inductance L. The characteristic impedance of multiple printed lines is composed of DC resistance R and self-inductance L, and is also affected by mutual inductance M. According to the formula for calculating mutual inductance M (4), in addition to being affected by the length and width of the printed circuit board, the distance between the printed lines also plays an important role. M=2l[In 2l/(b+s)-1] (4) Where: s is the distance between the two lines. Increasing the distance between the two lines can reduce mutual inductance. From the above analysis, it can be seen that when designing a PCB, the impedance of power lines and ground lines should be reduced as much as possible, because power lines, ground lines and other printed lines have inductance. When the power supply current changes significantly, a large voltage drop will be generated. The ground voltage drop is an important factor in forming common impedance interference. Therefore, the ground line should be shortened as much as possible, and the power lines and ground lines should be thickened as much as possible. 2 Conclusion Electromagnetic compatibility is a very complex issue. When designing a high-frequency switching power supply, the possible electromagnetic environment of the power supply should be fully estimated. The coupling paths between the high-frequency switching power supply and the external environment should be fully considered. Various interference suppression techniques should be used to eliminate interference coupling and enhance the anti-interference capability of the high-frequency switching power supply. The main measures include proper grounding, good connection, reasonable wiring and shielding, filtering, and limiting. Only by fully considering EMC design during the design process can the electromagnetic interference of high-frequency switching power supplies be minimized. References: [1] Qu Xueji, Qu Jingkai, Yu Mingyang. Power Electronic Filtering Technology and Application [M]. Beijing: Electronic Industry Press, 2008. [2] Zhou Zhimin, Zhou Jihai, Ji Aihua. Practical Circuits for Switching Power Supplies [M]. Beijing: China Electric Power Press, 2006. [3] Zhou Zhimin, Zhou Jihai, Ji Aihua. Modern Switching Power Supply Control Circuit Design and Application [M]. Beijing: People's Posts and Telecommunications Press, 2005. [4] Liu Shengli. Practical Technology of Modern High-Frequency Switching Power Supplies [M]. Beijing: Electronic Industry Press, 2001. [5] Qu Xueji, Qu Jingkai, Yu Mingyang. Power Electronic Rectification Technology and Application [M]. Beijing: Electronic Industry Press, 2007. For details, please click: EMC Design of High-Frequency Switching Power Supplies