Abstract: This paper briefly introduces the electromagnetic compatibility (EMC) requirements, domestic and international standards, causes of EMC, research and solutions, and the current status of EMC in domestic communication switching power supplies. Keywords: Communication switching power supply, electromagnetic compatibility, standards 1 Introduction Communication switching power supplies are widely used in program-controlled switching, optical data transmission, wireless base stations, cable television systems, and IP networks due to their advantages such as small size, light weight, high efficiency, reliable operation, and remote monitoring capabilities. They are the core power source for the normal operation of information technology equipment. With the development of information technology, information technology equipment is ubiquitous throughout China, from developed central cities to remote mountainous areas, providing great convenience for communication and information transmission. Due to differences between urban and rural areas, the power supply network for communication equipment includes both stable large power grid supply and independent small hydropower supply. Under the small hydropower station supply method, the large fluctuations in water volume, user electricity consumption, and the instability of power generation equipment cause severe waveform distortion and large voltage fluctuations in the power grid. At the same time, the non-standard wiring of the power distribution system poses a severe challenge to the switching power supply used in communication. Railway communication and power communication are developing and expanding. The passage of electric locomotives generates strong induced voltages, causing significant fluctuations in ground voltage and consequently, large fluctuations in the power grid voltage. This strong electric field can easily lead to momentary instability in the operation of switching power supply equipment. Communication switching power supplies operating near high-voltage power grids, even with stable grid voltage, are susceptible to interference from strong electromagnetic fields caused by changes in grid load. Communication switching power supplies used in base stations, often installed on tall buildings or mountaintops, are even more vulnerable to lightning strikes. Therefore, communication switching power supplies must possess strong electromagnetic interference resistance, particularly against lightning strikes, surges, and power grid voltage fluctuations. They must also have sufficient resistance to electrostatic interference, electric fields, magnetic fields, and electromagnetic waves to ensure their own normal operation and the stability of power supply to communication equipment. Furthermore, the power switching transistors, rectifier or freewheeling diodes, and main power transformers within communication switching power supplies operate under high voltage, high current, and high-frequency switching conditions, resulting in square wave voltage and current waveforms. During the switching process of these high-voltage, high-current square waves, severe harmonic voltages and currents are generated. These harmonic voltages and currents, on the one hand, are transmitted through the power input lines or output lines of the switching power supply, interfering with other equipment and the power grid itself powered on the same grid as the communication power supply. They also interfere with equipment powered by the communication power supply, such as program-controlled switching equipment, wireless base stations, optical transmission equipment, and cable television equipment, causing them to malfunction. On the other hand, severe harmonic voltages and currents generate electromagnetic interference within the switching power supply, leading to instability and reduced performance. Furthermore, some electromagnetic fields radiate into the surrounding space through gaps in the switching power supply casing, combining with radiated electromagnetic fields generated through power lines and DC output lines to interfere with other high-frequency equipment and equipment sensitive to electromagnetic fields, causing malfunctions. Therefore, for communication switching power supplies, it is necessary to limit conducted interference from load lines and power lines, as well as electromagnetic interference radiated from power lines, to ensure that telecommunications equipment in the same electromagnetic environment can operate normally without interfering with each other. 2. Domestic and International Electromagnetic Compatibility Standards Electromagnetic compatibility refers to the ability of a device or system to operate normally in its electromagnetic environment without causing unacceptable electromagnetic interference to anything in that environment. It is impossible to completely eliminate electromagnetic interference (EMI) from equipment and make it insensitive to all external EMI signals. Only by systematically establishing standards for the permissible magnitude of EMI between devices and their ability to resist EMI can the requirements of EMI be met for electrical equipment and systems. Numerous EMI standards, both domestic and international, have established constraints for achieving EMI compatibility between devices within a system. The International Special Committee on Radio Interference (CISP) is an EMI standardization organization under the International Electrotechnical Commission (IEC). It began researching EMI standards as early as 1934 and has six subcommittees. Subcommittee Six (SCC) is primarily responsible for developing standards for interference measurement receivers and measurement methods. CISP 16, "Specification for Radio Interference and Immunity Measurement Equipment," provides detailed requirements for the performance and calibration methods of EMI measurement receivers and auxiliary equipment. CISP 17, "Measurement of Suppression Characteristics of Radio Interference Filters and Suppression Components," establishes measurement methods for filters. CISP 22, "Limits and Measurement Methods for Radio Interference from Information Technology Equipment," specifies the limits for EMI generated by information technology equipment in the frequency range of 0.15–1000 MHz. CISPR24, "Immunity Limits and Measurement Methods for Information Technology Equipment," specifies the time-domain and frequency-domain immunity requirements for information technology equipment to external interference signals. CISPR16, CISPR22, and CISPR24 constitute the electromagnetic compatibility (EMC) testing content and methods for information technology equipment, including communication switching power supplies, and represent the most fundamental requirements for EMC design of communication switching power supplies. IEC has also recently published numerous basic EMC standards, the most representative being the IEC 61000 series. These standards specify EMC requirements for electronic and electrical equipment against lightning strikes, surges, electrostatic discharge (ESD), electrical fast transients and bursts (EFT), current harmonics, voltage dips, voltage transients and short interruptions, voltage fluctuations and flicker, radiated electromagnetic fields, conducted interference caused by radio frequency electromagnetic fields, conducted interference, and radiated interference. In addition, FCC15, established by the U.S. Federal Commission of Electricity and Communications Technology (FCC), and VDE0871-2A1, VDE0871-2A2, and VDE0878, established by the German Association for Electrical, Electronic & Information Technology (DEET), all set requirements for the electromagnetic compatibility (EMC) of communication equipment. my country's research on EMC standards started relatively late. The primary approach adopted was to introduce, digest, and absorb international standards, making them suitable for Chinese use – this was the main method for developing domestic EMC standards. In 1998, the Ministry of Information Industry, based on CISPR22, IEC 61000 series standards, and ITU-T .41, formulated YD/T983 1998, "Electromagnetic Compatibility Limits and Measurement Methods for Communication Power Supply Equipment," which detailed the specific test items, requirements, and test methods for the EMC of communication power supply equipment, including communication switching power supplies. This clarified the design objectives for the inspection, compliance, and network access testing of communication power supplies. The national standard also adopted the corresponding international standards. For example, the GB/T 17626.1～12 series standards are equivalent to the IEC 61000 series standards; GB 9254 1998 "Limits and Measurement Methods for Radio Interference of Information Technology Equipment" is equivalent to CISPR22; GB/T 17618 1998 "Limits and Measurement Methods for Immunity of Information Technology Equipment" is equivalent to CISPR24. 3. Electromagnetic Compatibility Issues of Switching Power Supplies Communication switching power supplies operate under high voltage and high current switching conditions, resulting in complex electromagnetic compatibility (EMC) issues. From the perspective of overall EMC, there are mainly several types: common impedance coupling, line-to-line coupling, electric field coupling, magnetic field coupling, and electromagnetic wave coupling. The three elements of EMC are: the interference source, the propagation path, and the object being interfered with. Common impedance coupling mainly occurs when the interference source and the object being interfered with share a common impedance, allowing the interference signal to enter the object being interfered with. Line-to-line coupling mainly occurs when the conductors or PCB lines that generate interference voltage and current are coupled together due to parallel wiring. Electric field coupling is mainly caused by the induced electric field generated by the potential difference, which couples to the object being interfered with. Magnetic field coupling is mainly caused by the low-frequency magnetic field generated near a high-current pulsed power line, which couples to the object being interfered with. Electromagnetic wave coupling is mainly caused by the high-frequency electromagnetic waves generated by pulsating voltage or current, which radiate outward through space and couple to the corresponding object being interfered with. In reality, each coupling method cannot be strictly distinguished; they are just different in emphasis. In switching power supplies, the main power switching transistors operate at very high voltages in a high-frequency switching mode. Both the switching voltage and current are square waves, and the spectrum of high-order harmonics contained in these square waves can reach more than 1000 times the square wave frequency. At the same time, due to the leakage inductance and distributed capacitance of the power transformer, as well as the non-ideal operating conditions of the main power switching devices, high-frequency, high-voltage spike harmonic oscillations are often generated during high-frequency switching. The high-order harmonics generated by these oscillations are transmitted into the internal circuit through the distributed capacitance between the switching transistor and the heat sink, or radiated into space through the heat sink and transformer. Switching diodes used for rectification and freewheeling are also an important cause of high-frequency interference. Because rectifier and freewheeling diodes operate in high-frequency switching mode, the presence of parasitic inductance in the diode leads, junction capacitance, and the influence of reverse recovery current cause them to operate under very high voltage and current change rates, resulting in high-frequency oscillations. Since rectifier and freewheeling diodes are generally close to the power output line, the high-frequency interference they generate is most easily transmitted through the DC output line. Communication switching power supplies employ active power factor correction circuits to improve the power factor. Simultaneously, to improve circuit efficiency and reliability and reduce the electrical stress on power devices, soft-switching technology is widely used. Zero-voltage, zero-current, or zero-voltage-zero-current switching technology is the most widely applied. This technology greatly reduces electromagnetic interference generated by switching devices. However, soft-switching lossless absorption circuits often utilize I/C for energy transfer, leveraging the unidirectional conductivity of diodes to achieve unidirectional energy conversion. Therefore, the diodes in this resonant circuit become a major source of electromagnetic interference. In communication switching power supplies, I/C filter circuits are generally composed of energy storage inductors and capacitors to filter differential-mode and common-mode interference signals and convert AC square wave signals into smooth DC signals. The distributed capacitance of the inductor coil reduces its self-resonant frequency, allowing a large amount of high-frequency interference signals to pass through the inductor and propagate outwards along AC power lines or DC output lines. As the frequency of the interference signal increases, the capacitance and filtering effect of the filter capacitor continuously decrease due to the inductance of the leads, until it completely loses its capacitor function and becomes inductive above the resonant frequency. Improper use of filter capacitors and excessively long leads are also causes of electromagnetic interference. Communication switching power supplies, due to their high power density, high level of intelligence, and inclusion of an MCU microprocessor, contain voltage signals ranging from nearly 1000 volts to a few volts, and from high-frequency digital signals to low-frequency analog signals, resulting in a highly complex internal field distribution. Inappropriate PCB wiring, structural design, power line input filtering, input/output power line wiring, and CPU and detection circuit design can all lead to system instability or reduce immunity to electrostatic discharge, electrical fast transient/burst, lightning strikes, surges, conducted interference, radiated interference, and radiated electromagnetic fields. Electromagnetic compatibility (EMC) research generally utilizes electromagnetic field testing instruments and various interference signal simulators and auxiliary equipment specified in CISPR 16 and IEC 61000. This is done in standard testing sites or laboratories through detailed testing and analysis, combined with an understanding of circuit performance. From the perspective of the three elements of EMC, solving the EMC of switching power supplies can be approached from three aspects: 1) reducing interference signals generated by interference sources; 2) cutting off the propagation path of interference signals; 3) enhancing the anti-interference capability of the affected components. When solving the internal EMC of switching power supplies, the above three methods can be used comprehensively, taking into account cost-effectiveness and ease of implementation. External interference generated by switching power supplies, such as power line harmonic currents, power line conducted interference, and electromagnetic field radiation interference, can only be addressed by reducing the interference source. On the one hand, this can be achieved by enhancing the design of input and output filter circuits, improving the performance of active power factor correction (APFC) circuits, reducing the voltage and current change rate of switching transistors and rectifier freewheeling diodes, and adopting various soft-switching circuit topologies and control methods. On the other hand, the shielding effect of the chassis should be strengthened, leakage at chassis gaps should be improved, and proper grounding should be implemented. For external interference immunity, such as surge and lightning strike protection, the lightning protection capabilities of the AC input and DC output ports should be optimized. Typically, for lightning strike waveforms combining 1.2/50μs open-circuit voltage and 8/20μs short-circuit current, due to their relatively low energy, a combination of zinc oxide varistors and gas discharge tubes can be used. For electrostatic discharge (ESD), TVS diodes and corresponding grounding protection are typically used in the small-signal circuits of communication and control ports; the electrical distance between the small-signal circuits and the chassis should be increased; or devices with anti-ESD interference capabilities should be selected. Fast transient signals contain a wide spectrum and can easily enter the control circuit in common-mode mode. The same anti-ESD methods should be used, along with reducing the distributed capacitance of the common-mode inductor and strengthening the common-mode signal filtering of the input circuit (e.g., adding common-mode capacitors or insertion-loss ferrite cores) to improve the system's immunity. To reduce internal interference in switching power supplies, achieve their electromagnetic compatibility, and improve their stability and reliability, the following aspects should be considered: Proper partitioning of digital and analog PCB wiring; proper decoupling of power supplies between digital and analog circuits; single-point grounding of digital and analog circuits, and single-point grounding of high-current and low-current circuits, especially current and voltage sampling circuits, to reduce common-resistance interference and the effects of ground loops; attention to spacing between adjacent lines and signal characteristics during wiring to avoid crosstalk; reducing ground impedance; reducing the area enclosed by high-voltage, high-current circuits, especially the transformer primary winding, switching transistors, and power supply filter capacitors; reducing the area enclosed by the output rectifier circuit, freewheeling diode circuit, and DC filter circuit; reducing the leakage inductance of the transformer and the distributed capacitance of the filter inductor; and using filter capacitors with high resonant frequencies. The high-frequency data and address lines of the MCU and LCD display are the main sources of interference and radiation. Small-signal circuits are the weakest link in resisting external interference; appropriately adding high-interference-resistance TVs, high-frequency capacitors, ferrite beads, and other components can improve their anti-interference capability. Small-signal circuits close to the chassis should be properly insulated and withstand voltage. The heat sinks of power devices and the electromagnetic shielding layer of the main transformer should be properly grounded. Comprehensive consideration of various grounding measures helps improve the overall electromagnetic compatibility (EMC) of the system. Large-area grounding shields between control units can improve the stability of the switching power supply's internal operation. On the rectifier rack, the electromagnetic coupling between rectifiers, the overall grounding layout, the correct relationship between the AC input neutral, ground, DC ground, and lightning protection ground, and the correct allocation of EMC levels must be considered. In the switching power supply's resistance to internal and external interference and its anti-interference capabilities, common-mode signals have a complex relationship with the operating mode of switching devices, the installation of heat sinks, and the connection between the PCB board and the chassis. Under certain conditions, common-mode signals can also be converted into differential-mode signals. The simplest way to solve common-mode interference is to properly address the issues between individual circuit units, overall ports, and the chassis. Overall shielding is difficult to implement and costly, and should only be used as a last resort. 5. Current Status of Electromagnetic Compatibility Improvement in Domestic Communication Switching Power Supplies Since the drafting of the YD/T983 standard, domestic communication power supply manufacturers have begun research on electromagnetic compatibility (EMC). Due to the high cost of EMC testing instruments and test site construction, and the need for experienced R&D personnel, many manufacturers lack their own laboratories, posing challenges to EMC research. The YD/T983 standard uses lower levels of immunity indicators from international standards. Except for lightning surge, ESD, and EFT indicators, other immunity indicators are relatively easy to meet. Electromagnetic interference indicators, such as conducted interference and radiated interference, are difficult to meet the standard requirements and are currently a hot topic in EMC research; only a very few domestic manufacturers can fully meet the relevant standard requirements. ZTE Corporation has established its own EMC laboratory and has been committed to EMC research since the early stages of communication switching power supply R&D. Its communication switching power supply employs state-of-the-art active power factor correction technology and lossless absorption circuitry in the front stage, and adopts zero-voltage zero-current (ZVZCS) phase-shift resonant soft-switching technology or dual-transistor forward lossless absorption soft-switching technology in the DC/DC stage. Through professional power input and output filter design and lightning protection design, as well as anti-static design and anti-fast transient/burst design for the overall safety of the machine and digital interface circuit, and appropriate electromagnetic shielding design for the overall structure, it not only ensures a good internal electromagnetic environment, stable operation, and improved reliability, but also makes the communication switching power supply meet or exceed the Class A requirements of the CISPR22 standard for external current harmonics, electrical fluctuations and flicker, conducted interference, and radiated interference. The AC input power line can withstand surge voltage interference of at least ±6kV (combined waves of 1.2/50μs and 8/20μs), and the DC power line can withstand surge voltage of at least ±2kV. Externally, the entire unit can withstand electrostatic discharge (ESD) interference of at least ±8kV, electrical fast transient/burst (EFT) interference of at least ±4kV, high-frequency electromagnetic field interference of 3V/m, and power frequency magnetic field interference of 300A/m. This wide AC input voltage range ensures that the switching power supply can operate normally after voltage drops, voltage transients, and short-term voltage interruptions. Power grid interference voltages professionally collected from across the country have been verified and analyzed on ZTE switching power supplies. The electromagnetic compatibility (EMC) indicators of ZTE's series of switching power supplies fully meet and exceed all the indicators specified in YD/T983-1998 "Electromagnetic Compatibility Requirements and Measurement Methods for Communication Switching Power Supply Equipment". Some products have passed all EMC indicators in CE and FCC certifications. They are truly environmentally friendly communication switching power supplies, especially suitable for use in mobile base stations, program-controlled switching equipment, IP phones, cable TV and other data communication transmission equipment, as well as in environments with strong electromagnetic interference such as railways, hydropower, and thermal power plants.