Abstract: This paper provides a detailed analysis and comparison of several aspects affecting the reliability of switching power supplies, and proposes solutions to improve the reliability of switching power supplies based on engineering practice. Keywords: Switching power supply; Reliability; Electromagnetic compatibility Introduction The quality of electronic products is a combination of technical aspects and reliability. As an important component of an electronic system, the reliability of the power supply determines the reliability of the entire system. Switching power supplies are widely used in various fields due to their small size and high efficiency. How to improve their reliability is an important aspect of power electronics technology. 1 Switching Power Supply Electrical Reliability Engineering Design Technology 1.1 Selection of Power Supply Method Power supply methods are generally divided into centralized power supply systems and distributed power supply systems. Modern power electronic systems generally adopt distributed power supply systems to meet the requirements of high-reliability equipment. 1.2 Selection of Circuit Topology Switching power supplies generally adopt eight topologies: single-ended forward, single-ended flyback, two-transistor forward, dual single-ended forward, dual forward, push-pull, half-bridge, and full-bridge. Among them, the switching transistors of the two-transistor forward converter, double forward converter, and half-bridge circuit are only voltage-bearing of the input power supply voltage. It is relatively easy to select a 600V switching transistor when derating by 60%, and there is no problem of unidirectional magnetic saturation. These three topologies are widely used in high-voltage input circuits. 1.3 Power Factor Correction Technology Harmonic currents from switching power supplies pollute the power grid, interfering with other grid-connected equipment. They may also cause excessive neutral current in three-phase four-wire systems, leading to accidents. One solution is to use switching power supplies with power factor correction technology. 1.4 Selection of Control Strategy In small and medium power supplies, current-type PWM control is widely used. In DC-DC converters, the output ripple can be controlled to 10mV, which is better than conventional power supplies with voltage-type control. Hard switching technology is limited by switching losses, and the switching frequency is generally below 350 kHz. Soft switching technology enables switching devices to switch at zero voltage or zero current, achieving zero switching losses, thereby increasing the switching frequency to the megahertz level. This technology is mainly used in high-power systems and is less common in low-power systems. 1.5 Component Selection Since components directly determine the reliability of the power supply, their selection is crucial. Component failures mainly focus on four aspects: manufacturing quality issues, component reliability issues, design issues, and losses. Sufficient attention should be paid to these during use. 1.6 Protection Circuits To ensure reliable operation of the power supply in various harsh environments, multiple protection circuits should be incorporated into the design, such as surge protection, over/under voltage protection, overload protection, short circuit protection, and overheat protection circuits. 2 Electromagnetic Compatibility (EMC) Design Technology Switching power supplies often employ pulse width modulation (PWM) technology. The pulse waveform is rectangular, and its rising and falling edges contain a large number of harmonic components. Furthermore, the reverse recovery of the output rectifier diodes also generates electromagnetic interference (EMI), which is a detrimental factor affecting reliability. This makes electromagnetic compatibility a critical issue for the system. As shown in Figure 1, three necessary conditions exist for generating electromagnetic interference: an interference source, a transmission medium, and a sensitive receiving unit. EMC design involves addressing one of these three conditions. For switching power supplies, the main focus is on suppressing the interference source, which is concentrated in the switching circuit and the output rectifier circuit. The technologies employed include filtering, layout and wiring, shielding, grounding, and sealing. 3. Thermal Design Technology for Power Supply Equipment Reliability Statistical data shows that for every 2°C increase in the temperature of electronic components, reliability decreases by 10%; the lifespan at a 50°C temperature rise is only 1/6 of that at a 25°C temperature rise. Besides electrical stress, temperature is the most important factor affecting equipment reliability. This necessitates technical measures to limit the temperature rise of the chassis and components—this is thermal design. The principles of thermal design are: first, to reduce heat generation, i.e., to select superior control methods and technologies, such as phase-shifting control technology and synchronous rectification technology; second, to select low-power devices, reduce the number of heat-generating components, increase the width of thick printed lines, and improve power supply efficiency. Third, to enhance heat dissipation, i.e., to transfer heat using conduction, radiation, and convection technologies. This includes heat sink design, air cooling (natural convection and forced air cooling) design, liquid cooling (water, oil) design, thermoelectric cooling design, and heat pipe design. Forced air cooling dissipates heat more than ten times that of natural cooling, but it requires additional fans, fan power supplies, interlocking devices, etc. The heat dissipation method must be selected based on the actual situation during the design phase. 4. Safety Design Technology For power supplies, safety has always been considered the most important performance characteristic. Unsafe products not only fail to perform their intended functions but may also cause serious accidents, even resulting in massive losses such as machine damage and loss of life. To ensure a high level of product safety, safety design is essential. The safety design of power supply products includes preventing electrical hazards and overheating hazards. For the commercial equipment market, representative safety standards include UL, CSA, and VDE, etc., with content varying depending on the application. The permissible leakage current is between 0.5 and 5 mA, and the leakage current specified in China's military standard GJB1412 is less than 5 mA. The magnitude of the power supply equipment's leakage current to ground depends on the capacitance of the Y capacitor in the EMI filter, as shown in Figure 2. From the perspective of the EMI filter, a larger Y capacitor capacitance is better; however, from a safety perspective, a smaller Y capacitor capacitance is better. The capacitance of the Y capacitor is determined according to safety standards. According to GJB151A, the capacitance should be less than 0.1 μF for 50 Hz devices and less than 0.02 μF for 400 Hz devices. If the safety performance of the X capacitor is poor, it may break down when transient peaks occur in the power grid. While this breakdown does not endanger personal safety, it will cause the filter to lose its filtering function. 5. Three-Proof Design Technology Three-proof design refers to moisture-proof design, salt spray-proof design, and mold-proof design. All power supplies used in areas south of the Yangtze River, coastal areas, and military applications in China should be designed with three-proof features. In humid marine atmospheres, electronic equipment surfaces will absorb a thin layer of moisture, i.e., a water film. However, when this water film reaches a thickness of 20-30 molecular layers, it forms an electrolyte film necessary for chemical corrosion. This salt-rich electrolyte has strong corrosive activity against exposed metal surfaces. Furthermore, sudden temperature changes and the formation of dew points in the air can decrease the insulation resistance between printed lines, cause mold growth on components, resulting in verdigris, corrosion, and breakage of leads. Humid and hot environments provide favorable conditions for mold growth. Mold feeds on organic matter in electronic devices, absorbing moisture and secreting organic acids, damaging insulation, causing short circuits, and accelerating metal corrosion. In engineering, corrosion-resistant materials can be selected, and then plating, coating, or chemically treating the electronic equipment and components with a metallic or non-metallic protective film to isolate them from the surrounding medium, thus achieving protection. Structurally, sealed or semi-sealed designs can be used to isolate them from adverse external environments. Applying a special conformal coating to printed circuit boards and components can effectively prevent corona discharge and breakdown between conductors, improving power supply reliability. Transformers should be impregnated and terminal-sealed to prevent moisture ingress and short circuits. Conformal design and electromagnetic shielding are often contradictory. Excellent conformal design provides good electrical insulation, but an electrically insulated casing may lack good shielding; these two aspects must be considered comprehensively. In the overall design, shielding and grounding requirements should be fully considered, and reasonable processes should be adopted to ensure long-term conductivity of surfaces with electrical contact. 6. Vibration Resistance Design Technology Vibration is also a significant cause of power supply failures. In vibration tests, tantalum capacitors and aluminum electrolytic capacitors often experience lead breakage, necessitating reinforced designs. Generally, tantalum capacitors can be secured with silicone, aluminum electrolytic capacitors exceeding 25cm in height and 12cm in diameter can be secured with clamps, and ribs can be added to the printed circuit board. 7. Conclusion The above suggestions apply only to industrial and military power supplies. For commercial-grade products, different choices may be made in certain aspects. In short, the reliability of power supply equipment depends not only on electrical design but also on assembly, manufacturing processes, structural design, and processing quality. Reliability is based on design; in practical engineering applications, feedback data obtained through various tests should be used to refine the design and further improve power supply reliability.