Abstract: Based on the development and classification of switching power supplies, this paper describes the topology and characteristics of DO/DO and AC/DC converters, discusses the new technology trends of these two major categories of converters at home and abroad, and describes the selection of switching power supplies. Keywords: Switching power supply; technology; trends and applications; selection 1 Introduction With the rapid development of power electronics technology, the relationship between power electronic equipment and people's work and life is becoming increasingly close. Power electronic equipment cannot function without a reliable power supply. In the 1980s, computer power supplies were fully converted to switching power supplies, taking the lead in completing the power supply replacement of computers. In the 1990s, switching power supplies were successively introduced into various electronic and electrical equipment fields, as well as program-controlled exchanges, communication, power detection equipment, and control equipment power supplies. Switching power supplies have been widely used, further promoting the rapid development of switching power supply technology. A switching power supply is a power supply that uses modern electronic technology to control the on and off time ratio of switching transistors to maintain a stable output voltage. Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and MOSFETs. Compared with linear power supplies, the cost inversion point is higher for both switching power supplies and linear power supplies as the output rate decreases. This point is called the cost inversion point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating, and the cost reversal point is increasingly shifting towards low-power output, which provides a broad development space for switching power supplies. High-frequency switching is the direction of its development, enabling miniaturization and allowing switching power supplies to enter a wider range of application fields. Especially in the application of high-tech fields, it has promoted the miniaturization and portability of high-tech products. Furthermore, the development and application of switching power supplies are of great significance in saving energy, conserving resources, and protecting the environment. 2. Classification of Switching Power Supplies In the field of switching power supply technology, people are developing related power electronic devices and switching frequency conversion technology simultaneously. The two promote and drive each other. Switching power supplies can be divided into two main categories: AC/DC and DC/DC. DC/DC converters have now achieved modularization, and their design technology and manufacturing processes are mature and standardized both domestically and internationally, and have been recognized by users; however, the modularization of AC/DC converters, due to their inherent characteristics, encounters more complex technical and manufacturing problems in the modularization process. 2.1 DC/DC Conversion DC/DC conversion transforms a fixed DC voltage into a variable DC voltage, also known as DC transient. There are two operating modes for tampers: one is pulse width modulation (PWM) mode where Ts remains constant and ton is changed (general); the other is frequency modulation mode where ton remains constant. Changing Ts (easily causes interference) can lead to the following types of circuits: (1) BUCK circuit - step-down tamper, where the average output voltage Vo is less than the input voltage Vt, and the polarities are the same. (2) BUCK circuit - voltage tamper, where the average output voltage v0 is greater than or less than the input voltage, and the polarities are the same. (3) BUCK circuit - voltage or boost tamper, where the average output voltage Vo is greater than or less than the input voltage v0, and the polarities are opposite, with inductive transmission. (4) BUCK circuit - step-down or boost transformer, where the average output voltage v0 is greater than or less than the input voltage U, and the polarities are opposite, with capacitive transmission. Modern soft-switching technology has brought about a qualitative leap in DC/DC converters. V-COR, an American company, designs and manufactures various EC-soft-switching DO/DO converters with maximum output powers of 300W, 600W, and 800W, corresponding to power densities of 6, 2, 10, and 17 watts per cubic centimeter, respectively. Efficiency is achieved at 200-300kHz, with a power density reaching 27 watts per cubic centimeter. Using the same rectifier (MOSFET instead of Schottky diode), the overall circuit power is increased by 90%. 2.2 AC/DC Conversion AC/DC conversion transforms alternating current (AC) into direct current (DC). The power flow can be bidirectional. Power flowing from the power source to the load is called "rectification." Power returning from the load to the power source is called "active inversion." AC/DC converters accept 50/60Hz AC input, which requires rectification and filtering. Therefore, relatively large filter capacitors are essential. Furthermore, due to safety standards (such as UI and CCE) and EMC directives (such as IEC, FCC, and CSA), EMC-compliant voltage regulation and safety-standard-compliant components must be used on the AC input side, thus limiting the miniaturization of the AC/DC power supply. Additionally, the high-frequency, high-voltage, and high-current switching operations increase the difficulty of addressing EMC issues, placing high demands on the internal high-density circuit design. For the same reason, high-voltage and high-current switching increases power losses, further restricting the modularization of AC/DC converters. Therefore, power system optimization methods must be employed to achieve satisfactory efficiency. AC/DC converters can be classified into half-wave and full-wave circuits according to their wiring method, single-phase, three-phase, and multi-phase according to the number of power phases, and quadrant (T) according to the circuit quadrant (one-quadrant, two-quadrant, three-quadrant, and four-quadrant). [b]3 Selection of Switching Power Supplies[/b] Due to the characteristics of its circuit (multi-stage series connection), switching power supplies are less susceptible to input interference such as surge voltages. They also have a significant advantage over linear power supplies in terms of output voltage stability, which can reach 0.5-1%. 3.1 Selection of Output Current Since switching power supplies have high efficiency, generally exceeding 80%, the maximum absorption current of the equipment should be accurately measured or calculated to ensure a high performance-price ratio. The typical output calculation formula is: 3.2 Grounding Switching power supplies generate more interference than linear power supplies. For equipment sensitive to common-film dry winding, grounding and shielding measures should be taken. According to EMC restrictions such as ICEIO000 and FCC, switching power supplies all adopt EMC electromagnetic compatibility measures. Therefore, switching power supplies generally have EMC electromagnetic compatibility filters. For example, Lihua Technology's HA series switching power supplies connect their FG terminals to ground or to the user's chassis to meet the above electromagnetic compatibility requirements. 3.3 Protection Circuit Switching power supplies need to have overcurrent, overheat, and short-circuit protection functions in their design. Therefore, the design should first ensure that the power module has complete protection functions, and the technical parameters of its protection circuit should match the operating characteristics of the electrical equipment to avoid damage to the electrical equipment or the switching power supply. [b]4 Development Trends of Switching Power Supply Technology[/b] The development trend of switching power supplies is high frequency, high reliability, low power consumption, low noise, anti-interference, and modularity. Since the key technology for making switching power supplies lightweight, small, and thin is high-frequency operation, major international switching power supply manufacturers are committed to the simultaneous development of new intelligent components, especially reducing the losses of secondary rectifier devices, and increasing technological innovation in power ferrite (Mn.Zn) materials to improve high magnetic performance at high frequencies and high magnetic flux densities (Bs). Capacitor miniaturization is also a key technology. The application of SMT technology has enabled significant progress in switching power supplies, allowing for the placement of components on both sides of the circuit board to ensure lightweight, small, and thin switches. The high-frequency operation of switching power supplies inevitably requires innovation in traditional PWM switching technology. Achieving ZVS and ZCS switching technologies has become the mainstream technology for switching power supplies, significantly improving their operating efficiency. Regarding reliability, US switching power supply manufacturers have greatly improved switch reliability by reducing operating current and temperature to decrease component stress. Modularization is the overall trend in switching power supply development. Modular power supply groups and distributed power supply systems can be adopted, and N+I redundant power supply systems can be designed, allowing for capacity expansion through various connection methods. While partial resonant switching circuit technology can theoretically achieve both high-frequency operation and noise reduction, its practical application still faces technical challenges. Therefore, further research is needed in this area to ensure the technology's widespread adoption. Click to download: Principles and Applications of New Power Switches