Currently, power electronics, as the foundation of energy conservation, energy saving, automation, intelligence, and mechatronics, is developing towards higher frequency applications, modular hardware structures, and greener product performance. In the near future, power electronics technology will make power supply technology more mature, economical, and practical, achieving a combination of high efficiency and high-quality power consumption. 1. Development of Power Electronics Technology The development direction of modern power electronics technology is shifting from traditional power electronics, which primarily addresses problems with low-frequency technology, to modern power electronics, which primarily addresses problems with high-frequency technology. Power electronics technology originated with silicon rectifier devices in the late 1950s and early 1960s. Its development has successively gone through the rectifier era, the inverter era, and the frequency converter era, promoting the application of power electronics technology in many new fields. The power semiconductor composite devices, represented by power MOSFETs and IGBTs, which integrate high frequency, high voltage, and high current, developed in the late 1980s and early 1990s, indicate that traditional power electronics technology has entered the modern power electronics era. 1.1 The Era of Rectifiers: High-power industrial electricity was provided by power frequency (50Hz) AC generators, but approximately 20% of the electricity was consumed in DC form. The most typical applications were electrolysis (non-ferrous metals and chemical raw materials required DC electrolysis), traction (electric locomotives, diesel locomotives with electric transmission, subway locomotives, urban trolleybuses, etc.), and DC drives (steel rolling, papermaking, etc.). High-power silicon rectifiers could efficiently convert power frequency AC to DC, leading to significant development and application of high-power silicon rectifier tubes and thyristors in the 1960s and 1970s. At that time, a wave of silicon rectifier factories was established across China, and the numerous semiconductor manufacturers producing silicon rectifiers nationwide today are products of that era. 1.2 The Era of Inverters: The 1970s saw a global energy crisis, and AC motor variable frequency speed control developed rapidly due to its significant energy-saving effects. The key technology of variable frequency speed control is converting DC to AC at 0-100Hz. In the 1970s and 1980s, with the popularization of variable frequency speed control devices, thyristors, giant power transistors (GTRs), and gate turn-off thyristors (GTOs) for high-power inverters became the mainstays of power electronic devices. Similar applications included high-voltage DC output and static var compensating for reactive power. At this time, power electronic technology could achieve rectification and inversion, but the operating frequency was low, limited to the low to medium frequency range. 1.3 The Era of Variable Frequency Drives Entering the 1980s, the rapid development of large-scale and very large-scale integrated circuit technology laid the foundation for the development of modern power electronic technology. The organic combination of fine processing technology and high-voltage, high-current technology of integrated circuits led to the emergence of a number of new fully controllable power devices. First, the advent of power MOSFETs led to the development of small and medium power supplies towards higher frequencies, and then the emergence of insulated-gate bipolar transistors (IGBTs) brought opportunities for the development of medium and large power supplies towards higher frequencies. The successive advent of MOSFETs and IGBTs marked the transformation from traditional power electronics to modern power electronics. According to statistics, by the end of 1995, power MOSFETs and GTRs had reached a point of equal dominance in the power semiconductor device market, and the replacement of GTRs with IGBTs had become a settled matter in the field of power electronics. The development of new devices not only provides higher frequencies for AC motor frequency conversion speed regulation, making its performance more perfect and reliable, but also drives modern electronic technology towards higher frequencies, providing an important technical foundation for efficient material and energy saving in electrical equipment, achieving miniaturization, lightweighting, mechatronics, and intelligence. 2. Application Areas of Modern Power Electronics 2.1 High-Efficiency Green Power Supply for Computers The rapid development of computer technology has led humanity into the information society, and has also promoted the rapid development of power supply technology. In the 1980s, computers fully adopted switching power supplies, taking the lead in completing the replacement of computer power supplies. Subsequently, switching power supply technology entered the fields of electronic and electrical equipment. The development of computer technology has led to the concepts of green computers and green power supplies. Green computers generally refer to personal computers and related products that are harmless to the environment. Green power supplies refer to high-efficiency and energy-saving power supplies related to green computers. According to the U.S. Environmental Protection Agency's "Energy Star" program of June 17, 1992, desktop personal computers or related peripherals that consume less than 30 watts in sleep mode meet the requirements of green computers. Improving power supply efficiency is the fundamental way to reduce power consumption. For example, a 200-watt switching power supply with an efficiency of 75% consumes 50 watts of energy. 2.2 High-Frequency Switching Power Supplies for Communication The rapid development of the communication industry has greatly promoted the development of communication power supplies. High-frequency miniaturized switching power supplies and their technology have become the mainstream of modern communication power supply systems. In the field of communication, rectifiers are usually called primary power supplies, while DC-DC converters are called secondary power supplies. The function of a primary power supply is to convert a single-phase or three-phase AC power grid into a DC power supply with a nominal value of 48V. Currently, in the primary power supplies used in program-controlled exchanges, traditional phase-controlled voltage regulators have been replaced by high-frequency switching power supplies. High-frequency switching power supplies (also known as switching rectifiers, SMRs) utilize the high-frequency operation of MOSFETs or IGBTs, with switching frequencies typically controlled within the 50-100kHz range, achieving high efficiency and miniaturization. In recent years, the power capacity of switching rectifiers has continuously increased, with single-unit capacities expanding from 48V/12.5A and 48V/20A to 48V/200A and 48V/400A. Due to the diverse types of integrated circuits used in communication equipment, their power supply voltages vary. High-power-density high-frequency DC-DC isolated power supply modules are used in communication power supply systems to convert the intermediate bus voltage (typically 48V DC) to the required DC voltages. This significantly reduces losses, facilitates maintenance, and is very convenient for installation and expansion. They can generally be directly mounted on standard control boards, requiring high power density in the secondary power supply. As communication capacity continues to increase, communication power supply capacity will also continue to increase. 2.3 DC-DC Converters DC/DC converters transform a fixed DC voltage into a variable DC voltage. This technology is widely used in the stepless speed control and regulation of trolleybuses, subway trains, and electric vehicles, enabling these controls to achieve smooth acceleration and rapid response, while also saving energy. Replacing a rheostat with a DC chopper can save (20-30)% of energy. The DC chopper not only functions as a voltage regulator (switching power supply) but also effectively suppresses harmonic current noise from the power grid. Secondary power supply DC/DC converters for communication power supplies are commercially available. These modules employ high-frequency PWM technology, with a switching frequency of around 500kHz and a power density of 5W-20W/in³. With the development of large-scale integrated circuits, miniaturization of power supply modules is required. Therefore, it is necessary to continuously increase the switching frequency and adopt new circuit topologies. Currently, some companies have developed and produced secondary power supply modules using zero-current switching and zero-voltage switching technologies, resulting in a significant increase in power density. 2.4 Uninterruptible Power Supply (UPS) An uninterruptible power supply (UPS) is a highly reliable, high-performance power supply essential for computers, communication systems, and other applications requiring uninterrupted power. AC mains input is rectified to DC; part of the energy charges the battery bank, and the other part is converted back to AC by an inverter and delivered to the load via a transfer switch. To ensure continued power supply to the load in case of inverter failure, a backup power supply is provided through a power transfer switch. Modern UPS systems commonly employ pulse width modulation technology and modern power electronic devices such as power MOSFETs and IGBTs, reducing noise and improving efficiency and reliability. The introduction of microprocessor hardware and software technology enables intelligent management of the UPS, allowing for remote maintenance and diagnostics. Currently, the maximum capacity of online UPS systems can reach 600kVA. Ultra-miniature UPS systems are also developing rapidly, with various specifications available, including 0.5kVA, 1VA, 2kVA, and 3kVA. 2.5 Inverter Power Supply Inverter power supplies are mainly used for variable frequency speed control of AC motors. Their role in electrical drive systems is becoming increasingly important, and they have achieved significant energy savings. The main circuit of the inverter power supply adopts an AC-DC-AC scheme. The mains frequency power supply is converted into a fixed DC voltage by a rectifier, and then a PWM high-frequency converter composed of high-power transistors or IGBTs inverts the DC voltage into a variable voltage and frequency AC output. The power output waveform approximates a sine wave, used to drive an AC asynchronous motor to achieve stepless speed regulation. Inverter power supply series products below 400kVA have already been launched internationally. In the early 1980s, Toshiba Corporation of Japan was the first to apply AC variable frequency speed control technology to air conditioners. By 1997, its market share had reached over 70% in Japanese household air conditioners. Variable frequency air conditioners have advantages such as comfort and energy saving. Domestic research on variable frequency air conditioners began in the early 1990s, and production lines were introduced in 1996 to produce variable frequency air conditioners, gradually forming a hot spot in the development and production of variable frequency air conditioners. It is expected that a peak will be reached around 2000. In addition to the variable frequency power supply, variable frequency air conditioners also require compressor motors suitable for variable frequency speed control. Optimizing control strategies and selecting functional components are further directions for the development of air conditioner variable frequency power supplies. 2.6 High-Frequency Inverter Rectifier Welding Power Supply The high-frequency inverter rectifier welding power supply is a high-performance, high-efficiency, and material-saving new type of welding power supply, representing the current development direction of welding power supplies. Due to the commercialization of large-capacity IGBT modules, this type of power supply has broad application prospects. Most inverter welding power supplies adopt an AC-DC-AC-DC conversion method. 50Hz AC power is rectified into DC by a full-bridge rectifier. The PWM high-frequency conversion section composed of IGBTs inverts the DC power into a 20kHz high-frequency rectangular wave, which, after coupling by a high-frequency transformer and rectification and filtering, becomes a stable DC power supply for arc power. Because welding power supplies operate under harsh conditions, frequently alternating between short circuits, arcing, and open circuits, the reliability of the high-frequency inverter rectifier welding power supply is the most critical issue and the most concerning to users. By using a microprocessor as the pulse width modulation (PWM) controller, and extracting and analyzing multiple parameters and information, the system's various operating states can be predicted, allowing for advance adjustments and processing, thus solving the reliability problem of current high-power IGBT inverter power supplies. Foreign inverter welding machines can achieve a rated welding current of 300A, a duty cycle of 60%, a full-load voltage of 60-75V, a current adjustment range of 5-300A, and a weight of 29kg. 2.7 High-Power Switching High-Voltage DC Power Supplies: High-power switching high-voltage DC power supplies are widely used in large equipment such as electrostatic precipitators, water quality improvement systems, medical X-ray machines, and CT scanners. Voltages range from 50 to 159kV, currents exceed 0.5A, and power can reach 100kW. Since the 1970s, some Japanese companies have adopted inverter technology, converting rectified mains power to a medium frequency of approximately 3kHz before boosting the voltage. In the 1980s, high-frequency switching power supply technology developed rapidly. Siemens of Germany used power transistors as the main switching element, increasing the switching frequency of the power supply to over 20kHz. They also successfully applied dry-type transformer technology to high-frequency high-voltage power supplies, eliminating the high-voltage transformer tank and further reducing the size of the transformer system. Domestic research has been conducted on high-voltage DC power supplies for electrostatic precipitators. The mains power is rectified into DC, and a full-bridge zero-current switching series resonant inverter circuit is used to invert the DC voltage into a high-frequency voltage. This voltage is then stepped up by a high-frequency transformer and finally rectified into a high-voltage DC. Under resistive load conditions, the output DC voltage reaches 55kV, the current reaches 15mA, and the operating frequency is 25.6kHz. 2.8 Active Power Filters Traditional AC-DC converters inject a large amount of harmonic current into the power grid during operation, causing harmonic losses and interference. Simultaneously, the power factor on the grid side deteriorates, a phenomenon known as "power pollution." For example, with uncontrolled rectification and capacitor filtering, the third harmonic content on the grid side can reach (70-80)%, and the power factor on the grid side is only 0.5-0.6. Active power filters are a new type of power electronic device capable of dynamically suppressing harmonics. They overcome the shortcomings of traditional LC filters and are a promising harmonic suppression method. The filter consists of a bridge switching power converter and specific control circuitry. Not only does it provide feedback on the output voltage, but it also provides feedback on the average input current; (2) The current loop reference signal is the product of the voltage loop error signal and the full-wave rectified voltage sampling signal. 2.9 Distributed switching power supply system The distributed power supply system uses small power modules and large-scale control integrated circuits as basic components. It utilizes the latest theoretical and technological achievements to form a modular and intelligent high-power power supply, thereby closely integrating strong and weak currents, reducing the development pressure of high-power components and high-power devices (centralized), and improving production efficiency. In the early 1980s, the research on distributed high-frequency switching power supply systems was mainly focused on the research on converter parallel technology. In the mid-to-late 1980s, with the rapid development of high-frequency power conversion technology, various converter topologies emerged one after another. Combined with large-scale integrated circuits and power component technology, the integration of small and medium power devices became possible, thus rapidly promoting the development of distributed high-frequency switching power supply system research. Since the late 1980s, this direction has become a research hotspot in the international power electronics community, with the number of papers increasing year by year and the application field continuously expanding. Distributed power supply has the advantages of energy saving, reliability, high efficiency, economy and convenient maintenance. High-frequency switching power supplies have been gradually adopted by large-scale computer, communication equipment, aerospace, and industrial control systems, and are the most ideal power supply method for low-voltage power supplies (3.3V) of ultra-high-speed integrated circuits. They also have broad application prospects in high-power applications such as electroplating, electrolysis power supplies, electric locomotive traction power supplies, medium-frequency induction heating power supplies, and motor drive power supplies. 3. Development Trends of High-Frequency Switching Power Supplies In the application of power electronics technology and various power supply systems, switching power supply technology occupies a core position. For large electrolytic electroplating power supplies, traditional circuits are very large and bulky. If high-frequency switching power supply technology is adopted, its size and weight will be significantly reduced, and power utilization efficiency can be greatly improved, materials saved, and costs reduced. Switching power supply technology is indispensable in electric vehicles and frequency converters, as it changes the power frequency to achieve near-ideal load matching and drive control. High-frequency switching power supply technology is also the core technology of various high-power switching power supplies (inverter welding machines, communication power supplies, high-frequency heating power supplies, laser power supplies, power operating power supplies, etc.). 3.1 High-Frequency Theory Analysis and Practical Experience show that the size and weight of transformers, inductors, and capacitors in electrical products are inversely proportional to the square root of the power supply frequency. Therefore, when we increase the frequency from the power frequency of 50Hz to 20kHz, a 400-fold increase, the size and weight of the electrical equipment will roughly decrease to 5-10% of the power frequency design. Both inverter-type rectifier welding machines and switching rectifiers used in communication power supplies are based on this principle. Similarly, various DC power supplies used in the traditional "rectifier industry," such as those for electroplating, electrolysis, electrical processing, charging, float charging, and power switching, can be modified according to this principle to become "switching power supplies," saving 90% or more of the main materials and 30% or more of electricity. The gradual increase in the upper limit of the operating frequency of power electronic devices has prompted the solid-state transformation of many traditional high-frequency devices that originally used vacuum tubes, bringing significant economic benefits in terms of energy saving, water saving, and material saving, and further demonstrating the value of technological content. 3.2 Modularization Modularization has two aspects: firstly, the modularization of power devices; and secondly, the modularization of power supply units. Commonly seen power modules, containing one, two, six, or even seven units, including switching devices and anti-parallel freewheeling diodes, are essentially "standard" power modules (SPMs). In recent years, some companies have integrated the drive and protection circuits of the switching devices into power modules, creating "intelligent" power modules (IPMs). This not only reduces the overall size of the device but also simplifies its design and manufacturing. However, due to continuously increasing frequencies, the effects of parasitic inductance and capacitance in the leads are becoming increasingly severe, causing greater electrical stress on the devices (manifesting as overvoltage, overcurrent, and glitches). To improve system reliability, some manufacturers have developed "user-specific" power modules (ASPMs). These modules integrate almost all the hardware of a complete system into a single module as chips, eliminating traditional lead connections between components. Such modules undergo rigorous and rational thermal, electrical, and mechanical design to achieve optimal performance. They are similar to application-specific integrated circuits (ASICs) in microelectronics. By writing the control software into the microprocessor chip within the module and fixing the entire module to a suitable heatsink, a new type of switching power supply device is created. Therefore, the purpose of modularization is not only to facilitate use and reduce the overall size of the device, but more importantly, to eliminate traditional wiring, minimize parasitic parameters, and thus minimize the electrical stress on the devices, thereby improving system reliability. Furthermore, high-power switching power supplies, due to limitations in device capacity and considerations for increasing redundancy to improve reliability, generally employ multiple independent module units operating in parallel, using current sharing technology. All modules share the load current, and if one module fails, the remaining modules share the load current evenly. This not only increases power capacity, meeting the requirements of high current output with limited device capacity, but also greatly improves system reliability by adding redundant power supply modules with relatively small power consumption compared to the entire system. Even if a single module fails, it will not affect the normal operation of the system, and sufficient time is provided for repair. 3.3 Digitalization In traditional power electronics technology, the control section is designed and operates based on analog signals. In the 1960s and 70s, power electronics technology was entirely based on analog circuits. However, digital signals and digital circuits are becoming increasingly important, and digital signal processing technology is becoming more and more mature, showing more and more advantages: facilitating computer processing and control, avoiding distortion of analog signals, reducing interference from stray signals (improving anti-interference capability), facilitating software package debugging and remote sensing, telemetry, and adjustment, and also facilitating the implementation of self-diagnosis, fault tolerance, and other technologies. Therefore, in the 1980s and 1990s, analog technology was still useful for the design of various circuits and systems, especially in solving problems such as printed circuit board layout, electromagnetic compatibility (EMC) issues, and power factor correction (PFC), which were inseparable from knowledge of analog technology. However, for intelligent switching power supplies that require computer control, digital technology is indispensable. 3.4 Green Power Systems The greening of power systems has two meanings: First, significant energy saving, which means saving power generation capacity. Since power generation is a major cause of environmental pollution, energy saving can reduce environmental pollution; second, these power sources should not (or should cause less) pollution to the power grid. The International Electrotechnical Commission (IEC) has formulated a series of standards for this, such as IEC555, IEC917, and IEC1000. In fact, many power electronic energy-saving devices often become sources of pollution to the power grid: injecting severe high-order harmonic currents into the grid, lowering the total power factor, causing voltage spikes and even voltage defects and distortion. At the end of the 20th century, various active filters and active compensators emerged, providing multiple methods for correcting the power factor. These laid the foundation for the mass production of various green switching power supply products in the 21st century. Modern power electronics technology is the foundation of switching power supply technology development. With the continuous emergence of new power electronic devices and circuit topologies suitable for higher switching frequencies, modern power supply technology will develop rapidly driven by practical needs. Under traditional application technologies, the performance of switching power supplies is affected by the limitations of power device performance. To maximize the characteristics of various power devices and minimize the impact of device performance on switching power supply performance, new power circuit topologies and new control technologies can enable power switches to operate in a zero-voltage or zero-current state, thereby greatly increasing the operating frequency, improving the efficiency of the switching power supply, and designing high-performance switching power supplies. In summary, power electronics and switching power supply technologies are constantly evolving due to application demands. The emergence of new technologies will lead to the upgrading of many application products and open up new application areas. The realization of high-frequency, modular, digital, and green switching power supplies will signify the maturity of these technologies, achieving a combination of high-efficiency and high-quality power consumption. In recent years, with the development of the communications industry, the domestic market demand for communication switching power supplies, with switching power supply technology at its core, is over 2 billion RMB, attracting a large number of domestic and foreign scientific and technological personnel to conduct research and development. The replacement of linear and phase-controlled power supplies by switching power supplies is an inevitable trend. Therefore, the domestic market for power operating power supply systems, which also has a demand of several billion RMB, is starting and will develop rapidly. Many other specialized power supplies and industrial power supplies with switching power supply technology at their core are also waiting to be developed.