Abstract: This paper introduces the current status of high-frequency switching power supply technology for communication in engineering applications, summarizes the development of converter topology, modeling and simulation, and magnetic integration, and clarifies the profound impact of digital control technology. Keywords: Communication power supply | Modeling | Magnetic integration 0 Introduction The rapid development of the communication industry has greatly promoted the development of communication power supplies. Switching power supplies occupy a core position in communication systems and have become the mainstream of modern communication power supply systems. In the field of communication, high-frequency rectifiers are usually referred to as primary power supplies, while DC/DC converters are referred to as secondary power supplies. With the development of large-scale integrated circuits, the miniaturization of power supply modules is required, which necessitates continuously increasing the switching frequency and adopting new circuit topologies. This places higher demands on high-frequency switching power supply technology. 1 Development of High-Frequency Switching Power Supply Technology for Communication The development of high-frequency switching power supply technology for communication can be basically reflected in several aspects: converter topology, modeling and simulation, digital control, and magnetic integration. 1.1 Converter Topology Soft switching technology, power factor correction technology, and multi-level technology are hot topics in converter topology in recent years. Soft-switching technology can effectively reduce switching losses and switching stress, contributing to improved converter efficiency. PFC technology can improve the input power factor of AC/DC converters and reduce harmonic pollution to the power grid. Multilevel technology, mainly used in three-phase input converters for communication power supplies, can effectively reduce voltage stress on switching transistors. Furthermore, due to the high input voltage, employing appropriate soft-switching technology to reduce switching losses is an important future research direction for multilevel technology. To reduce converter size, higher switching frequencies are needed to achieve high power density, requiring the use of smaller magnetic materials and passive components. However, increasing the frequency significantly increases the switching and drive losses of MOSFETs, while soft-switching technology can reduce these losses. Currently, the most widely used technologies in communication power supply engineering are active clamp ZVS technology, ZVS phase-shifted full-bridge technology (developed in the early 1990s), and synchronous rectification technology (proposed in the late 1990s). 1.1.1 ZVS Active Clamping Active clamping technology has gone through three generations, all of which have been patented. The first generation was the active clamping ZVS technology from VICOR Corporation in the United States, which increased the operating frequency of DC/DC converters to 1MHz and achieved a power density of nearly 200W/in³, but its conversion efficiency did not exceed 90%. To reduce the cost of the first-generation active clamping technology, IPD filed a patent for the second-generation active clamping technology, which used P-channel MOSFETs and employed active clamping on the secondary side of the transformer for the forward circuit topology, significantly reducing product costs. However, this method resulted in a narrower zero-voltage switching (ZVS) boundary condition for the MOSFETs, and the PMOS operating frequency was not ideal. To prevent magnetic energy from being wasted during core reset, a Chinese-American engineer filed a patent for the third-generation active clamping technology in 2001. Its key feature was that, building upon the second-generation active clamping technology, the energy released during core reset was transferred to the load, thus achieving higher conversion efficiency. It has three circuit schemes: one of the schemes can use N-channel MOSFETs, so the operating frequency can be higher. This technology can combine ZVS soft switching and synchronous rectification technology, so it achieves an efficiency of up to 92% and a power density of more than 250W/in3. 1.1.2 ZVS phase-shifted full bridge Since the mid-1990s, ZVS phase-shifted full bridge soft switching technology has been widely used in medium and high power supply fields. This technology has played a great role in improving the efficiency of the converter when the switching speed of MOSFET is not ideal, but it also has many disadvantages. The first disadvantage is that it adds a resonant inductor, resulting in a certain volume and loss, and the electrical parameters of the resonant inductor need to be kept consistent, which is difficult to control in the manufacturing process; the second disadvantage is that it loses the effective duty cycle [1]. In addition, since synchronous rectification is more convenient to improve the efficiency of the converter, the control effect of phase-shifted full bridge on the synchronous rectification of the secondary side is not ideal. The initial PWM ZVS phase-shifted full-bridge controllers, UC3875/9 and UCC3895, only controlled the primary side, requiring additional logic circuitry to provide accurate secondary synchronous rectification control signals. While the latest phase-shifted full-bridge PWM controllers, such as LTC1922/1 and LTC3722-1/-2, have added secondary-side synchronous rectification control signals, they still cannot effectively achieve ZVS/ZCS synchronous rectification on the secondary side, although this is one of the most effective measures to improve converter efficiency. Another significant improvement of the LTC3722-1/-2 is its ability to reduce the inductance of the resonant inductor, which not only reduces the size and losses of the resonant inductor but also improves duty cycle loss. 1.1.3 Synchronous Rectification Synchronous rectification includes self-driven and externally driven methods. Self-driven synchronous rectification is simple and easy to implement, but the secondary voltage waveform is easily affected by factors such as transformer leakage inductance, resulting in lower reliability in mass production and its limited application in actual products. For the conversion of output voltage from 12V to around 20V, a dedicated external driver IC is often used, which can achieve better electrical performance and higher reliability. TI proposed the predictive drive strategy chip UCC27221/2, which dynamically adjusts the dead time to reduce the conduction loss of the body diode. ST also designed a similar chip STSR2/3, which is not only used for flyback but also for forward, and improves the performance of continuous and discontinuous conduction modes. The Center for Power Electronics Systems (CPES) studied various resonant drive topologies to reduce drive losses[2], and in 1997 proposed a new type of synchronous rectification circuit called quasi-square wave synchronous rectification, which can significantly reduce the conduction loss and reverse recovery loss of the body diode of the synchronous rectifier tube, and easily realize the soft switching of the primary main switch[3]. The synchronous rectification control chips LTC3900 and LTC3901 launched by Linear Technology can be better applied to forward, push-pull and full-bridge topologies. ZVS and ZCS synchronous rectification technologies have also been applied, such as the synchronous rectification driver of active clamp forward circuit (NCPl560), and the synchronous rectification driver chips LTCl681 and LTCl698 of dual transistor forward circuit, but none of them have achieved the excellent effect of symmetrical circuit topology ZVS/ZCS synchronous rectification. 1.2 Modeling and Simulation Switching converters mainly have two modeling methods: small signal analysis and large signal analysis. Small signal analysis method: mainly the state space averaging method [4], which was proposed by Middlebrook RD of California Institute of Technology in 1976. It can be said that this is the first truly significant breakthrough in the modeling and analysis of power electronics. Later, methods such as current injection equivalent circuit method, equivalent controlled source method (proposed by Chinese scholar Zhang Xingzhu in 1986), and three-terminal switching device method appeared. These all belong to the category of circuit averaging method. The disadvantages of the averaging method are obvious. The signal is averaged and cannot be effectively analyzed for ripple. It cannot be accurately analyzed for stability. It may not be suitable for resonant converters. The key point is that the model obtained by the averaging method is independent of the switching frequency. The applicable condition is that the natural frequency generated by the inductor and capacitor in the circuit must be much lower than the switching frequency for the accuracy to be high. Large signal analysis methods: There are analytical methods, phase plane methods, large signal equivalent circuit model methods, switching signal flow methods, nth harmonic three-port model methods, KBM methods and general averaging methods. There is also the equivalent small parameter signal analysis method proposed by Professor Qiu Shuisheng of South my country University of Technology in 1994 [5]. It is applicable not only to PWM converters but also to resonant converters and can perform output ripple analysis. The purpose of modeling is to simulate and then perform stability analysis. In 1978, Keller R first used Middlebrook RD's state-space averaging theory to perform SPICE simulation of switching power supplies [6]. Over the past 30 years, numerous scholars have established various model theories for the average SPICE modeling of switching power supplies, resulting in a variety of SPICE models. These models each have their strengths, with representative examples including: Dr. Sam Ben Yaakov's switching inductor model; Dr. Ray Ridley's model; the average PSpice model for switching power supplies based on Dr. Vatche Vopperian's Orcad 9.1; the average Isspice model for switching power supplies based on Steven Sandier's ICAP4; and the average model for switching power supplies based on Dr. Vincent G. Bello's Cadence model, among others. Based on these models, macro-models are constructed by combining the main parameters of the converter, and DC/DC converters constructed using these models are then used for DC analysis, small-signal analysis, and closed-loop large-signal transient analysis on professional circuit simulation software platforms (Matlab, PSpice, etc.). Due to the rapid evolution and development of converter topologies, the requirements for converter modeling are becoming increasingly stringent. It can be said that converter modeling must keep pace with the development of converter topologies to be more accurately applied in engineering practice. 1.3 Digital Control The simplest applications of digital control are protection and monitoring circuits, as well as communication with the system, and it is currently widely used in communication power supply systems. It can replace many analog circuits, performing power supply startup, input and output over/under voltage protection, output overcurrent and short-circuit protection, and overheat protection. Through specific interface circuits, it can also achieve communication and display with the system. More advanced digital applications not only achieve comprehensive protection and monitoring functions, but also output PWM waves to control power switching devices through drive circuits and realize closed-loop control functions. Currently, companies such as TI, ST, and Motorola have launched dedicated motor and motion control DSP chips. At present, the digitalization of communication power supplies mainly adopts a combination of analog and digital methods. The PWM part still uses dedicated analog chips, while the DSP chip mainly participates in duty cycle control, frequency setting, output voltage adjustment, and protection and monitoring functions. To achieve faster dynamic response, many advanced control methods have been gradually proposed. For example, ON Semiconductor proposed an improved V2 control, Intel proposed Active-droop control, Semtech proposed charge control, Fairchild proposed Valley current control, IR proposed multiphase control, and many universities in the United States have also proposed various other control ideas [7,8,9]. Digital control can improve system flexibility, provide better communication interfaces, fault diagnosis capabilities, and anti-interference capabilities. However, in precision communication power supplies, control accuracy, parameter drift, current detection and current sharing, and control delay are practical problems that urgently need to be solved. 1.4 Magnetic Integration As the switching frequency increases, the size of the switching converter decreases and the power density is greatly improved, but the switching losses will increase accordingly, and more magnetic components will be used, thus occupying more space. Research on magnetic component integration technology is relatively mature abroad, and some manufacturers have applied this technology to actual communication power supplies. In fact, magnetic integration is not a new concept. As early as the late 1970s, Cuk proposed the idea of ​​magnetic integration when he proposed the Cuk converter. Since 1995, the Center for Power Electronics Systems (CPES) in the United States has conducted extensive research on the integration of magnetic devices, using the concept of coupled inductors to conduct in-depth studies on the integration of multiphase BUCK inductors [10,11,12], and applying it to various types of converters. In 2002, Yim-Shu Lee et al. from the University of Hong Kong also proposed a series of discussions and designs on magnetic integration technology [13,14,15]. Conventional magnetic component design methods are extremely cumbersome and require consideration from different perspectives, such as the selection of core size, the determination of materials and windings, and the evaluation of iron and copper losses. However, in addition to these considerations, magnetic integration technology must also consider the problem of magnetic flux imbalance, because the equivalent total magnetic flux is different in each part of the iron core, and some parts may saturate prematurely. Therefore, the analysis and research on magnetic device integration will be more complex and difficult. However, the high power density advantage it brings will undoubtedly be a major development trend in communication power supplies. 1.5 Manufacturing process The manufacturing process of high frequency switching power supply for communication is quite complex and directly affects the electrical function, electromagnetic compatibility and reliability of the power supply system. Reliability is the primary indicator of communication power supply. Complete testing methods, complete process monitoring points and anti-static measures in the production process have largely extended the best design performance of the product. The widespread use of SMD surface mount devices can greatly improve the reliability of soldering. From 2006, Europe and the United States will require lead-free processes for electronic products, which will put forward higher and stricter requirements for the selection of components and the control of the production process in communication power supply. At present, the more attention-grabbing technology is the concept of power electronic integrated module (IPEM) proposed by the Center for Power Electronic Systems (CPEC) in recent years [16], commonly known as "building blocks". Advanced packaging technology is used to reduce parasitic factors to improve voltage ringing and efficiency in the circuit, and the drive circuit and power device are integrated together to improve the driving speed and thus reduce switching losses. Power electronic integration technology can not only improve the regulation of transient voltage, but also improve power density and system efficiency. However, such integrated modules currently face many challenges, primarily in the integration of passive and active devices, and achieving optimal thermal design is difficult. CPEC has conducted years of research on power electronics integration technology, proposing many useful methods, structures, and models. 2. Conclusion The main trend for high-frequency switching power supplies in communication applications will be towards integration and miniaturization, with increasingly higher power densities and more stringent process requirements. Without new breakthroughs in semiconductor devices and magnetic materials, significant technological advancements may be difficult to achieve; the focus of technological innovation will be on improving efficiency and reducing weight. Therefore, process technology will play an increasingly important role in power supply manufacturing. Furthermore, the application of digital control integrated circuits is another direction for future switching power supply development, which will rely on further improvements in DSP operating speed and anti-interference technology.