Ten Points for Attention in the Development of Switching Power Supply Technology           

In the 1960s, switching power supply gradually replaced linear regulated power supply and SCR phase-controlled power supply. Over the past 40 years, switching power supply technology has developed and changed rapidly. It has gone through three stages: power semiconductor devices, high frequency and soft switching technology, and integrated technology of switching power supply system.    
        
The development of power semiconductor devices from bipolar devices (BPT, SCR, GTO) to MOS devices (power MOSFET, IGBT, IGCT, etc.) makes it possible for power electronic systems to achieve high frequencies, greatly reduce the conduction loss, and simpler circuits.         
   
Since the 1980s, the development and research of high frequency and soft switching technology have made the power converter have better performance, lighter weight and smaller size. High frequency and soft switching technology have been one of the hotspots in the international power electronics field in the past 20 years.

In the mid-1990s, the technology of integrated power electronics system and integrated power electronics module (IPEM) began to develop, which is one of the new problems to be solved urgently in the international power electronics field.

Focus 1: Performance of Power Semiconductor Devices           
In 1998, Infineon introduced cold MOSFET, which uses Super-Junction structure, so it is also called superjunction power MOSFET. The working voltage is 600 V to 800V, the on-state resistance is reduced by almost one order of magnitude, and the switching speed is still fast. It is a promising high-frequency power semiconductor device.     
       
When IGBT first appeared, the voltage and current ratings were only 600V and 25A. For a long time, the withstand voltage level is limited to 1200V-1700V. After a long period of research and improvement, the voltage and current ratings of IGBT have reached 330V/1200A and 4500V/1800A, respectively. The withstand voltage of high-voltage IGBT monolithic has reached 6500V. The upper limit of working frequency of general IGBT is 20 kHz-40 kHz. Based on PT structure, it should be used. IGBT manufactured with new technology can work at 150 kHz (hard switch) and 300 kHz (soft switch).            
The technical progress of IGBT is actually a compromise between on-state voltage drop, fast switching and high voltage withstanding capability. With the different process and structure, IGBT has the following types in the 20-year history: PT, NPT, SPT, trench and FS.            
Silicon carbide SiC is an ideal material for wafers of power semiconductor devices. Its advantages are: wide band gap, high working temperature (up to 600 C), good thermal stability, low on-state resistance, good thermal conductivity, low leakage current, high voltage resistance of PN junction, which is conducive to the manufacture of high-temperature, high-frequency and high-power semiconductor devices.           
 It can be predicted that silicon carbide will be the most promising new power semiconductor device material for successful application in the 21st century.

Focus 2: Power density of switching power supply            
Increasing the power density of switching power supply, making it miniaturized and lightweight, is the goal that people constantly strive to pursue. The high frequency of power supply is one of the hotspots in the international power electronics field. Miniaturization of power supply and weight reduction are particularly important for portable electronic devices (such as mobile phones, digital cameras, etc.). Specific ways to miniaturize switching power supply are as follows:           
 One is high frequency. In order to achieve high power density of power supply, the working frequency of PWM converter must be increased to reduce the volume weight of energy storage elements in the circuit.            
The second is the application of piezoelectric transformer. The application of piezoelectric transformer can make the high frequency power converter light, small, thin and high power density.           
 Piezoelectric transformer transmits energy by using the characteristic of "voltage-vibration" and "vibration-voltage" transformations of piezoelectric ceramics. Its equivalent circuit is like a series-parallel resonant circuit, which is one of the research hotspots in the field of power conversion.            
Third, new capacitors are used. In order to reduce the volume and weight of power electronic equipment, it is necessary to improve the performance of capacitors and energy density, and to develop new capacitors suitable for power electronic and power supply systems, which require large capacitance, small equivalent series resistance ESR and small size.

Focus 3: High Frequency Magnetism and Synchronous Rectification Technology            
A large number of magnetic elements are used in power supply system. The material, structure and performance of high frequency magnetic elements are different from those of power frequency magnetic elements. There are many problems to be studied. The magnetic materials used for high frequency magnetic components have the following requirements: low loss, good heat dissipation and excellent magnetic properties. Magnetic materials suitable for megahertz frequencies have attracted much attention. Nanocrystalline soft magnetic materials have also been developed and applied.            
After high frequency, in order to improve the efficiency of switching power supply, soft switching technology must be developed and applied. It is a research hotspot in the international power industry in the past decades.            
For soft-switching converters with low voltage and high current output, the way to further improve their efficiency is to try to reduce the on-state loss of the switches. For example, synchronous rectifier SR technology, which uses power MOS transistor reverse connection as a switching diode for rectification instead of Schottky diode (SBD), can reduce the voltage drop of the transistor and improve the circuit efficiency.

Focus 4: Distributed Power Supply Architecture            
Distributed power supply system is suitable for large workstations (such as image processing stations) and large digital electronic switching systems consisting of ultra-high speed integrated circuits. Its advantages are: modularization of DC/DC converter components; easy realization of N+1 power redundancy, improving system* performance; easy expansion of load capacity; and reduction of 48V bus load. Current and voltage drop; easy to achieve uniform thermal distribution, easy to heat dissipation design; good transient response; online replacement of failure modules. At present, there are two types of distributed power system structure, one is two-level structure, the other is three-level structure.

Focus 5: PFC Converter            
Because the input of AC/DC converter has rectifier elements and filter capacitors, when sinusoidal voltage is input, the power factor of single-phase rectifier power supply is only 0.6-0.65. With PFC (Power Factor Correction) converter, the network side power factor can be increased to 0.95-0.99, and the input current THD is less than 10%. It not only controls the harmonic pollution of power grid, but also improves the overall efficiency of power supply. Active Power Factor Correction (APFC) single-phase APFC has been developed earlier at home and abroad, and the technology is mature. Although there are many kinds of topology types and control strategies for three-phase APFC, it still needs further research and development.            
Generally, high power factor AC/DC switching power supply consists of two-stage topology. For low power AC/DC switching power supply, two-stage topology is generally inefficient and costly.            
If the input power factor is not particularly high, the PFC converter and the post-stage DC/DC converter are combined to form a single-stage high power factor AC/DC switching power supply. With only one main switch, the power factor can be corrected to more than 0.8 and the output DC voltage can be adjusted. This topology structure is called single-transistor single-stage AC/DC switching power supply. That is, the S4PFC converter.

Focus 6: Voltage Regulator Module VRM            
Voltage regulator module is a kind of low voltage and high current output DC-DC converter module, which provides power to microprocessor. Nowadays, the speed and efficiency of data processing system are increasing day by day. In order to reduce the electric field strength and power consumption of microprocessor IC, it is necessary to reduce the logic voltage. The logic voltage of the new generation microprocessor has been reduced to 1V, while the current is up to 50A-100A. Therefore, the requirements for VRM are: low output voltage, high output current, and current change rate. High, fast response, etc.

Focus 7: Full Digital Control            
The control of power supply has been controlled by analog and analog-digital mixtures, and has entered the stage of full digital control. Full digital control is a new development trend and has been applied in many power conversion devices.           
 However, in the past, digital control was rarely used in DC/DC converters. In the past two years, high-performance digital control chips for power supply have been developed, and the cost has been reduced to a reasonable level. Many companies in Europe and America have developed and manufactured digital control chips and software for switching converters.            
The advantages of full digital control are: compared with mixed analog signals, digital signals can calibrate smaller quantities and lower chip prices; accurate digital correction of current detection errors and more accurate voltage detection can be achieved; fast and flexible control design can be realized.

Focus 8: Electromagnetic Compatibility            
The EMC problem of high frequency switching power supply has its particularity. Di / dt and DV / dt generated by power semiconductor switch during switching process cause strong conducted electromagnetic interference and harmonic interference. In some cases, strong electromagnetic field (usually near field) radiation is also caused. It not only seriously pollutes the surrounding electromagnetic environment and causes electromagnetic interference to nearby electrical equipment, but also may endanger the safety of nearby operators. At the same time, the internal control circuit of power electronic circuit (such as switching converter) must also be able to withstand the EMI caused by switching action and the interference of electromagnetic noise applied in the field. The above particularities, together with the specific difficulties in EMI measurement, in the field of EMC of power electronics, there are many frontier topics of intersection science to be studied. Many universities at home and abroad have carried out research on EMI and EMC of power electronic circuits, and achieved many gratifying results. Research results in recent years show that the main source of electromagnetic noise in switching converters is the voltage and current changes caused by the switching action of autonomous switching devices. The faster the speed of change, the greater the electromagnetic noise.

Focus 9: Design and testing techniques            
Modeling, simulation and CAD are new design tools. In order to simulate the power supply system, first of all, a simulation model should be established, including power electronic devices, converter circuits, digital and analog control circuits, magnetic components and magnetic field distribution models, and the thermal model, feasibility model and EMC model of the switch tube should also be considered. Various models differ greatly, and the development direction of modeling is: digital-analog hybrid modeling, hybrid hierarchical modeling, and the formation of various models into a unified multi-level model.  

The CAD of power supply system includes the design of main circuit and control circuit, device selection, parameter optimization, magnetic design, thermal design, EMI design, PCB design, predictability, computer-aided synthesis and optimization design, etc. Using simulation-based expert system for power supply system CAD can optimize the performance of the designed system, reduce the cost of design and manufacture, and do manufacturability analysis. It is one of the development directions of simulation and CAD technology in the 21st century. In addition, the development, research and application of thermal test, EMI test and * testability test of power supply system should also be vigorously developed.

Focus 10: System Integration Technology            
The manufacturing characteristics of power supply equipment are: many non-standard parts, high labor intensity, long design cycle, high cost, low availability, and so on. The users require the power supply products produced by manufacturers to be more practical, more adaptable, lighter and cheaper. These situations make power manufacturers bear tremendous pressure, and it is urgent to develop integrated power module research and development, so as to achieve the goal of standardization, modularization, manufacturability, large-scale production and cost reduction of power products.   
         
In fact, in the development of power integration technology, it has gone through the stages of modularization of power semiconductor devices, integration of power and control circuits, and integration of passive components (including magnetic integration technology). In recent years, the development direction is to integrate the low power supply system on a chip, which can make the power supply more compact, smaller, and reduce the length of the lead, thus reducing the parasitic parameters. On this basis, integration can be achieved. All components are integrated in one module together with control and protection.

In the 1990s, with the development of large-scale distributed power supply system, the concept of integrated design was extended to the integration of power supply system with larger capacity and higher voltage, which improved the integration degree and led to the emergence of integrated power electronic module (IPEM). IPEM integrates power devices with circuit, control, detection and execution components to obtain standard, manufacturable modules, which can be used for both standard design and special design. The advantage is that it can provide products for users quickly and efficiently, significantly reduce costs and improve accessibility. 

 In short, power system integration is one of the new problems that need to be solved urgently in the international power electronics industry.