In the 1960s, the advent of switching power supplies gradually replaced linear regulated power supplies and SCR phase-controlled power supplies. Over the past 40 years, switching power supply technology has developed and changed rapidly, and has experienced three development stages: power semiconductor devices, high frequency and soft switching technology, and integration technology of switching power supply systems.
Power semiconductor devices have developed from bipolar devices (BPT, SCR, GTO) to MOS devices (power MOSFET, IGBT, IGCT, etc.), making it possible for power electronic systems to achieve high frequency and significantly reduce conduction losses, circuits It's also simpler. Since the 1980s, the development and research of high-frequency and soft-switching technologies have enabled power converters to have better performance, lighter weight, and smaller size. High frequency and soft switching technology have been one of the hot topics in the international power electronics community in the past 20 years. In the mid-1990s, integrated power electronic systems and integrated power electronic module (IPEM) technology began to develop. It is one of the new issues that needs to be solved urgently in the international power electronics community today.
In 1998, Infineon launched a cold MOS tube, which adopts a "Super-Junction" structure, so it is also called a super-junction power MOSFET. The operating voltage is 600V ~ 800V, the on-state resistance is almost reduced by an order of magnitude, and the switching speed is still maintained. 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 was limited to 1200V ~ 1700V. After a long period of exploration, research and improvement, the voltage and current ratings of IGBT have now reached 3300V/1200A and 4500V/1800A respectively, and the high-voltage IGBT single-chip withstand voltage has reached 6500V, the upper limit of the operating frequency of general IGBT is 20kHz ~ 40kHz. IGBTs based on the punch-through (PT) structure and applying new technologies can work at 150kHz (hard switching) and 300kHz (soft switching).
The technological progress of IGBT is actually a compromise between on-state voltage drop, fast switching and high withstand voltage capability. With different processes and structural forms, IGBT has the following types during its 20-year historical development: punch-through (PT) type, non-punch-through (NPT) type, soft punch-through (SPT) type, trench type and electric field cutoff (FS) type. Silicon carbide SiC is an ideal material for power semiconductor device wafers. Its advantages are: forbidden bandwidth, high operating temperature (up to 600°C), good thermal stability, small on-state resistance, good thermal conductivity, extremely small leakage current, PN junction High voltage resistance, etc., is conducive to manufacturing high-frequency and high-power semiconductor devices that can withstand high temperatures. It is foreseeable that silicon carbide will be the most likely new power semiconductor device material to be successfully applied in the 21st century.
Improving the power density of switching power supplies and making them smaller and lighter are the goals that people are constantly striving to pursue. The high frequency of power supply is one of the hot topics in the international power electronics community. Miniaturization and weight reduction of power supplies are particularly important for portable electronic devices (such as mobile phones, digital cameras, etc.). Specific methods to miniaturize switching power supplies include: First, high frequency. In order to achieve high power density of the power supply, the operating frequency of the PWM converter must be increased to reduce the volume and weight of the energy storage components in the circuit. The second is the application of piezoelectric transformers. The application of piezoelectric transformers can enable high-frequency power converters to be light, small, thin, and have high power density. Piezoelectric transformers use the unique "voltage-vibration" transformation and "vibration-voltage" transformation properties of piezoelectric ceramic materials to transmit energy. Its equivalent circuit is like a series-parallel resonant circuit, which is one of the research hotspots in the field of power conversion. The third is to use new capacitors. In order to reduce the size and weight of power electronic equipment, we must find ways to improve the performance of capacitors, increase energy density, and research and develop new capacitors suitable for use in power electronics and power systems, which require large capacitance, small equivalent series resistance ESR, and small volume. Wait.
A large number of magnetic components are used in power systems. The materials, structures and properties of high-frequency magnetic components are different from power frequency magnetic components. There are many issues that need to be studied. The magnetic materials used in high-frequency magnetic components have the following requirements: low loss, good heat dissipation performance, and superior magnetic properties. Magnetic materials suitable for megahertz frequencies have attracted people's attention, and nanocrystalline soft magnetic materials have also been developed and applied. After high frequency, in order to improve the efficiency of switching power supplies, soft switching technology must be developed and applied. It has been a research hotspot in the international power industry in the past few decades. For soft-switching converters with low voltage and high current output, a measure to further improve their efficiency is to try to reduce the on-state loss of the switch. For example, synchronous rectification SR technology uses a power MOS tube reversely connected as a switching diode for rectification instead of a Schottky diode (SBD), which can reduce the tube voltage drop and thereby improve circuit efficiency.
The distributed power supply system is suitable for use as the power supply for large-scale workstations (such as image processing stations) and large-scale digital electronic switching systems composed of ultra-high-speed integrated circuits. Its advantages are: it can realize modularization of DC/DC converter components; it is easy to realize N+ 1 Power redundancy improves system reliability; it is easy to expand load capacity; it can reduce the current and voltage drop on the 48V bus; it is easy to achieve uniform heat distribution and facilitate heat dissipation design; it has good transient response; it can replace failed modules online, etc. . There are currently two structural types of distributed power systems, one is a two-level structure and the other is a three-level structure.
Since the input end of the AC/DC conversion circuit has rectifier components and filter capacitors, when sinusoidal voltage is input, the power factor of the electronic equipment powered by a single-phase rectified power supply on the grid side (AC input end) is only 0.6 to 0.65. Using PFC (power factor correction) converter, the grid-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 the power grid, but also improves the overall efficiency of the power supply. This technology is called active power factor correction APFC. Single-phase APFC was developed earlier at home and abroad, and the technology is relatively mature. Although there are many topology types and control strategies for three-phase APFC, they still need to be continuously researched and developed.
Generally, high power factor AC/DC switching power supplies consist of a two-stage topology. For low-power AC/DC switching power supplies, the overall efficiency of the two-stage topology is low and the cost is high. If the power factor requirements at the input end are not particularly high, the PFC converter and the subsequent DC/DC converter can be combined into a topology to form a single-stage high power factor AC/DC switching power supply. Only one main switch tube is used. The power factor is corrected to above 0.8 and the output DC voltage is adjustable. This topology is called a single-tube single-stage S4PFC converter.
The voltage regulator module is a type of low-voltage, high-current output DC-DC converter module that provides power to the microprocessor. Nowadays, the speed and efficiency of data processing systems are increasing day by day. In order to reduce the electric field strength and power consumption of microprocessor ICs, the logic voltage must be reduced. The logic voltage of new generation microprocessors has been reduced to 1V, while the current is as high as 50A~100A. Therefore, the requirements for VRM are: low output voltage, large output current, high current change rate, fast response, etc.
Power supply control has evolved from analog control, analog-digital hybrid control, to a fully digital control stage. Full digital control is a new development trend and has been applied in many power conversion equipment. But digital control has been less used in DC/DC converters in the past. In the past two years, high-performance fully digital control chips for power supplies have been developed, and the cost has been reduced to a more reasonable level. Many companies in Europe and the United States have developed and manufactured digital control chips and software for switching converters. The advantages of full digital control are: compared with mixed analog-digital signals, digital signals can be calibrated to smaller quantities, and the chip price is also cheaper; current detection errors can be accurately digitally corrected, and voltage detection is also more accurate; rapid, Flexible control design.
The electromagnetic compatibility EMC problem of high-frequency switching power supply has its own particularities. The di/dt and dv/dt generated by the power semiconductor switch during the switching process cause strong conducted electromagnetic interference and harmonic interference. Some conditions can also cause radiation from strong electromagnetic fields (usually near field). It not only seriously pollutes the surrounding electromagnetic environment, causes electromagnetic interference to nearby electrical equipment, but may also endanger the safety of nearby operators. At the same time, the control circuit inside the power electronic circuit (such as a switching converter) must also be able to withstand the EMI generated by the switching action and the interference of electromagnetic noise at the application site. The above-mentioned particularities, coupled with the specific difficulties in EMI measurement, there are many cutting-edge topics in communication science waiting to be studied in the field of electromagnetic compatibility of power electronics. Many universities at home and abroad have carried out research on electromagnetic interference and electromagnetic compatibility issues of power electronic circuits, and have achieved many gratifying results. Research results in recent years have shown that the source of electromagnetic noise in switching converters mainly comes from voltage and current changes generated by the switching action of the main switching device. The faster the change, the greater the electromagnetic noise.
Modeling, simulation and CAD are new design tools. In order to simulate the power system, we must first establish a simulation model, including power electronic devices, converter circuits, digital and analog control circuits, magnetic components and magnetic field distribution models, etc., and also consider the thermal model, reliability model and EMC model of the switching tube. . Various models vary greatly, and the development direction of modeling is: digital-analog hybrid modeling, hybrid hierarchical modeling, and combining various models into a unified multi-level model. CAD of power supply system, including main circuit and control circuit design, device selection, parameter optimization, magnetic design, thermal design, EMI design and printed circuit board design, reliability prediction, computer-aided synthesis and optimization design, etc. Using a simulation-based expert system for CAD of power systems can optimize the performance of the designed system, reduce design and manufacturing costs, and enable 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 technologies such as thermal testing, EMI testing, and reliability testing of power supply systems should also be vigorously developed.
The manufacturing characteristics of power supply equipment are: many non-standard parts, high labor intensity, long design cycle, high cost, low reliability, etc. However, users require power supply products produced by manufacturers to be more practical, higher reliability, lighter and smaller , lower cost. These circumstances have put power supply manufacturers under tremendous pressure, and there is an urgent need to carry out research and development of integrated power modules to achieve the goals of standardization, modularization, manufacturability, scale production, and cost reduction of power supply products. In fact, in the development process of power integration technology, it has gone through development stages such as modularization of power semiconductor devices, integration of power and control circuits, and integration of passive components (including magnetic integration technology). The development direction in recent years is to integrate low-power power supply systems on one chip, which can make the power supply products more compact and smaller, and also reduce the lead length, thereby reducing parasitic parameters. On this basis, integration can be achieved, with all components together with control and protection integrated into one module.
In the 1990s, with the development of large-scale distributed power systems, the integrated design concept was promoted to the integration of larger-capacity and higher-voltage power systems, which improved the degree of integration and led to the emergence of Integrated Power Electronic Modules (IPEM). IPEM integrates and packages power devices with circuit, control, detection, execution and other components to obtain standard, manufacturable modules that can be used for standard designs as well as dedicated and special designs. The advantage is that products can be provided to users quickly and efficiently, significantly reducing costs and improving reliability.
In short, power system integration is one of the new issues that need to be solved urgently in the international power electronics community today.
