Ten focus points in the development of switching power supply technology

In the 1960s, the advent of switching power supplies gradually replaced linear voltage regulators and SCR phase-controlled power supplies. Over the past 40 years, switching power supply technology has developed and changed rapidly, going through three stages of development: power semiconductor devices, high frequency and soft switching technology, and integrated technology of switching power supply systems.

Power semiconductor devices have evolved from bipolar devices (BPT, SCR, GTO) to MOS devices (power MOSFET , IGBT , IGCT, etc.), making it possible for power electronics systems to achieve high frequencies, significantly reduce conduction losses, and make circuits simpler.

Since the 1980s, the development and research of high-frequency and soft-switching technologies have made power converters better in performance, lighter in weight and smaller in size. High-frequency and soft-switching technologies have been one of the hot topics in the international power electronics research community in the past 20 years.

In the mid-1990s, integrated power electronic systems and integrated power electronic modules (IPEM) technology began to develop. It is one of the new problems that need to be solved urgently in the international power electronics community today.

Focus 1: Power semiconductor device performance

In 1998, Infineon launched the cold MOS tube, which uses a "super-junction" structure, so it is also called a super-junction power MOSFET. The working voltage is 600V to 800V, and the on-state resistance is almost reduced by an order of magnitude, while still maintaining the characteristics of fast switching speed. 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 reached 3300V/1200A and 4500V/1800A respectively. The withstand voltage of high-voltage IGBT has reached 6500V. The upper limit of the operating frequency of general IGBT is 20kHz~40kHz. IGBT manufactured based on the new technology of punch-through (PT) structure can work at 150kHz (hard switching) and 300kHz (soft switching).

The technical progress of IGBT is actually a compromise between on-state voltage drop, fast switching and high withstand voltage. With the difference of process and structure, IGBT has the following types in the 20-year history: punch-through (PT), non-punch-through (NPT), soft punch-through (SPT), trench and field stop (FS).

Silicon carbide SiC is an ideal material for power semiconductor device wafers. Its advantages are: wide bandgap, high operating temperature (up to 600°C), good thermal stability, small on-resistance, good thermal conductivity, extremely small leakage current, high PN junction withstand voltage, etc., which is conducive to the manufacture of high-frequency, high-power semiconductor devices that are resistant to high temperatures.

It can be foreseen that silicon carbide will be the new power semiconductor device material most likely to be successfully applied in the 21st century.

Focus 2: Switching power supply power density

Improving the power density of switching power supplies and making them smaller and lighter is a goal that people are constantly striving to achieve. High frequency of power supplies is one of the hot topics in the international power electronics research community. Miniaturization and weight reduction of power supplies are particularly important for portable electronic devices (such as mobile phones, digital cameras, etc.). Specific methods for miniaturizing switching power supplies are:

The first is high frequency. In order to achieve high power density of the power supply, the operating frequency of the PWM converter must be increased, thereby reducing the volume and weight of the energy storage components in the circuit.

The second is to use piezoelectric transformers. The use of piezoelectric transformers can make high-frequency power converters light, small, thin and high-power density.

Piezoelectric transformers use the unique "voltage-vibration" and "vibration-voltage" conversion 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, it is necessary to improve the performance of capacitors, increase energy density, and research and develop new capacitors suitable for power electronics and power supply systems, which require large capacitance, small equivalent series resistance ESR, and small size.

Focus 3: High-frequency magnetic and synchronous rectification technology

A large number of magnetic components are used in power supply systems. The materials, structures and performances of high-frequency magnetic components are different from those of industrial frequency magnetic components, and 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 supply, soft switching technology must be developed and applied. It has been a research hotspot in the international power supply industry in the past few decades.

For soft-switching converters with low voltage and high current output, the measure to further improve their efficiency is to try to reduce the conduction loss of the switch. For example, synchronous rectification SR technology, which uses the reverse connection of the power MOS tube as the switching diode for rectification , instead of the Schottky diode (SBD), can reduce the tube voltage drop, thereby improving the circuit efficiency.

Focus 4: Distributed power structure

The distributed power system is suitable for use as a power supply for large workstations (such as image processing stations) composed of ultra-high-speed integrated circuits , large digital electronic switching systems, etc. Its advantages are: modularization of DC/DC converter components can be achieved; N+1 power redundancy can be easily achieved to improve system reliability; load capacity can be easily expanded; current and voltage drops on the 48V bus can be reduced; heat distribution can be easily uniform and heat dissipation design can be facilitated; transient response is good; failed modules can be replaced online, etc. There are currently two types of distributed power systems, one is a two-level structure and the other is a three-level structure.

Focus 5: PFC converter

Since there are rectifier elements and filter capacitors at the input end of the AC/DC conversion circuit, when a sinusoidal voltage is input, the power factor of the electronic equipment powered by a single-phase rectifier power supply is only 0.6 to 0.65 on the grid side (AC input end). By using a PFC (power factor correction) converter, the grid-side power factor can be increased to 0.95 to 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 types of topology types and control strategies for three-phase APFC, there is still room for further research and development.

Generally, a high power factor AC/DC switching power supply is composed of a two-stage topology. For a low-power AC/DC switching power supply, the overall efficiency is low and the cost is high when a two-stage topology is used.

If the input power factor requirement is not particularly high, the PFC converter and the subsequent DC/DC converter are combined into a topology to form a single-stage high power factor AC/DC switching power supply. Only one main switch tube is used to correct the power factor to above 0.8 and make the output DC voltage adjustable. This topology is called a single-tube single-stage or S4PFC converter.

Focus 6: Voltage Regulator Module VRM

The voltage regulator module is a type of low-voltage, high-current output DC-DC converter module that provides power to the microprocessor. The speed and efficiency of data processing systems are now increasing. In order to reduce the electric field strength and power consumption of the microprocessor IC, the logic voltage must be reduced. The logic voltage of the new generation of microprocessors has been reduced to 1V, while the current is as high as 50A~100A. Therefore, the requirements for VRM are: very low output voltage, large output current, high current change rate, fast response, etc.

Focus 7: Fully digital control

The control of power supply has evolved from analog control to analog-digital hybrid control to full digital control. Full digital control is a new development trend and has been applied in many power conversion devices.

However, digital control was rarely 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 dropped to a relatively 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: digital signals can calibrate smaller quantities than mixed analog-digital signals, and the chip price is also lower; current detection errors can be accurately digitally corrected, and voltage detection is also more accurate; fast and flexible control design can be achieved.

Focus 8: Electromagnetic compatibility

The electromagnetic compatibility (EMC) problem of high-frequency switching power supply has its own particularity. The di/dt and dv/dt generated by the power semiconductor switch tube during the switching process cause strong conducted electromagnetic interference and harmonic interference. In some cases, it will also cause strong electromagnetic field (usually near field) radiation. Not only will it seriously pollute the surrounding electromagnetic environment, cause electromagnetic interference to nearby electrical equipment, but it may also endanger the safety of nearby operators. At the same time, the control circuit inside the power electronic circuit (such as the 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 particularity, coupled with the specific difficulties in EMI measurement, in the field of electromagnetic compatibility of power electronics, there are many cross-scientific frontier topics waiting to be studied. 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 the voltage and current changes generated by the switching action of the main switching device. The faster the change speed, the greater the electromagnetic noise.

Focus 9: Design and testing technology

Modeling, simulation and CAD are new design tools. 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. We must also consider the thermal model, reliability model and EMC model of the switch tube. The various models are very different, 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 for power supply systems includes main circuit and control circuit design, device selection, parameter optimization, magnetic design, thermal design, EMI design, printed circuit board design, feasibility estimation, computer-aided synthesis and optimization design, etc. Using simulation-based expert systems for CAD of power supply systems can optimize the performance of the designed system, reduce design and manufacturing costs, and perform manufacturability analysis. This 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 feasibility testing of power supply systems 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 reliability, etc., while users require the power supply products produced by the manufacturer to be more practical, more reliable, lighter and lower cost. These situations put power supply manufacturers under great 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, large-scale production and cost reduction of power supply products.

In fact, in the development process of power integration technology, it has gone through the development stages of modularization of power semiconductor devices, integration of power and control circuits, and integrated passive components (including magnetic integration technology). The development direction in recent years is to integrate low-power power supply systems on a chip, which can make power products more compact and smaller, and also reduce the lead length, thereby reducing parasitic parameters. On this basis, integration can be achieved, and all components together with control protection are integrated into one module.

上世纪90年代，随着大规模分布电源系统的发展，一体化的设计观念被推广到更大容量、更高电压的电源系统集成，提高了集成度，出现了集成电力电子模块(IPEM)。IPEM将功率器件与电路、控制以及检测、执行等元件集成 封装 ，得到标准的，可制造的模块，既可用于标准设计，也可用于专用、特殊设计。优点是可快速高效为用户提供产品，显著降低成本，提高可*性。

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