Top Ten Explanations on the Technical Concerns of Switching Power Supply Technology

Switching power supply has always been a very popular technology in the electronics industry, and its development trend is a problem that everyone must pay attention to, otherwise it will not keep up with the pace of technological development. Electronic component technology has made a focus on the development of switching power supply technology, and has come to the following ten hot spots.

Focus 1: Power semiconductor device performance

In 1998, Infineon introduced the cold mos tube, which uses the Super-JuncTIon structure, so it is also called the super junction power MOSFET. The operating voltage is 600V ~ 800V, the on-state resistance is reduced by an order of magnitude, and the switching speed is still fast. It is a promising high-frequency power semiconductor electronic device.

When the 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 IGBTs have reached 3300V/1200A and 4500V/1800A respectively, and the high voltage IGBT monolithic withstand voltage has reached 6500V, the upper limit of the operating frequency of general IGBT is 20kHz ~ 40kHz, IGBT based on punch-through (PT) type structure application new technology can work at 150kHz (hard switch) and 300kHz (soft switch).

The technical progress of IGBT is actually a compromise between on-state voltage drop, fast switching and high withstand capability. Depending on the process and structure, IGBTs have the following types in the 20-year history: punch-through (PT), non-punch-through (NPT), soft-through (SPT), gully and electric field cut-off (FS) type.

Silicon carbide SiC is an ideal material for power semiconductor device wafers. It has the advantages of forbidden bandwidth, high operating temperature (up to 600 ° C), good thermal stability, low on-state resistance, good thermal conductivity, minimal leakage current, and PN junction. High voltage resistance is conducive to the manufacture of high-frequency, high-power semiconductor electronic components with high temperature resistance.

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

Focus 2: Switching power supply power density

Increasing the power density of the switching power supply to make it compact and lightweight is a goal that people are constantly striving for. The high frequency of power supply is one of the hotspots in the international power electronics industry. The miniaturization and weight reduction of power supplies are especially important for portable electronic devices such as mobile phones, digital cameras, and the like. Specific methods for miniaturizing switching power supplies are:

One is high frequency. In order to achieve high power density of the power supply, it is necessary to increase the operating frequency of the PWM converter, thereby reducing the volumetric weight of the energy storage components in the circuit.

The second is the application of piezoelectric transformers. The application of a piezoelectric transformer enables the high frequency power converter to achieve light, small, thin and high power density. Piezoelectric transformers use the characteristics of "voltage-vibration" transformation and "vibration-voltage" transformation of piezoelectric ceramic materials to transmit energy. The equivalent circuit is like a series-parallel resonance 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 try to improve the performance of capacitors, increase the energy density, and research and develop new capacitors suitable for power electronics and power systems, requiring large capacitance, small equivalent series resistance ESR, and volume. Small.

Focus 3: High frequency magnetic and synchronous rectification technology

A large number of magnetic components are used in the power system. The materials, structure and performance of the high-frequency magnetic components are different from those of the power frequency magnetic components, and many problems need to be studied. The magnetic material used for the high-frequency magnetic component has the following requirements: low loss, good heat dissipation performance, and superior magnetic properties. Magnetic materials suitable for megahertz frequencies are of interest, and nanocrystalline soft magnetic materials have also been developed.

After high frequency, in order to improve the efficiency of the switching power supply, soft switching technology must be developed and applied. It is a research hotspot in the international power 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 reduce the on-state loss of the switch. For example, the synchronous rectification SR technology, in which the power MOS transistor is reversely connected as a rectifying switching diode, instead of a Schottky diode (SBD), can reduce the tube voltage drop, thereby improving circuit efficiency.

Focus 4: Distributed power structure

The distributed power system is suitable for use as a power source for large workstations (such as image processing stations) and large digital electronic switching systems composed of ultra-high-speed integrated circuits. The advantages are: modularization of DC/DC converter components; easy implementation of N+ 1 power redundancy, easy to amplify the load capacity; can reduce the current and voltage drop on the 48V bus; easy to achieve uniform heat distribution, easy to heat design; transient response is good; can replace the failed module online.

There are currently two types of distributed power systems, one is a two-stage structure and the other is a three-level structure.

Focus 5: PFC converter

Since the input end of the AC/DC conversion circuit has a rectifying element and a filter capacitor, when the sinusoidal voltage is input, the power factor of the power supply of the single-phase rectified power supply, the power supply side (AC input end) is only 0.6 to 0.65. With PFC (Power Factor Correction) converter, the power factor on the grid side 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 is 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 is still to be researched and developed.

The general high power factor AC/DC switching power supply consists of a two-stage topology. For a low-power AC/DC switching power supply, the two-stage topology is low in overall efficiency and high in cost.

If the power factor requirement of the input terminal is not particularly high, the PFC converter and the rear-stage DC/DC converter are combined into one topology to form a single-stage high power factor AC/DC switching power supply, and only one main switching tube can be 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 or S4PFC converter.

Focus 6: Voltage Regulator Module VRM

The voltage regulator module is a 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. To reduce the electric field strength and power consumption of microprocessor ICs, the logic voltage must be reduced. The logic voltage of the new generation microprocessor has been reduced to 1V, and the current is as high as 50A to 100A. Therefore, the requirements for VRM are: low output voltage, large output current, high current rate of change, and fast response.

Focus 7: Full digital control

The control of the power supply has been controlled by analog control, analog-digital hybrid control, and entered the stage of full digital control. Full digital control is a new trend that has been applied in many power conversion devices.

But in the past, digital control was used less in DC/DC converters. In the past two years, the high-performance full-digital control chip of the power supply has been developed, and the cost has been reduced to a reasonable level. Several companies in Europe and the United States have developed and manufactured digital control chips and software for switching converters.

The advantage of full digital control is that the digital signal can be calibrated to a smaller amount than the mixed analog-to-digital signal, and the chip price is also lower; the current detection error can be accurately digitally corrected, and the voltage detection is more accurate; Flexible control design.

Focus point 8: Electromagnetic compatibility

The electromagnetic compatibility EMC problem of high frequency switching power supplies has its particularity. The di/dt and dv/dt generated by the power semiconductor switching tube during the switching process cause strong conducted electromagnetic interference and harmonic interference. In some cases, it also causes strong electromagnetic fields (usually near-field) radiation. It not only seriously pollutes the surrounding electromagnetic environment, but also causes electromagnetic interference to nearby electrical equipment, and 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 electromagnetic noise interference in the application field. The above special characteristics, coupled with the specific difficulties in EMI measurement, in the field of electromagnetic compatibility of power electronics, there are many frontier issues in the field of science and technology to be studied. Many universities at home and abroad have carried out research on electromagnetic interference and electromagnetic compatibility of power electronic circuits, and have achieved many gratifying results. The research results in recent years show that the electromagnetic noise source in the switching converter mainly comes from the voltage and current changes generated by the switching action of the main switching device. The faster the change, the greater the electromagnetic noise.

Focus 9: Design and test techniques

Modeling, simulation, and CAD are a new design tool. To simulate a power system, first establish a simulation model, including power electronics, converter circuits, digital and analog control circuits, and magnetic components and magnetic field distribution models. Also consider the thermal model, the reliability model, and the EMC model of the switch. . The various models vary widely. The development direction of modeling is: digital-analog hybrid modeling, mixed hierarchical modeling, and the formation of a unified multi-level model.

Power system CAD, including main circuit and control circuit design, device selection, parameter optimization, magnetic design, thermal design, EMI design and printed circuit board design, predictability, computer-aided synthesis and optimization design. Using the simulation-based expert system for power system CAD, the designed system performance can be optimized, design and manufacturing costs can be reduced, and manufacturability analysis can be done. It is one of the development directions of 21st century simulation and CAD technology. In addition, the development, research and application of thermal testing, EMI testing, and reliability testing of power systems should be vigorously developed.

Focus 10: System Integration Technology

The manufacturing characteristics of the power supply equipment are: non-standard parts, labor intensity, long design cycle, high cost, low availability, etc., and the user requires the power supply products produced by the manufacturer to be more practical, more flexible, lighter and smaller. , the cost is lower. These conditions have put tremendous pressure on power supply manufacturers, and it is urgent to carry out research and development of integrated power modules, so that the goals of standardization, modularization, manufacturability, scale production, and cost reduction of power products can be realized. In fact, in the development of power integration technology, it has experienced the development 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 has been to integrate a low-power power system on a single chip, which makes the power supply product more compact, smaller in size, and reduces the lead length, thereby reducing parasitic parameters. On this basis, integration can be achieved, with all components integrated with control protection in one module.