In the 1960s, the introduction of switching power supplies gradually replaced linear regulated power supplies and SCR phase-controlled power supplies. For more than 40 years, switching power supply technology has developed and changed rapidly. It has experienced three development stages of power semiconductor devices, high-frequency and soft-switching technologies, and integration technologies for switching power supply systems.
Power semiconductor devices have evolved from bipolar devices (BPT, SCR, GTO) to MOS devices (power MOSFETs, IGBTs, IGCTs, etc.), making it possible for power electronics systems to achieve high frequency and significantly reduce conduction losses. It's also simpler.
Since the 1980s, the development of high-frequency and soft-switching technologies has made power converters better in performance, lighter in weight, and smaller in size. High-frequency and soft-switching technology is one of the hotspots in the international power electronics industry in the past 20 years.
In the mid-1990s, integrated power electronics systems and integrated power electronics module (IPEM) technology began to develop, and it is one of the new problems in the international power electronics industry.
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 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. The high withstand voltage is conducive to the manufacture of high-frequency, high-power semiconductor devices 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, improve system availability; easy to amplify load capacity; reduce current and voltage drop on 48V bus; easy to achieve uniform heat distribution, easy heat dissipation design; good transient response; online replacement of failed modules, etc. .
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.
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