Ten focuses for development of switch power technology
In the early '60s, switching power supply was appeared and gradually replaced linear stabilized voltage supply and SCR phase control power supply. And after these 40 years, the switching power supply has rapidly developed and changed, and experienced three development stages which are power semiconductor apparatus, high-frequency and soft switch technology, and integration technology of switch power system.
The power semiconductor apparatus is developed from a dual-polar apparatus (BPT, SCR, GTO) to MOS type apparatus (power MOSFET, IGBT, IGCT, and so on), thus the power electronics system may achieve high frequency, and the conduction loss is greatly reduced, and circuit becomes more simple. Since the ‘80s, the power converter is better in performance, lighter in weight, and smaller in size due to the development and study of high frequency and soft switch technology. The high frequency and soft switch technology is one of the study hot spot of international power electronics cycle in the last 20 years. In the middle term of the ‘90s, the integrated power electronics system and integrated power electronics module (IPEM) technology are developed, and it is the one of new problems urgently to be solved in the current international power electronics cycle.
Focus 1: power semiconductor apparatus performance: in 1998, Infineon Company introduced cold MOS tube also called a super-junction power MOSFET for it applies “super-junction” structure. The working voltage is 600V-800V, and the on-state resistance is nearly reduced by one order of magnitude, but still keeps the feature of quick switch speed; therefore, it is a promising high-frequency power semiconductor apparatus. When the IGBT is just appeared, voltage and current rated values are only 600V and 25A. Within a very long period, the withstand voltage is limited in 1200V-1700V; through the long-time exploration study and improvement, the voltage and power rated values of the IGBT at present have been respectively reached 3300V/1200A and 4500V/1800A, the withstand voltage of high-voltage IGBT single piece has been reached 6500V, and the upper limit of the working frequency of general IGBT is 20kHz-40kHz; the IGBT based on the new technology of punchthrough (PT) type structural application can be worked for 150kHz (hard switch) and 300 kHz (soft switch). The technological development of IGBT is the compromise of on-state voltage drop, fast switch and high voltage withstand ability. Along with the difference of technical and structural forms, the IGBT has several types as below during the history development progress for 20 years: punchthrough (PT) type, non-punchthrough (NPT) type, soft punchthrough (SPT) tpe, channel type and field stop (FS) type. The silicon carbide SiC is an ideal material of power semiconductor apparatus wafer, and has the advantages of wide forbidden band, high working temperature (up to 600℃), good thermal stability, small on-state resistance, good heat conductivity, small leaking current, and high PN junction voltage withstand, and others; it is good for manufacturing the heat-resisting high frequency big power semiconductor apparatus; it can be foreseen that the silicon carbide will be the novel power semiconductor apparatus material which is most possible successfully applied in the 21st century.
Focus 2: Switch power density: It improves the power density of the switching power supply, so that it is small-sized and light, and is the goal that people continuously strives for. The high frequency of power supply is one of the hot spot for the study of international power electronics cycle. The small-sized and light power supply is very important to the portable electronic device (such as mobile phone, digital camera etc.). The specific methods for minimizing the switching power supply include:
I.High frequency. In order to realize the high power density of power supply, the working frequency of the PWM converter must be improved, thus the volume weight of energy storage element in the circuit can be reduced.
II.Application of piezoelectric transformer. Application of the piezoelectric transformer can make that the high-frequency power converter achieves light, small, thin and high-power density. The piezoelectric transformer conveys energy by using the specific “voltage-vibration” conversion and “vibration-voltage” conversion” of the piezoelectric ceramic material, and its equivalent circuit is similar to a serial-parallel resonate circuit, and it is also the one of research hotspot in the power conversion field.
III.Application of novel capacitor. In order to reduce the volume and weight of the power electronic equipment, the performance of the capacitor must be improved and energy density need to be improved; besides, the novel capacitor suitable for power electronics and power system shall be studied and developed; it requires that the electricity capacity is big, the equivalent series resistance ESR is small, volume is small, and so on.
Focus 3: high frequency magnet and synchronous rectification technology: the power system applies a lot of magnetic components, the materials, structures and performances of high frequency magnetic components are different from power frequency magnetic element; so there are many problems to be studied. There are requirements on magnetic materials for the high frequency magnetic elements as below: small loss, good radiating performance and excellent magnetic performance. The magnetic material applicable to megahertz frequency is concerned by people, and the nano-crystal soft magnetic material has also been developed and applied. After the high frequency, soft switch technology must be developed and applied in order to improve the efficiency of the switching power supply. It is one research hot spot of the international power cycle in the past dozens of years. As for the soft switch converter with low voltage and big current outputs, the measure for further improving its efficiency is to reduce the on-state loss of the switch. E.g. synchronous rectification SR technology, namely, the power MOS tube transposition is acted as the switch diode for rectification to replace Schottky diode (SBD), thus the tube voltage drop can be reduced, and the circuit efficiency is improved.
Focus 4: Distributed power structure: the distributed power structure is applicable to be used as the power supply of large work station (such as image processing station) consisting of super-high speed integrated circuits and large digital converting system, and others; the distributed power structure has the advantages of realizing the modularization of DC/DC converter component; easily realizing the N+1 power redundancy, improving system availability; easily expanding load capacity; reducing current and voltage drop on 48V busbar; easily reaching uniform heat distribution and facilitating radiating design; good transient response; online replacement of failure module, and so on. The distributed power system today has two structural types: the one is two-grade structure while the other one is three-grade structure.
Focus 5: PFC converter: For the input end of the AC/DC converting circuit is provided with a rectifying element and a filtering capacitor; when the sinusoidal voltage is input, the electronic equipment is supplied by single-phase rectifying power, and the power factor of the grid side (alternating current input end) is only 0.6-0.65. By applying the PFC(power factor correction) converter, the network side power factor can be improved to 0.95-0.99, and the input current THD is less than 10%. Thus the harmonic pollution of the power grid is governed and the integral efficiency of the power supply is also improved. This technology is called as active power factor correction APFC, the single-phase APFC is developed early at home and abroad, and the technology has been matured; although there are many types of topological type and control strategy of the three-phase AFPC, they still need study and development. Generally, high power factor AC/DC switching power supply consists of two-grade topology; as for small power AC/DC switching power supply, the two-grade topology structure is low in overall efficiency and high in cost. If the requirement on the power factor of the input end is not very high, the PFC converter and the back-grade DC/DC converter are combined to be one topology, thus the single-grade high power factor AC/DC switching power supply is formed; the power factor can be corrected to 0.8 above only by one main switch tube, and the output DC voltage is adjustable; this topology structure is called as single-tube and single-grade, or S4PFC converter namely.
Focus 6: voltage regulator module VRM: the voltage regulator module is a kind of DC-DC converter module with low voltage and big current output, which provides power for a microprocessor. The speed and efficiency of the current data processing system are increasingly improved; the logic voltage must be reduced for reducing the field intensity and power loss of the microprocessor IC; the logic voltage of the new generation of microprocessor has been reduced to 1V, and the current is up to 50A-100A, so the requirement on VRM is very low output voltage, big output current, high current change rate, and rapid response, and so on.
Focus 7: full digital control: the power control enters a full digital control stage from the simulative control and modulus mixed control. The full digital control is a new developing trend, and has been applied to many power converters. However, the digital control in the past is less applied to the DC/DC converter. In recent two years, the high-performance full digital control chip of the power supply has been developed, and the cost has also reduced to a reasonable level; many companies in Europe and America have been developed and manufactured digital control chips and software of the switch converter. The advantage of the full digital control is: that a smaller quantity can be calibrated by comparing the digital signal with mixed modulus signal, and the chip price is lower; the current detection bias can be accurately and digitally corrected, the voltage detection can be more accurate; the rapid and flexible control design can be realized.
Focus 8: Electromagnetic compatibility: the electromagnetic compatibility EMC problem of the high frequency switching power supply is special. Di/dt and dv/dt generated by the power semiconductor switch tube during the switching process may cause powerful conduction electromagnetic disturbance and resonate disturbance. Some conditions may also cause the strong electromagnetic field (near field generally) radiation. Thus it may seriously pollute surrounding electromagnetic environment, cause electromagnetic disturbance to electrical equipment nearby and also threaten safety of operators nearby. Meanwhile, the control circuit at inner part of the power electronic circuit (such as switch converter) must bear the disturbance of EMI and application field electromagnetic noise generated by the switch action. Above particularity and specific difficulty on EMI measurement in the electromagnetic compatibility field of the power electronic circuit have many leading tasks of electricity science to be studied. Many universities at home and abroad have carried out the study of electromagnetic disturbance and electromagnetic compatibility of power electronic circuit, and got many good achievements. The research results in recent years indicate that the electromagnetic noise source in the switch converter is mainly originated from the voltage and current changes of the switch function of the switch appliance. The change speed is faster and the electromagnetic noise is bigger.
Focus 9: design and testing technology: modeling, simulation and CAD are new design tools. For simulating a power system, a simulation model shall be set up firstly, and it includes a power electronic apparatus, a converter circuit, a digital and simulation control circuit, a magnetic element and magnetic field distribution model, and others; the thermal model, available model and EMC model of the switch tube shall be considered. There is big difference of various models; the development direction of the molding is: digital-simulation mixed molding, mixed layer molding and uniform multi-layered model consisting of various models. The CAD of the power system mainly includes main circuit and control circuit design, apparatus selection, parameter optimization, magnetic design, thermal design, EMI design and printed circuit board design, availability evaluation, computer assistant comprehensive and optimized design, and others. The CAD of the power system based on the simulated expert system can make the system performance optimize, the design and manufacturing costs are reduced, and the manufacturing analysis is made; it is the one of the development direction of the 21st century simulation and CAD technology. Besides, development, study, and application of the heat test, EMI test, and availability test, and other technologies of the power system should also be rapidly developed.
Focus 10: system integration technology: the power equipment is featured by multiple nonstandard parts, big labor intensity, long design cycle, high cost, and low availability, and others; the power product produced by the manufacturing factory as required by users are more practicable, higher in operability, lighter and smaller, and lower in cost. Due to these conditions, power manufacturer bears huge pressure, and urgently needs to carry out the study and development of the integrated power module, thus the power product can be produced in a standard, modular, manufacturing, and scale manner, the cost is reduced. Actually, the power integrated technology has been experienced the modularization of electric semiconductor apparatus, integration of power and control circuit, integrated inactive element (including magnetic integration technology) and other development stages. The development direction in recent years is to integrate the small power system to one chip, thus the power product is more compact, the volume is smaller; the lead length is also reduced, thus the parasitic parameter is reduced. On this basis, the integration can be realized, and all element appliances and the control protection are integrated in one module. In the last 90s, the integration design concept has been popularized to the power system integration with larger capacity and higher voltage along with the development of the large-scale distributed power system, thus the integration degree is improved, and the integrated power electronic module (IPEM) is appeared. The IPEM integrates and packs power apparatus and circuit, control and detection, execution, and other elements, thus the standard and manufacturing module can be obtained. It can be applied to standard design as well specific and special design. It has the advantage of rapidly and efficiently providing product for users, significantly reducing cost and improving operability. In a word, the power system integration is the one of new problems urgently to be solved by the international power electronic cycle today.