1. High performance silicon carbide (SiC) power semiconductor device
It can be predicted that silicon carbide will be a new type of power semiconductor device material which is most likely to be successfully applied in 2 1 century. Its advantages are: band gap, high working temperature (up to 600℃), low on-state resistance, good thermal conductivity, extremely low leakage current, high voltage resistance of PN junction, etc.
2. High frequency magnetic technology
There are many kinds of magnetic components used in high frequency switching converters, and there are many basic problems to be studied.
(1) With the high frequency of switching power supply, some parasitic parameters that can be ignored at low frequency will have an important impact on some circuit performance (such as switching peak energy and noise level) at high frequency. In particular, the eddy current, leakage inductance, winding AC resistance Rac and distributed capacitance of magnetic components are very different at low frequency and high frequency. As a frontier subject, the theory of high-frequency magnetic technology is still widely valued by people, such as: mathematical modeling of iron loss, simulation modeling of hysteresis loop, computer simulation modeling and CAD of high-frequency magnetic components, one-dimensional and two-dimensional simulation models of high-frequency transformers, etc. There are still problems to be studied: the design of high-frequency magnetic components determines the performance, loss distribution and waveform of high-efficiency switching power supply. People hope to give the design criteria and methods, the dependence of magnetic parameters and structural parameters on circuit performance, and clarify the freedom and constraints of design.
(2) The following requirements are required for high-frequency magnetic materials: small loss, good heat dissipation performance and superior magnetic performance. People pay attention to magnetic materials suitable for megahertz frequency, such as 5~6? When 1MHz(Bm=0. 1T), the loss of M ultra-thin cobalt-based amorphous ribbon is only 0.7~ 1W/cm3, which is1/3 ~14 of MnZn high frequency ferrite. Nanocrystalline soft magnetic films are also under study.
(3) Research on high density integration of ferrite or other thin film materials on silicon wafer. Or silicon material is integrated on ferrite, which is a magnetoelectric hybrid integration technology. Magnetoelectric hybrid integration also includes the hybrid integration of magnetic elements and capacitors by using the distributed capacitance between inductor foil windings.
3. New capacitor
Research and development of new capacitors and super-large capacitors suitable for power supply system. It requires large capacitance, small equivalent resistance (ESR) and small volume. According to reports, the United States in the late 1990s, has developed 330? As a new type of solid tantalum capacitor, its equivalent series resistance is obviously reduced.
4. AC DC switching technology with power factor correction
High power factor AC DC power supply generally consists of two stages: a pre-stage power factor corrector is added before DC-DC converter, which requires at least two main switches and two sets of control and drive circuits. Therefore, for low-power switching power supply, the overall efficiency is low and the cost is high.
When the input power factor is not particularly high, a low-power AC -DC switching power supply is composed of PFC and converter combined circuit, and PF can be corrected to above 0.8 with only one main switching tube, which is called single-tube single-stage PF correction AC -DC converter, or S4 for short. For example, the isolated S4PF rectifiers AC/DC converter, the Boost converter working in DCM is used as the pre-power factor corrector, and the flyback converter is used as the main circuit of the post-regulator, which works according to CCM or DCM. Two-stage circuits share a main switch tube.
5. Research on electromagnetic compatibility of high frequency switching power supply.
The electromagnetic compatibility of high frequency switching power supply has its particularity. It usually involves di/dt and dv/dt generated in the switching process, which causes strong conducted electromagnetic interference and harmonic interference. In some cases, it will also cause strong electromagnetic field radiation. It not only seriously pollutes the surrounding electromagnetic environment and causes electromagnetic interference to nearby electrical equipment, but also may endanger the safety of nearby operators. At the same time, the control circuit inside the switching power supply must also be able to withstand the interference of electromagnetic noise from the main circuit and industrial application site. Due to the above particularity and specific measurement difficulties, the research on electromagnetic compatibility of switching power supply is still in its infancy. Obviously, in the field of electromagnetic compatibility, there are many interdisciplinary frontier topics to be studied. Such as: modeling of near-field, conducted interference and radiated interference of typical circuits and systems; Electromagnetic compatibility optimization design software for printed circuit board and switching power supply: the influence of low intermediate frequency, ultra-audio frequency and high frequency strong magnetic field on human health; Research on electromagnetic compatibility measurement method of high power switching power supply.
6. Design and test technology of switching power supply
Modeling, simulation and CAD are new, convenient and economical design tools. In order to simulate switching power supply, simulation modeling should be carried out first. The simulation model should include power electronic devices, converter circuits, digital and analog control circuits, magnetic components and magnetic field distribution models, circuit distribution parameter models, etc. In addition, the thermal model, reliability model and electromagnetic compatibility model of the switch tube are also considered. Various models are very different, so the development direction of modeling should be: mixed digital and analog modeling; Mixed layered modeling; And combine all kinds of models into a unified multi-level model (similar to circuit model, block diagram, etc. ); Automatically generate the model, so that the simulation software has the function of automatic modeling, saving users' time. On this basis, a model base can be established.
The CAD of switching power supply includes 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. Using simulation-based expert system to CAD switching power supply can optimize the performance of the designed system, reduce the design and manufacturing costs, and analyze the manufacturability, which is one of the development directions of simulation and CAD technology in the 2/kloc-0 century. At present, an expert system for designing DC-DC switching converters and MATSPICE software for simulation have been developed abroad.
In addition, we should also vigorously develop the development, research and application of thermal test, EMI test and reliability test of switching power supply.
7. Development of low-voltage and high-current switching power supply
(1) requirements for low-voltage and high-current switching converters
The speed and efficiency of data processing system are increasing day by day. The logic voltage of the new generation microprocessor is as low as 1. 1~ 1.8V, while the current reaches 50~ 100A. Its power supply is a DC-DC converter module with low voltage and high current output, also known as voltage regulation module (VRM). The requirements of the new generation microprocessor for VRM are: low output voltage, large output current, high current change rate and fast response speed.
① In order to reduce the electric field strength and power consumption of ic, the power supply voltage of microprocessor must be reduced, so the output voltage of VRM should be reduced from the traditional 3V to below 2V, or even1v..
② During operation, the input current of power supply is >; 100A, because of parasitic l and c parameters, the voltage disturbance is large, so l should be reduced as much as possible.
③ Microprocessors frequently start and stop, constantly start and work from the dormant state, and then enter the dormant state. Therefore, it is required that the VRM current suddenly changes from 0 to 50A, and then suddenly drops to 0, and the current change rate reaches 5A/ns.
④ During design, the disturbance voltage should be controlled ≤ 10%, and the output voltage is allowed to change by 2%.
(2) Using waveform interleaving technology.
The parasitic impedance of line, ESR and ESL of capacitor have great influence on the voltage regulation of VRM when the load changes. It is necessary to develop a new voltage regulation module with high frequency, high power density and high speed. At present, there are many topologies, such as: synchronous rectifier Buck converter (using power MOS tube instead of switching diode); In order to prevent the disturbance of high-frequency parasitic parameters on the output voltage when the current changes greatly, the literature introduces the use of multi-input channels or multi-phase DC-DC converters, and the application of interleaving technology to ensure that the output ripple of VRM is small, improve the output transient response, and reduce the output filter inductance and capacitance.
(3) Voltage ripple and impulse voltage.
① Voltage ripple and ESR. For the load with voltage below 1V and current above 100A, the load resistance is below10mΩ, which is lower than the internal equivalent series resistance of the filter capacitor, the voltage ripple problem will occur. Now suppose that this power supply can be realized by boost or boost converter, but the ripple current flowing through the capacitor is above 100A, and the efficiency is less than 50%. In this regard, the buck converter includes a series filter inductor, which can suppress ripple current. However, the load resistance is equivalent to ESR, and the ripple current flows through the capacitor and the load respectively, which works differently from the current filter circuit.
In order to discuss the action mode of ripple voltage, the equivalent circuit is given for simulation. According to the value of Crc, there are four kinds of ripple voltages in the simulation. There are also four action modes in the relationship curve between voltage ripple value and RC/R. The greater C, the smaller ripple rate. In order to further reduce the output voltage ripple of low voltage and high current, that is, to reduce the ESR value of filter capacitor, certain methods and strategies must be adopted.
② Impulse voltage caused by sudden load change. For the load of digital circuit, in order to quickly respond to the conversion of various modes, the transient response characteristics of output voltage corresponding to load change are very important. At this time, if the change rate of current is large and the pulse generation time is shorter than the switching period Ts, it is difficult to expect the output voltage stabilization effect brought by feedback. At present, the technology has no way, and it is in the stage of simulation research.
(4) Explore the possibility of omitting the filter capacitor.
If the output voltage fluctuates due to sudden load change and the fluctuation duration exceeds the switching period, it can be adjusted to some extent through feedback, and the LC filter circuit plays a decisive role in this voltage adjustment effect. In order to achieve the purpose of voltage regulation, it is necessary to increase the switching frequency, reduce the values of L and C, and make the cutoff frequency extend as far as possible to the high domain end. Some people consider using two asymmetric inverters (with transformers) to output two-phase square waves, and the output voltage of each inverter is connected to the same load through half-wave rectification, extending the cutoff frequency to the high domain end.
The switching frequency is determined by the switching time of MOSFET. In order to improve the switching efficiency and exceed its limit, multiphase switch can be used to increase the switching frequency equivalently in practice. However, the number of stages is also limited. In addition, the reason for the change is only on the load side, so it is also very effective to keep the cutoff frequency as low as possible. In order to achieve this goal, the use of electric double layer capacitor filter may be the future development direction. Of course, we should consider how to reduce the equivalent series resistance and equivalent series inductance of electric double layer capacitor at the same time.
(5) Portable equipment and fuel cells
For portable appliances such as notebook computers, mobile phones and digital cameras, power supply is the most problematic part. The power supply of portable equipment has always been based on traditional batteries, which can not fully meet the requirements of users in terms of portability and long-term use. Therefore, fuel cells made of solid polymer materials have recently attracted everyone's attention. Fuel cell uses methanol as fuel, platinum as catalyst, and the structure is electrolyte membrane between electrodes, and the energy density can reach 10 times of that of lithium battery. The working temperature below 100℃ includes normal temperature power generation, and the voltage of a single section is about 1~2V. Originally, hydrogen is the most ideal fuel, but from a practical point of view, it is more convenient to use the combination of methanol and platinum catalyst. However, it has problems in following the load changes, so it needs to be used in combination with capacitors to protect the electrodes.
The advantage of fuel cell is that it is easy to maintain and can be used for a long time. When the power is insufficient, you only need to replenish the fuel, and you don't need to charge for a long time.
Based on the low-voltage and high-current switching power supply, the future technical development direction of switching power supply is discussed. According to Moore's law, the integration of IC will increase by 2 times every 18 months, so it is difficult to determine to what extent the voltage will decrease. If this trend continues indefinitely, it can be expected that the demand for power supply will be higher and higher. To meet these requirements, first of all, the development of new semiconductors and capacitors is the premise. In addition, the establishment of microstructure models of components from the perspective of circuits may also become the key point to solve the problem. Therefore, it will be more and more necessary to break the boundaries of disciplines and conduct multi-level collaborative research in the future.
8. Low voltage and high current DC-DC converter module