In 1957, the General Electric Company of the United States invented the thyristor based on the three-terminal and three-layer semiconductor structure of the transistor, marking the birth of power electronic technology. The birth and development of this technology have revolutionized the use of electrical energy by humans, and greatly changed people's concept of using electrical energy. In the world, the proportion of total power consumption that has been transformed and adjusted by power electronic devices has become an important indicator to measure the level of electricity consumption. At present, the average number of this indicator worldwide is 40 °%, and it will reach 80%. This poses new challenges to power electronics technology.
The application range of power electronic technology is very wide. It is not only used in general industry, but also widely used in transportation, power systems, communication systems, computer systems, new energy systems, etc. It is also widely used in household appliances such as lighting and air conditioning and other fields. In recent years, with the increasing capacity of high-power power electronic devices, the problem of heat generation of the device has emerged. In certain application environments where the temperature exceeds 200600K at room temperature, such as various aircraft circuits, automotive electronics, underground drilling exploration, and nuclear reactors, the heat dissipation conditions of power electronic switching devices and other passive devices begin to deteriorate, which may cause overall system efficiency. Decreased, the performance becomes worse; more importantly, it may damage the device and paralyze the entire system. At this time, it is necessary to improve and optimize the three parts of the device, circuit structure and control method for the high-temperature working environment to adapt to these harsh working environments.
On the other hand, as people continue to explore the material world, new working environments are constantly emerging. In high altitude, high latitude areas, and in exploration and investigation of outer space, the ambient temperature is often 100200K lower than room temperature, and the performance of power electronic devices under these low temperature conditions will also change differently. People's desire for the unknown world has promoted the deepening of scientific investigations. To cope with this trend, the development of low-temperature power electronic technology must also be deepened.
This article mainly introduces some technological developments and latest developments in the application of power electronics technology at extreme temperatures. Since the scope of power electronics is too extensive, only the areas where the research is more in-depth and there have been many technological breakthroughs are described here, including: silicon carbide devices in high-temperature applications, new cooling and heat dissipation technologies, high-temperature passive devices and integration Circuit, as well as low-temperature power electronic technology and high-temperature superconducting technology.
2 Power circuit devices at high and low temperatures The vast majority of power electronic equipment currently operates in the temperature range of 0100 ° C (considering the heat generated by the equipment itself). All circuit components, circuit structures, and control methods are based on this The scope of work is based. However, when the operating temperature is too high or too low, the characteristics of each component will change to varying degrees, so that the performance of the entire circuit will also change accordingly. Therefore, before discussing various new technologies, it is necessary to explain the different performance of various devices in power electronic circuits under high and low temperature environments.
First, the iconic and most important devices in power electronics are various switching devices, and the performance of these devices is also most affected by temperature. Now the MOSFET commonly used in high-frequency power electronic circuits is a positive temperature coefficient device, which means that when the temperature of the device increases, its on-state resistance should increase accordingly, and high temperature will cause low current. This is important when MOSFETs are required to work in parallel, because through a good thermal path between devices, the positive temperature coefficient can reduce the current in the hotter device and force the current to flow more to the lower temperature device, thereby avoiding thermal runaway . The actual MOSFET can be regarded as being operated by thousands of micro power FET units in parallel. According to the above theory, the parallel unit array can ensure the reliable operation of the device. But if the MOSFET has a negative temperature coefficient, the current parallel cell structure will cause serious reliability problems, in fact, this possibility exists. The figure below is the transconductance curve of IRFP450 produced by IR.
It can be seen that the transconductance curves at different temperatures have an intersection. When the gate-source voltage is lower than this intersection voltage value, the temperature coefficient is negative; when it is greater than the intersection voltage, the temperature coefficient is positive. In this way, when the MOSFET works in the saturation region, it may be in the range of negative temperature coefficient. However, in power electronic circuits, MOSFETs mostly work in the switching state, and the gate-source voltage when the switch is turned on is generally greater than the cross-point voltage in the transconductance curve, so the on-state resistance has a positive temperature coefficient, which can automatically avoid temperature runaway . In general, the power MOSFET will not have the problem of secondary breakdown, which is one of its major advantages, but in actual use, care should be taken to leave an appropriate margin.
The most important passive devices in power electronic devices are magnetic components such as inductors and transformers. In traditional applications, ferromagnetic components, due to their large size and weight, generate heat during operation, thus becoming a bottleneck in the overall circuit design. In the case of high temperature and large capacity applications, as the heat dissipation conditions deteriorate and the current in the inductor winding increases, the heat generated by the inductor and transformer will be very large. When the temperature of the ferromagnetic material rises to a certain level, the ferromagnetic material will be converted into a weak magnetic substance, and the temperature at which the conversion begins is called the Curie point. According to the magnetic domain theory, when the temperature rises and exceeds the Curie point, the magnetic domain structure in the ferromagnetic material is destroyed due to thermal motion, so that it is completely disintegrated. At this time, the ferromagnetic substance is converted into a weak magnetic permeability close to 1 Magnetic, and remain unchanged.
Therefore, the design and manufacture of magnetic components are even more important. Without the ferromagnetic components with small size, light weight, and high temperature resistance, the role of the new switching device cannot be fully exerted. At present, the working temperature of widely used magnetic components generally does not exceed 105C, and the highest is only 150C. In order to improve this temperature index, many countries around the world have done a lot of research: NASA Lewis Research Center has successfully studied the inversion of the operating temperature of 200C Transformer; the US Air Force Research Institute has also developed a transformer in the 1.5kW, 27028V forward converter, which can operate for 600h in an environment greater than 290C. The core of this transformer uses manganese zinc material with a Curie temperature above 310C, And after repeated simulation and. It can be seen that SiC devices are significantly better than traditional devices in this index. The following will introduce SiC devices and various applications of these devices.
SiC materials are used in many types of power electronic devices, which can be divided into three categories according to device types: â‘ SiC Schottky diodes are relatively mature SiC power electronic devices, and their structures are as shown. It has almost no reverse recovery current when it is turned off, and it is attractive outside the application field of 3003000V and switching frequency higher than 50kHz. There are already 600V / 6A grade products on the market. â‘¡The structure of SiCMOSFET is not much different from that of traditional silicon devices, but due to the high breakdown field strength of SiC semiconductors, the use of 4H-SiC in 2000 has achieved a blocking voltage of more than 2000V, up to 7000V, and its on-state The impedance is 250 times lower than silicon devices. â‘¢ In addition, SiC can also be made into bipolar devices with high blocking voltage, such as high-voltage pin diodes and thyristors. Recently, the research and development activities of SiC thyristors have begun to concentrate on GTO. In 2000, there have been reports of 4H-SiCGTO with a blocking voltage of 3100V and a turn-off gain of up to 41 at 50C.
Since SiC devices are superior to the current traditional silicon devices in terms of breakdown voltage, on-state resistance, switching frequency and device capacity, on the basis of further improvement of manufacturing technology and further cost reduction, SiC devices can completely replace all current Silicon devices, and is expected to play an important role in aerospace, high temperature radiation environment, petroleum exploration, automation, radar and communications, automotive electronics, etc. As far as aerospace is concerned, high-performance military flight equipment, jet aircraft, and civil aircraft all need sensors, electronic controllers, and power electronics that can withstand high temperatures to improve the reliability of the aircraft, eliminate cooling devices, and reduce weight. The next-generation turbine control system must use electronic equipment that operates at a high temperature of 350C. In addition, high-temperature power electronic devices are also urgently needed for civil aircraft. High-temperature radiation environments, including nuclear reactors, tritium system preparation devices, and nuclear waste storage devices, often have temperatures as high as several hundred degrees and are irradiated with 7 rays and neutrons. These require high-temperature and radiation-resistant electronic systems. In addition, high-power phased array radar is mainly limited by heat dissipation, SiC power devices are expected to solve this problem.
It can be seen that the new SiC devices have outstanding characteristics such as high temperature resistance, large capacity, low loss, and high frequency. The application prospects are very broad, but there are still many problems that need to be resolved, especially the reliability of the device. It is expected that it will take some time to become practical.
4 New cooling and heat dissipation technology With the increasing capacity and frequency of power electronic components, the problem of device heating has become prominent. Especially in some high-temperature applications, without proper heat dissipation measures, the temperature of the device may exceed the maximum allowable junction temperature, resulting in deterioration of device performance and damage. Therefore, in the circuit design, choosing appropriate cooling and heat dissipation methods and rationally designing is one of the indispensable important links to make the full potential of the device and improve the reliability of the circuit. The basic task of heat dissipation design of power electronic devices is to design a heat flow path with the lowest possible thermal resistance for the device according to the principle of heat transfer, so that the heat emitted by the device can be dissipated as soon as possible, thereby ensuring the internal junction temperature of the device during operation Always keep within the allowable temperature range. The following will introduce several commonly used heat dissipation technologies according to the device structure design and the cooling medium.
For a long time, the use of air cooling is the easiest and easiest method, and it is widely used. From the form of cooling, air cooling can be divided into two types of self-cooling and forced air cooling. The self-cooling type is the traditional way of adding a radiator. Although the efficiency of heat dissipation is very low, it has a simple structure, no noise, and easy maintenance. Especially without rotating parts, it has high reliability and is suitable for devices with a rated current of 20A or less Or high-current devices in simple devices. On the contrary, the forced cooling method mainly refers to the heat dissipation device equipped with a fan, which can be qualified for large-capacity occasions with a rated current of 50500A, but the noise, low reliability, and difficult maintenance hinder the development of this method.
For the above air cooling method, increasing the surface area of ​​fin heat dissipation and increasing the air volume can reduce the thermal resistance of convection and the thermal resistance of air temperature rise. However, this is exactly contradictory to reducing the conduction thermal resistance. Therefore, the traditional heat sink takes the largest possible heat dissipation area among the limited conduction thermal resistance. The use of heat pipes can solve this contradiction. The heat pipe is a hermetically sealed and sealed evaporative cooling device. It is composed of a sealed pipe, a liquid wick and a steam channel, and uses the circulating effect of the working liquid filled in it to conduct and radiate heat. The heat pipe was born in 1963 and was quickly applied to artificial satellites, but due to its high cost, it has not been widely used. At present, with the trend of high frequency and miniaturization of power electronic devices and the price reduction of heat pipe radiators, the prospect of using heat pipe radiators for power circuits is very broad.
If power electronic devices are applied at high temperatures, the traditional heat dissipation method cannot meet the increasingly strict heat dissipation requirements. Therefore, liquid cooling is proposed for large capacity and high operating temperature applications. according to.
Standard module hardware manager Application manager The countries of the world have conducted in-depth research on integrated power electronics technology and made a lot of progress: Germany ’s EUPEC has developed IGBT modules for medium power transmission. The current sampling resistance is built into the module ’s substrate. Measured the current of 35kW inverter; German Semikron company developed the MiniSKp type CIB (rectification, inverter, brake chopper) module in 1996, and has been launched to the market; In 2002, Semikron launched a new generation of MiniSKp -Type CIB module, using today's cutting-edge chip packaging technology to improve thermal resistance and reduce size, in order to adapt to high temperature environment, a temperature sensor is also integrated in the module to detect the temperature inside the module; three-terminal offline PWM launched by PowerIntegration of the United States TOPSwitch is a special module for high-frequency switching power supply. In 2002, the fourth-generation product TOPSwitch-GX has been developed. It is characterized by simple peripheral circuits and low cost. It can be seen that current power electronic products are gradually developing towards modularization and integration to adapt to changing application environments, especially in aerospace and harsh geological environments.
Although the modular power circuit can simplify the circuit design and reduce the manufacturing cost, the accompanying problems are also more prominent: as the output power increases and the device volume decreases, the power density is greatly improved, resulting in the entire The amount of heat generated by the device surges, resulting in decreased stability, reduced efficiency, and increased failure rate. At this time, the traditional air cooling and liquid cooling technology can no longer meet the heat dissipation requirements of the new device, so a thermoelectric module (TEM) with a more compact structure and heat dissipation efficiency has appeared.
The thermoelectric module used in the power electronic module is a solid-state energy converter. Its basic function is to follow the Peltier effect and use electrical energy to transfer heat. Specifically, its working principle is as follows: A basic hotspot converter is composed of a series of N-type and P-type thermocouples alternately and connected in series on the circuit. These units are sandwiched by two ceramic diaphragms. This ceramic material can achieve electrical insulation and good thermal conductivity. When a positive voltage is applied across the TEM, the electrons move from the N region to the P region, and the potential decreases and releases heat. Conversely, when a negative voltage is applied across the TEM, the electrons move in the opposite direction, and the potential rises and absorbs heat. Based on this principle, the heat at both ends of the module can be controlled by applying different DC voltages to the TEM. When applied to power electronic modules, a suitable DC voltage can be added to the TEM to make the temperature difference between the high temperature area inside the module and the ambient temperature as low as possible, and dissipate the excessive heat inside to the surrounding environment through the TEM.
At present, TEM has been widely used in refrigeration or occasions requiring precise temperature control. Its thermodynamic efficiency is much higher than traditional resistance heaters. However, TEM also has some problems, such as a DC power supply that requires low voltage and large current, a limited temperature adjustment range, and a relatively expensive price. In the long run, TEM is the first choice for heat dissipation of high-power power electronic modules due to its compact structure and high heat dissipation efficiency.
6 Low-temperature power electronics At high temperatures, power electronic devices will have unstable performance or even damage due to the deterioration of heat dissipation conditions. In contrast, in a low-temperature working environment, because the on-state impedance of the power switching device will be significantly reduced, its switching loss will also be reduced, plus low temperature is conducive to the heat dissipation of the device, so the whole device The efficiency, transient response and power density will be greatly improved than the devices at high temperature and room temperature. Since high temperature superconductors (HTS) are the most popular new materials in the field of low temperature, this section mainly discusses some characteristics of power electronic devices working in low temperature environments.
At present, with the deepening of power electronics technology in various fields, it has appeared in many low-temperature occasions. For example, geological surveys, biological population research, petroleum exploration in the depths of the ocean, and scientific surveys in high-altitude and high-altitude areas, the temperature of these working environments is basically between -800C. The power electronic equipment in these workplaces includes various scientific expeditions, power devices for deep-sea submarines and polar ships, and power generation equipment in these areas.
For the above reasons, at low temperatures, the heat dissipation conditions of these power electronic devices are very good, and sometimes they can work stably without the aid of additional heat dissipation devices. However, the most widely studied and in-depth low-temperature applications at present are various devices in the outer space environment. Their working environment is not available on the surface of the earth. The usual operating temperature is between -200-100C. At this time, the devices and circuits The characteristics are different from the surface conditions.
Most aviation power electronic systems can be seen as a power electronic converter, solar cell array and DC power distribution network. DC direct current and DC-AC converters are used to provide electrical energy for loads of different power levels. The applications of aviation power systems that are already in use include the International Space Station (ISS), aircraft and satellite power systems, servo systems, and various spacecraft, including outer space probes, planetary landing vehicles, and some planet surface probes. In the application of power electronic devices and devices to perform space missions, especially in outer space and deep space exploration, due to the very low ambient temperature (see Table 2), high efficiency and stability must be guaranteed.
Table 2 Typical operating temperatures of spaceships mission temperature / C Mars Jupiter Saturn Uranus Neptune Pluto NASA ’s Saturn Aura exploration program, the operating temperature of various electronic devices is about -183C. Currently, some of the universes working in these low-temperature environments The spacecraft uses a radioisotope heating device (RHU) to maintain the working temperature of the power electronic device in the spaceship at about 20C. At the same time, the normal work of the RHU needs other equipment to cooperate, and even when the temperature of the entire device is high, the RHU It will still heat, which requires a temperature control system to adjust the working temperature. Therefore, if this type of aviation power device can directly work in an ultra-low ambient temperature, the RHU and related devices and temperature control system can be eliminated. In this way, the size and weight of the entire device can be greatly reduced, the cost of system development and launch can be reduced, and the reliability and life of the device can be improved.
And Mukhopadhyay and others have discovered that among various switching devices, insulated gate MOSFETs have the most development prospects for low temperature applications. In 1994, the research reports of these scientists pointed out that MOSFETs operating at an ambient temperature of 77K have low on-state resistance, fast turn-on and turn-off, low diode reverse recovery current, small device size, and high thermal conductivity of semiconductor materials. advantage.
In contrast, although traditional passive devices such as resistors, capacitors, inductors, and transformers can also operate at low temperatures, their respective characteristics have changed differently. The metal film resistor can work normally at low temperature, but the capacitor is different: the electrolyte of the electrolytic capacitor will solidify, making the capacitor useless; while the polypropylene and polystyrene capacitors will show increased withstand voltage and reduced loss ; Although polyester capacitors can also work normally, but the capacitance value will vary greatly. At present, there is already a ceramic capacitor that can work at an ambient temperature of 77K, and the DC withstand voltage value exceeds 2000V, which is called low temperature super capacitor (CHC). Traditional ferrite and permalloy magnetic components can work at 77K, but their losses are almost unchanged compared to room temperature environments, so it is urgent to find low-temperature ferromagnetic materials with low loss and proper permeability.
Researchers at the NASALewis Research Center pointed out that the two core issues of low-temperature power electronics applications are high-temperature superconductor devices and low-temperature semiconductor switching technology. At present, they have successfully developed a DC converter composed of customized superconductor inductors and other components, and a high electron mobility transistor switch that can adapt to low temperature and large current loads.
7 High temperature superconductor (HTS) As early as 1911, when Kamerlingh Onnes of Leiden Laboratory in the Netherlands discovered the superconductor, people expected that the superconductor could be quickly applied in power technology. It was not until the 1960s that practical and non-ideal superconductors NbTi and Nb3Sn were discovered one after another, that the research and development of superconductivity technology could be widely carried out, and the conceptual design schemes and experimental prototypes of various superconductivity devices were published . In the early 1980s, after the successful development of ultra-fine filament composite multi-core NbTi superconducting materials with low AC loss, the research and development of superconducting power technology became active again. In 1986, IBM Swiss Research Center. G. Bednorz and KAMueller discovered La series high temperature superconducting materials. In the following two years, Y series, Bi series and Tl series high temperature superconducting materials (the critical temperature was 85125K) were discovered one after another. The high-temperature superconducting equipment can operate at the temperature of liquid nitrogen (77K). Compared with the low-temperature superconducting equipment (operating at 4.2K of liquid helium temperature), not only the operating cost is greatly reduced, but also the magnetic-thermal stability is greatly improved. However, as a ceramic material, high-temperature superconductors are not as easy as NbTi superconducting materials to make flexible wires with high current carrying capacity. Until the mid-to-late 1990s, the preparation of practical high-temperature superconducting tapes had achieved significant progress. For example, using the high-temperature superconducting composite wire that is more important in superconducting power technology: At present, the technology for preparing Ag-based Bi-based multicore composite wires with a length of 1.02.0 km using PIT is relatively mature. Engineering current density (;) Bi series multi-core composite conductors began to be commercialized, priced at 200,300 US dollars / kA, m. Table 3 is the main units and technical levels of international preparation of high-temperature superconducting tape.
Table 3 Units and technical level of HTS strips developed internationally Unit technical level American superconducting company Denmark Nordic Superconducting Germany Vacuum Smelting Japan Sumitomo Electric Australia Superconducting Company (AST) Northwest China Nonferrous Metal Research Institute Beijing Inner Superconducting Technology The company's superconducting materials have now penetrated into all aspects of power technology and are widely used in transmission line grids, power electronic devices and circuits, power transmission and other fields. The following will introduce the relatively fast-developing application fields.
High-temperature superconductor transformer: Compared with conventional transformers, HTS transformers have a small total loss (30 °% of conventional), light weight (45 °%), low total cost (80 °%), small size, strong overload capacity, no Oil operation can avoid the problems of fire and environmental pollution, low operating impedance, limited current function, reduce the impact of faults, improve the voltage stability of the power grid and so on. The largest prototype to date has been developed by ABB and Woukasha, Wisconsin, USA. The three-phase 630kVA (18.7kV / 420V) transformer jointly developed by ABB has been operating in the Geneva power grid for 1 year; the Ministry of Energy is currently funding another 11 million US dollars Of the US partners are the second set of US public utility research, manufacturing, installation, and testing close to 10MVA. It is estimated that for transformers with more than 30MVA, there will be a market of 3 billion US dollars in one year.
: SMES can improve power quality and reliability, improve power grid capacity, can be used to balance power loads, and can respond quickly and continuously to different power requirements without reducing power quality. At the EUCAS99 meeting, Israel reported energy storage devices 60 (77K) and 130 (64K); Finland and Germany reported devices operating at 20K energy storage 5k. The other is the flywheel energy storage device, which is a magnetic floating frictionless bearing made of Y series blocks and permanent magnets, which is also a research hotspot. Compared with other energy storage methods, its advantages are: high energy storage density (about 30 times), lower refrigeration cost and life cycle cost (5 times lower than battery-based UPS system). The United States (Houston and Boeing), Europe (Germany, France and Spain; Finland and Germany; United Kingdom) and Japan (CHUBU Electric and Mitsubishi Heavy Industries) have projects in progress. Among them, Boeing has developed a small flywheel energy storage device that can be used for aerospace landing, storing hundreds of watt-hours of energy; the European joint research project is "MTGYBCO material for magnetic levitation and energy conversion", and the energy storage of devices in Finland and Germany reaches 300Wh. The UK and others have studied the 15kWh test system; the model system developed by Japan has reached 1.4kWh of energy, using 9 pieces of YBCO with a diameter of 60mm, and the flywheel is made of carbon fiber reinforced plastic (CFRP) with a speed of 21000r / min and is currently studying 10MWh Mockup.
In 1996, the American Institute of Electric Power, with the support of the US Department of Energy, and Pirelli Cable Company and Southwire Company used ASC's Bi-2223 / Ag ribbon conductor to develop a 30m, 115kV, 2kA three-phase AC high-temperature superconducting cable model. The goal is to develop a 1km high-temperature superconducting cable. Southwire has installed a 30m, 12.5kV, 1.25kA three-phase AC high-temperature superconducting cable for testing at its headquarters. The cable has been operating at full load for more than 3000h. Southwire started using the first industrial high-temperature superconducting power transmission system in the company in January 2001. The system uses three 30m-long superconducting cables and transmits power to three factories under its control for 12,700 hours. Edison Power Company of Detroit, USA has now laid three 120m-long superconducting cables to supply electricity to 14,000 residents in the city center. In Europe, in May 2001, Copenhagen, Denmark used a 30m high-temperature superconducting cable on the power grid, which is currently supplying 150,000 residents. In 2002, some users of the Paris Power Company also obtained electricity through high-temperature superconducting cables.
At present, the industrialization of superconducting power technology is no longer a question of whether it can be realized, but a question of how long it takes to realize it. The active superconducting current limiter, superconducting energy storage system and superconducting current limiting-energy storage system formed by combining high-temperature superconducting technology, power electronic technology and modern control technology will first be industrialized. However, the large-scale application of superconducting power technology still depends on making breakthroughs in the following two aspects:-the preparation of high-temperature superconducting wires with low cost, low loss, and good mechanical properties; the second is to improve the long-term reliability of low-temperature systems and refrigeration systems Cost and reduce the cost of construction and operation and maintenance.
8 Conclusion This article reviewed the main directions and development trends of modern power electronic technology in high-temperature and low-temperature applications from several aspects. In general, the application fields involved in power electronics technology are more and more extensive, and began to expand to high-altitude, polar and even outer space and other special working environments. In these extreme applications, the impact of high and low temperatures on power electronic systems has risen to the forefront. Therefore, in order to further expand the living space of human beings and inquire about scientific discoveries that are not yet known, it is necessary to conduct more in-depth research on power devices, power electronic circuits, and control methods.
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