Power conversion technology and semiconductor power converter

The theory of power electronics (or power electronics technology) is based on three disciplines: electronics, electricity and control. At first, it was considered a marginal discipline between electronics, electricity and control, but with the continuous development of power electronics technology, it has become a discipline involving a wide range of fields. It can be said that it has its place in any occasion involving the application of electric energy. Today, it has not only developed into a branch of high technology, but also the support of many high technologies.

The reason why power electronics technology is connected with the word "electricity" is that its initial application scope was mainly in electrical engineering and power systems to control and transform the mains or strong electricity. Its function is to transform the mains and strong electricity in various forms (mainly frequency) according to the special requirements of load and load, so that the electrical equipment can get the best power supply and the power system can be in the best operating state, so that the electrical equipment and the power system can achieve efficient, safe and economical operation. Today, power electronics technology has developed, and it not only involves the transformation and application of "electricity", but also involves the transformation and application of chemical energy sources (batteries) and solar cell electricity. Although it has broken through the original boundary of pure "electricity", it is still within the scope of power conversion.

As far as power electronics technology itself is concerned, it mainly includes two aspects, namely power semiconductor device manufacturing technology and power semiconductor conversion technology. The former is the foundation of power electronics technology, and the latter is the core of power electronics technology. The two complement each other, are interdependent, and promote each other, making the momentum of power electronics technology development higher and higher, making it play an increasingly important role in scientific and technological progress and economic construction.

1Power semiconductor devices

The development of semiconductor power conversion technology is based on the development of power semiconductor devices, which gradually developed from the production of silicon rectifiers (SR) in 1956 and thyristors (SCR) in 1958 in the United States.

After more than 40 years of development, the device manufacturing technology has been continuously improved. It has gone through three development periods: discrete devices represented by thyristors, power integrated devices (PID) represented by gate turn-off thyristors (GTOs), giant transistors (GTRs), power MOSFETs, and insulated gate bipolar transistors (IGBTs), and power integrated circuits (PICs) represented by intelligent power integrated circuits (SPICs) and high-voltage power integrated circuits (HVICs). From the semi-controlled devices of thyristors that turn off by zero-crossing the commutation current to the fully controlled devices of PID and PIC that can turn on and off the devices through gate or gate control pulses, the real thyristor has been realized. In terms of the control mode of the device, the development from the current control mode to the voltage control mode not only greatly reduces the control power of the gate (gate), but also greatly improves the switching speed of the device on and off, so that the operating frequency of the device is continuously improved from industrial frequency → medium frequency → high frequency.

In terms of device structure, it has evolved from discrete devices to primary modules that combine discrete devices into power conversion circuits, and then to complex modules that combine power conversion circuits with trigger control circuits, buffer circuits, detection circuits, etc. The speed at which power integrated devices have evolved from single devices to modules is even faster, and today modules with intelligent functions (IPM) have been developed.

Table 1 Overview of the development of representative power semiconductor devices

All of this lays the foundation for the development of high-frequency conversion technology, for the high-frequency, miniaturization and lightweight of converters, and for energy and material conservation, and for improving efficiency and reliability.

For an overview of the development of representative power semiconductor devices and modules, see Table 1.

Summarizing the development of power electronic devices over the past 40 years, it has gone through three periods, which can be divided into four stages.

(1) Phase 1

In the development stage represented by rectifiers and thyristors, their application in the field of low-frequency and high-power conversion had advantages and soon completely replaced mercury arc rectifiers.

(2) Second stage

The development stage represented by fully controlled devices such as GTO and GTR, although still belonging to the current type control mode, its application makes the quasi-high frequency of the converter possible.

(3) The third stage

The development stage represented by voltage-type fully controlled devices such as power MOSFET and IGBT can be directly driven by IC, and have better high-frequency characteristics. It can be said that device manufacturing technology has entered the primary stage of combining with microelectronics technology. That is, power electronic devices and electronic devices have gone their separate ways for a period of time on the road of development, and now they have come together again.

(4) Stage 4

The development stage represented by power integrated circuits such as SPIC and HVIC has made power electronics technology and microelectronics technology more closely integrated. It is a highly intelligent power integrated circuit that integrates fully controlled power electronic devices with drive circuits, control circuits, sensor circuits, protection circuits, logic circuits, etc. It realizes the integration of devices and circuits, strong electricity and weak electricity, power flow and information flow, and becomes an intelligent interface between machines and electricity, and a basic unit of mechatronics. It is expected that the development of PIC will enable power electronics technology to achieve a second revolution and enter a new era of intelligence. This stage is still in the early stages of development.

2Semiconductor Power Converter

2.1 Application scope of power conversion technology

With the development of power conversion technology today, its application scope can be roughly divided into five aspects.

(1) Rectification: realize AC/DC conversion;

(2) Inversion: realize DC/AC conversion;

(3) Frequency conversion: realize AC/AC (AC/DC/AC) conversion;

(4) Chopping: realizing DC/DC (AC/DC/DC) conversion;

(5) Static solid-state circuit breaker: realizes the functions of contactless switch and circuit breaker, and controls the on and off of electric energy.

2.2 Development of power conversion technology

The development of power conversion technology has gone through three stages.

(1) Phase 1

The first stage was based on the development and application of electron tubes and ion tubes (thyratrons, mercury arc rectifiers, high-pressure mercury arc valves). At that time, this discipline was called Industrial Electronics. The research work in this stage was mainly focused on the development of rectification, inversion and frequency conversion technology. The application areas of conversion technology are mainly DC transmission, DC traction, electrification, electrometallurgy, medium frequency, high frequency quenching, heating, high-voltage DC transmission, etc. Since DC transmission, DC traction, electrochemical electrometallurgy have an overwhelming advantage in the application of conversion technology, DC transmission, traction and electrification were called the three pillars of the conversion industry at that time. In fact, from the classification of conversion technology, it belongs to rectification conversion, which is a small part of conversion technology.

(2) Second stage

The second stage is based on the development and application of silicon rectifiers and thyristors, mainly thyristors. In my country, it began in the early 1960s, when power electronics came into being and replaced industrial electronics. Since the basic theory of power conversion technology - the research on rectification, inversion, and frequency conversion technology, can be said to have been completed in the first stage, this is no longer the research topic of the second stage. This stage is mainly aimed at new problems that have emerged after silicon rectifiers and thyristors replaced electron tubes and ion tubes (such as silicon rectifiers and thyristors have low blocking voltages, low on-state currents, and weak overvoltage and overcurrent impact tolerance. If there is a slight abnormality in the application, it will cause permanent damage to the device). The application research, such as: research on trigger circuits, research on current balancing measures for parallel connection of devices, research on voltage balancing measures for series connection of devices, research on buffer (damping) circuits to prevent overcurrent and overvoltage during device commutation, research on overvoltage protection, overcurrent protection, and overheating protection of converters, and research on the protection coordination between the thermal capacity of the device and the short-circuit current of the system and the short-circuit capacity of the fast fuse when the converter system fails. With the continuous improvement of device manufacturing level and the continuous improvement of protection measures for converters, the application technology of silicon rectifiers and thyristors in converters has become increasingly mature.

Just as the development of any new business is unstoppable, the application and development of silicon rectifiers and thyristors in power conversion technology is also unstoppable. It quickly replaced the position of mercury arc rectifiers in power conversion technology, allowing my country to enter the development and application stage of power electronics technology. After completing its historical mission, my country's mercury arc rectifier manufacturing industry officially stopped production in 1972. It not only completely replaced mercury arc rectifiers in the so-called three pillar industries of power conversion technology, but also has greater power. Even in the field of high-voltage direct current transmission, the world's first high-voltage thyristor converter valve was put into operation in the Gotland Island DC transmission project in Sweden in 1970, announcing the end of the historical mission of high-voltage mercury arc valves in the field of high-voltage direct current transmission. In addition, it also replaced the motor-generator set used for electroplating, battery charging, power plants (stations) and substation DC systems; replaced the DC excitation unit of the generator.

During this period, with the continuous improvement of the manufacturing technology level of rectifier tubes, especially thyristors, the application fields involved in semiconductor current conversion technology have been continuously expanded. For example, the development of fast thyristors has greatly promoted the development of medium-frequency induction heating, smelting, and quenching power supplies (1kHz~8kHz); thyristor low-frequency power supplies, 400Hz medium-frequency power supplies, high-precision voltage-stabilized power supplies and current-stabilized power supplies serving national defense construction and high-tech research have been developed one after another; there are many other application fields, which will not be elaborated.

In this development stage with thyristor application as the core, whether it is rectification, inversion or frequency conversion, the conversion is achieved by phase shift control (α, β) of the gate of the thyristor, that is, phase-controlled conversion technology. Since thyristor is a non-self-shutoff (full-controlled) device and it is a current-controlled device, it cannot replace thyristors and electron tubes in the field of high-frequency applications, and only has an advantage in the field of low-frequency and high-power.

At this stage, research on chopping technology for DC/DC conversion has been carried out and first applied in DC traction speed regulation. The thyristor speed regulation used in public buses and trolleybuses is an example of DC/DC conversion application. However, since thyristors are semi-controlled devices, using them in DC/DC conversion requires more complex main circuits and control circuits in order to force them to shut down, but their energy-saving effect is significant.

(3) The third stage

The third stage is based on the development and application of fully controlled power semiconductor devices. It is the stage of semiconductor power converters developing towards high frequency, and the stage of the control mode of converters developing from phase shift control (PhaseshiftControl) to time ratio control (TimeRatioControl—TRC). At present, it is not accurate to simply call converters (power supplies) using the above two control methods phase-controlled power supplies and switching power supplies. This is because in semiconductor power converters, power electronic devices that undertake power conversion are used as contactless switches. Both phase-controlled power supplies and time ratio controlled power supplies work in the switching state. Therefore, it is more accurate to call them phase-shift controlled power supplies and time ratio controlled power supplies.

There are three types of TRC, namely pulse width modulation (PWM), pulse frequency modulation (PFM), and mixed modulation (PWM + PFM). PWM is the most commonly used TRC because the modulation frequency is fixed, that is, the modulation period T is constant (or basically unchanged), and the conversion circuit is adjusted by changing the duty cycle D of the control pulse, thereby making the design of the filter circuit relatively simple.

The third stage of development gradually unfolds with the development of fully controlled devices.

First, the application of bipolar full-control devices such as GTO and GTR greatly simplifies the structure of inverter, frequency conversion and chopper conversion circuits, and increases the conversion frequency to about 20kHz, laying the foundation for the high frequency, miniaturization, high efficiency, energy saving and material saving of electrical equipment. However, since GTO and GTR are current-type control devices, the control circuit has high power and the conversion frequency cannot be very high.

As the conversion frequency continues to increase, the shortcomings of the PWM circuit are gradually exposed. Since the PWM circuit is a hard switching circuit, on the one hand, the voltage stress and current stress of the conversion device in the circuit are large when it is working. At the same time, the high dv/dt and di/dt in the conversion process will produce serious electromagnetic interference, which makes the electromagnetic compatibility of electrical and electronic equipment prominent; on the other hand, the problem of device turn-on and turn-off loss is gradually becoming thorny, which seriously restricts the further increase of the conversion frequency. Therefore, the soft switching circuit based on the resonance and quasi-resonance principle, the so-called zero voltage switch (ZVS) and zero current switch (ZCS) circuit came into being. It is a new type of current conversion circuit that uses resonance for phase switching, which realizes the conduction of the device at zero voltage and the shutdown at zero current, thereby greatly reducing the switching loss of the device. In this way, TRC technology + soft switching technology further improves the conversion frequency.

Later, represented by the application of voltage-controlled and hybrid fully-controlled devices such as power MOSFET and IGBT, high frequency was truly realized, with the conversion frequency reaching 100kHz~500kHz or even higher, creating conditions for electrical and electronic equipment to be more high-frequency, miniaturized, efficient, energy-saving and material-saving.

From the above description, we can know that the third stage is mainly the period of power semiconductor devices developing towards full control, modularization, integration, and intelligence, and semiconductor conversion technology developing towards high frequency. As a result, it has achieved a leap from traditional power electronics technology (thyristor and phase shift control) to modern power electronics technology (full control devices and TRC + soft switching technology), which is of epoch-making significance. The technological progress and energy and material saving brought by high frequency alone have a huge impact on reducing unit power consumption and improving comprehensive economic benefits.

Today, most of the application areas of thyristors have been or will be replaced by power integrated devices. However, thyristors cannot be replaced for the time being in the application areas of high-power and extra-high-power electrification, electrometallurgical power supplies, and high-voltage direct current transmission (HVDC) related to power systems, static dynamic reactive power compensation devices (SVC), and series controlled capacitor compensation devices (SCC).

3. The relationship between semiconductor conversion technology and power supply technology

It is unscientific to describe the relationship between semiconductor conversion technology and power supply technology as the relationship between two independent disciplines. In fact, power supply technology should belong to the category of power electronics technology, and is a small part of it. This is because:

(1) The semiconductor power conversion devices used in power supply technology belong to electrical

Power semiconductor devices;

(2) The problems that power supply technology needs to solve are still inseparable from power conversion.

Its theoretical basis is semiconductor conversion technology;

(3) Power supply technology involving AC and DC stable power supply, UPS, etc.

All of them are related to semiconductor power converters. As for AC/DC, DC/AC, AC/AC, and DC/DC conversion technologies, they are also issues that have long been solved by semiconductor conversion technology.

(4) Chemical power sources used in power technology—batteries, physical

Power sources—generators and solar cells—each belong to a discipline and an industry, and power technology is just used to use them;

(5) The issue of electromagnetic compatibility is a big topic and falls within the scope of radio technology. Power supply technology also uses general electromagnetic compatibility technology in the process of information transmission. It is mainly used to solve the electromagnetic interference problem caused by high frequency to the power supply itself and other electronic equipment.

Due to its specific application scenarios, the power of power supply technology is not very large and belongs to small and medium power. Therefore, high-frequency conversion technology based on time ratio control + soft switching technology has broad development prospects in the application of power supply technology. It is only a matter of time before it completely replaces phase-controlled conversion technology.

4Static solid-state circuit breaker

The controllability of power electronic devices in turning on and off makes people think of the possibility of using them as circuit switches. In particular, the characteristic of power electronic devices that they do not produce arcs when turned off has important use value, which is of great significance for solving power transmission and distribution problems in environments containing flammable and explosive gases and dust.

At present, low-voltage and low-power solid-state (body) switches using power electronic devices have been widely used with good results. With the forward and reverse blocking voltages of power electronic devices reaching 10kV to 12kV and the on-state current reaching 3kA or even higher, and with the continuous decline in device manufacturing costs, high-voltage static solid-state circuit breakers for power systems are now in the development and application stage. This will be another new application area for power electronic technology.