Power Electronics, converters, research and Applications: A Brief overview

Power electronics is the application of solid-state electronics for the control and conversion of electric power. It also refers to a subject of research in electronic and electrical engineering which deals with design, control, computation and integration of nonlinear, time varying energy processing electronic systems with fast dynamics. The capabilities and economy of power electronics system are determined by the active devices that are available. Their characteristics and limitations are a key element in the design of power electronics systems. Formerly, the mercury arc valve, the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices. Power electronic devices may be used as switches, or as amplifiers. An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load. Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors (as well as the mercury valve and thyratron) allow control of the start of conduction, but rely on periodic reversal of current flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them. Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts. The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source. As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz. Devices such as MOSFETS and BJTs can switch at tens of kilohertz up to a few megahertz in power applications, but with decreasing power levels. Vacuum tube devices dominate high power (hundreds of kilowatts) at very high frequency (hundreds or thousands of megahertz) applications. Faster switching devices minimize energy lost in the transitions from on to off and back, but may create problems with radiated electromagnetic interference. Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating. Power electronic circuit’s process and control electrical energy, and are critical elements in many kinds of systems. The rapid evolution of technology is generating a demand for power electronics whose capabilities greatly exceed what is presently achievable. Challenges of particular importance include miniaturization and integration of power electronics, and improving their cost and dynamic performance. Miniaturization is difficult in part because the magnetic components used in most power circuits scale down poorly in size. Likewise, achieving integration and low cost is difficult because of the diverse materials and assembly methods that are required for contemporary designs. My research interest includes working to address these challenges through a combination of new technologies. One research focus is on the development of improved power passive components. Passive components such as inductors and capacitors often dominate the size and cost of power circuits, and limit their efficiency, noise attenuation, and transient performance. In one effort, we are developing means to improve the performance of passive filter components by compensating for their parasitic. These efforts have led to new integrated filter components with much better performance than conventional passives. Likewise, we are developing new types of power passive components that better scale to small sizes and high frequencies. Construction of these components using micro fabrication techniques is also being explored, with the goal of enabling integrated fabrication of power converters. A second research focus is the development of techniques to achieve greatly increased switching frequencies in power converters. Higher frequencies are desirable because they enable faster transient response and reduce passive component requirements. Moreover, at sufficiently high frequencies, batch fabrication of many circuit components may become possible, enabling higher levels of integration to be achieved. We are exploring new system architectures, circuit designs, and control methods that together enable substantial increases in operating frequency over the present state of the art. It is anticipated that the technologies under development will lead to miniaturized, highly integrated power electronics. In addition to developing fundamental power conversion technologies, we are applying them in a variety of applications. Automotive power generation and control is one such area. For example, we have investigated the application of power electronics to enhance the efficiency, power, and transient performance of automotive alternators. We have also developed dc/dc converters and other power electronics for automotive applications, with the goal of enabling improved performance, safety, and comfort in vehicles. Other areas of interest include power components and circuits for industrial, commercial, consumer, and medical applications where improved size, efficiency, and performance are of importance. Applications of power electronics range in size from a switched mode power supply in an AC adapter, battery chargers, fluorescent lamp ballasts, through variable frequency drives and DC motor drives used to operate pumps, fans, and manufacturing machinery, up to gigawatt-scale high voltage direct current power transmission systems used to interconnect electrical grids. Power electronic systems are found in virtually every electronic device. For example: • DC/DC converters are used in most mobile devices (mobile phones, PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of the battery is. These converters are also used for electronic isolation and power factor correction. A power optimizer is a type of DC/DC converter developed to maximize the energy harvest from solar photovoltaic or wind turbine systems. • AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television etc.). These may simply change AC to DC or can also change the voltage level as part of their operation. • AC/AC converters are used to change either the voltage level or the frequency (international power adapters, light dimmer). In power distribution networks AC/AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids. • DC/AC converters (inverters) are used primarily in UPS or renewable energy systems or emergency lighting systems. Mains power charges the

DC battery. If the mains fail, an inverter produces AC electricity at mains voltage from the DC battery. Solar inverter, both smaller string and larger central inverters, as well as solar micro-inverter are used in photovoltaic as a component of a PV system. Motor drives are found in pumps, blowers, and mill drives for textile, paper, cement and other such facilities. Drives may be used for power conversion and for motion control. For AC motors, applications include variable-frequency drives, motor soft starters and excitation systems. In hybrid electric vehicles (HEVs), power electronics are used in two formats: series hybrid and parallel hybrid. The difference between a series hybrid and a parallel hybrid is the relationship of the electric motor to the internal combustion engine (ICE). Devices used in electric vehicles consist mostly of dc/dc converters for battery charging and dc/ac converters to power the propulsion motor. Electric trains use power electronic devices to obtain power, as well as for vector control using pulse width modulation (PWM) rectifiers. The trains obtain their power from power lines. Another new usage for power electronics is in elevator systems. These systems may use thyristors, inverters, permanent magnet motors, or various hybrid systems that incorporate PWM systems and standard motors.