Power Electronics for Energy Efficiency

Power electronics deals with the conversion and control of electrical energy with the help of power semiconductor devices that operate in a switching mode, and, therefore, efficiency of power electronic apparatus may approach as high as 98–99%.

Power electronics is one of the contemporary subjects of electrical engineering, which has seen a lot of advancements in recent times and has impacted human life in almost every sphere. We use so many power electronic applications in our daily life without even realizing it. From laptop computer to kitchen appliances at home, most of the electronic equipment that makes our lives so convenient today relies on gadgets called power electronics. This technology uses electronic components such as inverters and transformers to convert the electricity from wall outlet into the right voltage and current to power each of handy or handheld devices. It is also used more widely to distribute electricity throughout the power grid to connect renewable energy systems, and to charge up electric vehicles. Electrical energy has already become our lifeline so we want to get a regulated form of this energy which in itself is not usable until it is converted into a tangible form of energy such as motion, light, sound, heat etc. In order to regulate these forms of energy, an effective way is to regulate the electrical energy itself and this forms the content of the subject power electronics. Power electronics is the study of switching electronic circuits in order to control the flow of electrical energy, its generation, transportation and applications. Power Electronics is the technology behind switching power supplies, power converters, power inverters, motor drives, and motor soft starters. Power electronics is an enabling technology, its development, together with internal developments, such as wide bandgap semiconductors, will be driven externally by applications in the future.

Power electronics deals with the conversion and control of electrical energy with the help of power semiconductor devices that operate in a switching mode, and, therefore, efficiency of power electronic apparatus may approach as high as 98–99%. Power electronics is very important in modern high-efficiency energy processing systems, such as HVDC, SVC, flexible ac transmission system (FACTS) for active and reactive power flow control, uninterruptible power supply (UPS), and industrial process control with variable-frequency drives for improving productivity and quality of products in modern automated factories. Variable-frequency drives are now used extensively in pumps and compressor drives, paper and textile mill drives, subway and locomotive propulsion, electric and hybrid vehicles, elevators, metal rolling and textile mills, home appliances, machine tools, and robotics, variable-speed air conditioners, wind generation systems, ship propulsion. Power electronics plays a very important role in solving or mitigating our global warming problem, which is a very serious concern in our society.

Power Electronic Devices

We can trace the overwhelming advancement in the subject back to the development of commercial thyristors or silicon controlled rectifiers (SCR). Before this the control of electrical energy was mainly done using thyratrons and mercury arc rectifiers which work on the principle of physical phenomena in gases and vapours. After SCR, a great many power electronic devices have emerged like GTO, IGBT, SIT, MCT, TRIAC, DIAC, IEGT, IGCT and so on. These devices are rated for several hundreds of volt and ampere unlike the signal level devices which work at few volts and mill amperes. In order to achieve the purpose of power electronics, the devices are made to work as nothing more than a switch. All the power electronic devices act as a switch and have two modes, i.e. ON and OFF. For example, a BJT (Bipolar Junction Transistor) has three regions of operation in its output characteristics cut-off, active and saturation. In analogue electronics where the BJT is supposed to work as an amplifier, the circuit is so designed to bias it in active region of operation. However, in power electronics BJT will work in cutoff region when it is OFF and in saturation region when it is ON. Now that the devices are required to work as a switch, they must follow the basic characteristic of a switch, i.e. when the switch is ON, it has zero voltage drop across it and carries full current through it, and when it is in OFF condition, it has full voltage drop across it and zero current flowing through it. The power electronic devices alone are not that useful in practical applications and hence require to be designed with a circuit along with other supporting components. These supporting components are like the decision making part which controls the power electronic switches in order to achieve the desired output. This includes the firing circuit and the feedback circuit. The Control Unit takes the output feedback from sensors and compares it with references and accordingly gives input to the firing circuit. Firing circuit is basically a pulse generating circuit which gives pulse output in a fashion so as to control the power electronic switches in the main circuit block. The net result is that the load receives the desired electrical power and hence delivers the desired result. A typical example of the above system would be speed control of motors.

Si vs. GaN vs. SiC

Role of Materials

Power electronics plays an important role in our today society. It is present almost all power supplies, in servo systems and variable drives. The flexible energy conversion of power electronics is also present in energy saving applications and in almost all renewable energy applications, from small photovoltaic to large wind turbines. It is also penetrating our transport means, from railway to electrical bikes, ship propulsion to more performing and electric cars. Silicon (Si), gallium- nitride (GaN), and silicon-carbide (SiC) are the commonly used materials for power electronic devices with their specific advantages. SiC is also used to manufacture power transistors, but because SiC does not have an electron-gas structure, only vertical conduction devices are practical. With a vertical conduction device in GaN or SiC, 1- to 2-kV breakdown voltage levels are easier to reach than with Si. As silicon (Si), gallium-nitride (GaN), and silicon-carbide (SiC) processes are maturing, so, too, are their suppliers’ expertise and creativity towards power electronics. GaN and SiC are wide-bandgap (WBG) materials, which mean the energy required for an electron to jump from the top of the valence band to the bottom of the conduction band within the semiconductor is typically larger than one or two electron volts (eV). GaN transistors are extremely fast. As a result, the system is far more sensitive to the layout than it is with slower Si devices. In particular, stray inductance plays a larger role in the overall system efficiency. GaN needs no package—it is inert to its environment. This approach greatly reduces any resistive, inductive, and thermal problems.

Researchers have discovered that when two oxide compounds—strontium titanate (STO) and neodymium titanate (NTO)—are joined together, they make an extraordinary conductive material. The main application for a material with this level of conductivity would be in power transistors that regulate electrical current in electronic devices such as televisions and cellphones. The researchers have shown that these two materials—which on their own operate as insulators—are up to five times more conductive than silicon. Currently, gallium nitride serves as the material used for transistors in power supplies. While gallium nitride is highly conductive as well, many believe that years of optimization have brought the material to its limits. This new STO/NTO material still has room for further optimization and improvement and eventually may serve as an attractive replacement for gallium nitride. These materials could be used in power transistors to enable much smaller devices, because the power supplies would be far more efficient. Take the external power supplies that come with our laptops and the big black box halfway down the electrical cord. By building far smaller power supplies inside the laptop, the need for these large external power supplies could be eliminated.

Future Visions

Technologies have specific life cycles that are driven by internal innovation, subsequently reaching maturity. Power electronics appears to be a much more complex case, functioning as an enabling technology. Till now, the development of power electronics has been driven chiefly by internal semiconductor technology and converter circuit technology, approaching maturity in its internally set metrics, such as efficiency. Critically examination of the fundamental functions found in electronic energy processing, the constituent technologies comprising power electronics, and the power electronics technology space in light of the internal driving philosophy of power electronics and its historical development is very important. Although with the approaching limits of its internal metrics indicates internal maturity, the external constituent technologies of packaging, manufacturing, electromagnetic and physical impact, and converter control technology still present opportunities for development.

The primary trends are: increased efficiency; higher power density; and cost reduction. With cost concerns, efficiency requirements and need of environmental friendly technology, power supply efficiency is a key selection criterion and is supported by legislation.

Independent of the environmental angle, OEMs are seeking to increase the performance of their end equipment and are consequently looking for power supplies that dissipate less heat and take up less space. There are several methods to improving efficiency and these include: developing new topologies; improved power devices, control of electronic and magnetic component designs entering the market; the availability of new materials; and the application of digital control loops. Depending on the type of power supply and its end use, some or all of these methods will be used. For lower power applications, it’s all about high power density with more and more power being claimed on industry standard pc board sizes, such as 2 x 4in and 3 x 5in. For cost reasons, a flyback circuit is commonly used but this has its limitations in terms of achieving efficiency improvements. New chips are now available that enable efficiencies of up to 92%, although one must appreciate that at an efficiency of 92%, a 200W power supply will still dissipate 27% more excess power than a previous generation 100W product at 88% efficiency. Indeed, many components will be larger for the 200W power supply so, if both are designed to be the same size, then the parts are packed much more tightly and significant thermal challenges arise.

Addressing the growing energy challenges faced by our society requires advances in how we create, manipulate, store, and utilize electrical energy. Energy-processing circuits – or power electronics – are a key element in each of these areas. Researchers are exploring how power electronics can be better designed and applied to meet the energy needs of our society. For example, ongoing research explores the design of power electronics to better extract energy from solar, mechanical, and thermal sources. Likewise, the development of power electronics to improve efficiency and energy utilization is being explored in applications ranging from lighting to computation to communications. To meet the needs of future systems, it is important to miniaturize and better integrate power electronic circuits. The size and cost of power conversion circuitry is a major factor preventing improved energy utilization and efficiency in many applications. Moreover, power electronics are not easily integrated with other electronic elements, and often limit the miniaturization of entire systems. Miniaturization and integration of power electronics are difficult because the necessary energy storage components scale down poorly in size and are not well suited to the planar geometries of most integrated fabrication processes.

Researchers are working to develop power electronics providing miniaturization and integration. A key focus of this work is the development of system architectures and circuit topologies that permit greatly increased operating frequencies. Higher frequencies are desirable because they reduce energy storage requirements, thereby reducing size and enabling better component integration. However, higher frequencies have traditionally been associated with major practical obstacles, including low efficiency. New circuit designs under development will greatly reduce frequency-dependent losses by recovering energy that is traditionally lost in device switching and gating. These designs also seek to eliminate fixed loss components that reduce light load efficiency. Additional research focuses on design of semiconductor devices and passive components that are compatible with these circuits and that operate efficiently at very high frequencies. Together, these approaches enable up to two orders of magnitude increase in operating frequency, with commensurate improvements in energy storage. It is anticipated that such design approaches will enable small, highly integrated power controls that benefit size, efficiency and energy utilization in a tremendous range of future systems.

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