Chapter 1

Introduction

1.1 Introduction

Power electronics may be considered a revolutionary field in electrical engineering because of the new insights obtained during its development. This has actually been the case from the beginning, when mercury arc rectifiers and thyratrons were employed in grid-controlled circuits. After this first generation of power devices and converters, power electronics with silicon power diodes and thyristors was developed to overcome many of the problems of the first generation, such as the operation in low efficiency. As mentioned in Reference 1, the so-called power electronics, with gas tube and glass-bulb electronics, was known as industrial electronics, and the power electronics with silicon-controlled rectifiers began emerging in the market in the early 1960s.

The different definitions of power electronics lead to the same concept or idea: that the control of power flow between an apparatus that furnishes electrical energy and another one that demands electrical energy. For instance, the definition given in References 2 and 3 say, respectively: ".power electronics involves the study of electronic circuits intended to control the flow of electrical energy. These circuits can handle power flow at levels much higher than the individual devices ratings." and ".power electronics deal with conversion and control of electrical power with the help of electronic switching devices."

Power electronics involves several academic disciplines creating a complex system, including semiconductor physics, control theory, electronics, power systems, and circuit principles. The comprehensive aspect of power electronics makes the presentation of its contents difficult. The interdisciplinary nature of power electronics requires the integration of the practices and assumptions of all the academic disciplines involved, as well as calling for significant prerequisites on the part of the students enrolled for the course. Figure 1.1 illustrates this by analogy, with the prerequisite skills needed for a power electronics course being shown as the roots of a tree, the various power electronics devices as the trunk, and the resulting technologies and applications (power quality, renewable energy systems, etc.) as the branches.

Figure 1.1 Interdisciplinary nature and new insights obtained from power electronics.

Since the dawn of solid-state power electronics, the use of semiconductor devices has been the major technology to drive power processors. A comparison of the semiconductor devices formerly used in controlled rectifiers with new technologies underlines this dramatic development. In addition to the improvement of power switches, there has also been great activity in terms of circuit topology innovations.

A power electronic converter is the centerpiece of many electrical systems. Common applications include, but are not limited to, motor drive systems, renewable energies, robotics, electrical and hybrid vehicles, and circuits promoting power quality. These applications have required considerable research worldwide to develop semiconductor devices, configurations that process ac and dc variables, control and diagnosis, fault-tolerant systems, and the like.

In addition to the technical side mentioned already, the educational aspects have considerable importance, as students usually consider power electronics courses to be particularly difficult, perhaps because of their interdisciplinary nature. Achieving student motivation is thus a fundamental task of educators involved in the field of power electronics.

In this context, this book discusses a novel methodology for presenting an important set of power electronics converters, that is, topologies that process ac voltage. The common approach to teaching converters is to consider each type individually, in a separated and isolated manner. The direct consequence is that the learning process becomes passive as the power electronics configurations are presented without any consideration of their origin and development. Since the teaching process is based on the topology itself, students develop no ability to construct new topologies, different from the conventional ones. Section 1.2 outlines this new methodology.

1.2 Background

Although presenting the basics of power devices as well as an overview of the main power converter topologies in Chapter 2, this book focuses primarily on configurations processing ac voltage through a dc-link stage. This book is ideally suited for students who have already taken an introductory course in power electronics. It also serves as a reference book to senior undergraduate and graduate students in electrical engineering courses. However, students can easily manage despite the lack of knowledge of power devices and basic concepts of converters, because they are explained in Chapter 2.

Systems with power electronics conversion have been used to guarantee grid and load requirements in terms of controllability and efficiency of the electrical energy demanded, especially in industrial applications. Power electronics topologies convert energy from a primary source to a load (or to another source) requiring any level of processed energy.

Classifications of the power electronics topologies can be done in terms of the type of variable under control (i.e., ac or dc), as well as the number of stages of power conversions used, as observed in Fig. 1.2. Figure 1.2(a) shows, in a general way, many of the possibilities related to energy conversion. Figure 1.2(b) highlights a direct ac-ac conversion, which converts an ac voltage (v1) with a specific frequency (f1) to another ac voltage with a different (or same) voltage (v2) and frequency (f2); this converter is normally called a cycle converter. Figure 1.2(c) depicts the ac-dc or dc-ac conversion, while Fig. 1.2(d) shows a dc-dc converter. Even admitting that Fig. 1.2(e) and 1.2(f) could be considered as extended versions of the previous cases, those conversion systems (ac-dc-ac and dc-ac-dc) are presented in Fig. 1.2 because of the large use in different applications.

Figure 1.2 Power conversion: (a) all possibilities of conversion, (b) cycle converter, (c) rectifier or inverter, (d) chopper, (e) ac-dc-ac, and (f) dc-ac-dc.

Special attention is given to the conversion systems presented in Fig. 1.2(c) and 1.2(e), dealing with configurations that process ac voltage (at input and/or output converter sides) with one dc stage. A systematic approach is taken for the presentation of those configurations, instead of just showing them separately, as is normally done in a conventional presentation. Another aspect of this book is that only the subjects related to the converters themselves will be considered, which means that the contents dealing with either ac filters or transformers will be omitted. This will give more room for exploring the details of each topology and its concept. In this way, the method of conceptual construction of power electronics converters can be highlighted appropriately.

1.3 History of Power Switches and Power Converters

Configurations of power electronics converters have provided an attractive alternative for the applications needing energy processing, considering the acceptable level of losses associated with the conversion process itself, as well as improvement in reliability. As previously mentioned, power electronics converters must control the power flow, which means that the development of the devices used in those converters is crucial to guarantee the expected features. In this section, a historic view of the power electronics devices will be furnished, highlighting the main events that contributed to the current development.

The history of power electronics predates the development of the semiconductor devices employed nowadays. The first converters were conceived in the early 1900s, when the mercury arc rectifiers were introduced. Until the 1950s the devices used to build power electronics converters were grid-controlled vacuum tube rectifier, ignitron, phanotron, and thyratron. There were two important events in the power electronics development: (i) in 1948, when Bell Telephone Laboratories invented the silicon transistor, with applications in very low power devices such as in portable radios and (ii) in 1958, when the General Electric Company developed the thyristors or SCR, first using germaniums and later silicon. It was the first semiconductor power device.

Besides these two events, many developments have been achieved in terms of switching development. Between 1967 and 1977, the gate turnoff (GTO) (gate-controlled switch) and gate-assisted turnoff thyristor (GATT) (gate-assisted turnoff switch) were invented. Power transistors, MOSFETs (metal oxide semiconductor field-effect-transistors), MCTs (MOS-controlled thyristor) and IGBTs (insulated-gate-bipolar transistors) have been invented since the end of 1970s. In addition, it is worth mentioning that the area of power electronics was deeply influenced by microelectronics development, and the history of power electronics is closely related to advances in integrated circuits to control switching power supplies. Figure 1.3 depicts the timeline showing the development of power electronics devices.

Figure 1.3 Timeline of historical events in the power electronics...