Power Electronics

No doubt that power electronics is now considered one of the most vital enabling technologies in electrical engineering. In fact, large part of all electrically powered devices, circuits, or systems has close connection with the field of power electronics. Its scope is broad and covers very wide spectrum, with the paramount among them is its ever-increasing role in integrating renewable energy sources and electric storage to the grid. Power electronics is the “glue” that makes the ushering of a new kind of smart energy technology revolution possible. It is because of the engineering field of power electronics that we are able to encompass the efficient and cost-effective use of electronic components, circuit and control theory, modern analytical tools, and design techniques to make this smart energy revolution possible. This revolution will modernize our electric grid, give birth to massive electric transportation, allow for large solar energy penetration, help solve climate change, and enable the deployment of the highest possible energy efficiency systems. In short, power electronics has emerged as the enabling technology that transformed the field of energy and power engineering from a high-tech frontier to smart-tech frontier. Arriving at today’s remarkable important role of power electronics took more than 100 years of innovation and hard work by many scientists and engineers coupled with strong partnerships between the private sector, professional societies, and governments.

In this chapter, several general concepts will be discussed to provide necessary background material in power electronics. The review material will cover switching diode circuits, power computation, harmonic analysis, and component concepts.

In this chapter we discuss converter circuits that are used in power electronic circuits and systems to change the voltages from one dc level to another dc level. Once again, switching devices will be used to process energy from the input to the output. Since the input here is dc, which comes from a post-filtering stage, these devices are normally operated at much higher frequencies than the line frequency, reaching as high as a few hundred kilohertz. This is why such converter circuits are known as high-frequency dc-dc switching converters or regulators. The term regulator is used since the circuit’s main commercial application is in systems that require a stable and regulated dc output voltage. Depending on whether or not an output transformer is used, high-frequency dc-dc switching converters are classified as isolated or non-isolated. In this chapter and in the next, the emphasis will be on the steady-state analysis and design of several well-known second- and fourth-order dc-dc converters, each having its own features and applications. We will consider those topologies that do not use high-frequency isolation transformers as part of their power stage. Moreover, a large number of applications require output electrical isolation and multiple outputs that cannot be achieved using the basic topologies discussed in this chapter. The isolated and magnetically coupled topologies will be discussed in Chap. 5. Such topologies are the most popular in the power supply industry and are used in various types of electronic equipment whose design requires outputs with electrical isolation and multi-outputs.

The discussion in Chap. 4 showed that the four basic converter topologies have their output conversions determined by the duty ratio, D, and that they consist of a single input and a single output with a common reference point. For applications in which the output voltage does not differ from the input voltage by a large factor, those topologies can be used. However, for many applications, the input voltage is normally derived from an off-line half- or full-wave rectifier, and the output voltage is normally a very small fraction of the rectified dc input voltage. As a result, transformers must be added between the power stage and the output for the purpose of voltage scaling. Unlike line-frequency transformers, these transformers are high frequency and much smaller in size and weight. In addition, transformers are used in switching mode converters for electrical isolation between the input and output; reduction of stresses in switching devices; and provision multi-output connections. With isolation transformers, the output voltage polarity reversal does not become a design restriction. Also in some application, system isolation may be required by the certain regulatory body. However, the benefits obtained by adding isolation transformers come with a price. The major drawbacks include high converter volume and weight, reduced efficiency, and added circuit complexity to limit the effect of leakage inductance and avoid core saturation.

A class of dc-dc converters, known in the literature as soft-switching resonant converters, has been thoroughly investigated in recent years for its various attractive features. Soft switching means that one or more power switches in a dc-dc converter have either the turn-on or turn-off switching losses eliminated. This is in contrast to hard switching, where both turn-on and turnoff of the power switches are done at high current and high voltage levels. One approach is to create a full-resonance phenomenon within the converter through series or parallel combinations of resonant components. Such converters are generally known as resonant converters. Another approach is to use a conventional PWM buck converter, boost, buck-boost, Cuk, and SEPIC and replace the switch with a resonant switch that accomplishes the loss elimination. Because of the nature of the PWM circuit, resonance occurs for a shorter time interval compared to the full-resonance case. This class of converters, combining resonance and PWM, is appropriately known as quasi-resonance converters. In this chapter, our focus will be on the latter method, mainly using the resonance PWM switch to achieve soft switching. For simplicity, here we use the term soft switching to refer to dc-dc converters, quasi-resonance converters, and other topologies that employ resonance to reduce switching losses. Two major techniques are employed to achieve soft switching: zero-current switching (ZCS) and zero-voltage switching (ZVS). This chapter will focus on ZCS and ZVS types of PWM dc-dc resonant switches and their steady-state analyses.

We saw in the preceding chapter how diodes can be used to rectify an ac input voltage to produce an uncontrolled dc output. These circuits—whether half-wave or full-wave configurations under resistive, inductive, or capacitive loads—have one common feature: The level of the output voltage is a function only of the circuit parameters and the peak voltage and frequency of the applied voltage source. For this reason such circuits are known as uncontrolled rectifier circuits. In this chapter, controlled rectifier circuits, using the silicon-controlled rectifier (SCR) instead of the diode, will be discussed. Unlike in diode rectifier circuits, in controlled circuits, the power may flow from the load side (dc side) to the source side (ac side) under some control condition. This negative direction of power flow is known as inversion, and the circuits are known as controlled inverter circuits.