D–S node for switches S1 and S2, as shown in Figure 1.25b. Then, we replace S1 and S2 with a Π‐type grafted switch S12, as shown in Figure 1.25c. Since the currents through switches S1 and S2 are identical when they are operated in unison, the two circulating‐current diodes DF1 and DF2 can be removed from the Π‐type grafted switch, and the circuit becomes the one shown in Figure 1.25d. Note that detailed explanation for the diode degeneration will be presented in later chapter. From the circuit shown in Figure 1.25d, we can recognize that diodes D1 and D2 are just in series connection, and they can be replaced with a single one D12, as shown in Figure 1.25e. By redrawing the circuit, we can have the one shown in Figure 1.25f. Since switches S12 and diode D12 can be moved from the return path to the forward path without changing its operational principle, we can have a well‐known form of the buck‐boost converter shown in Figure 1.25g. Note that the output voltage polarity is naturally different from that of the input through the switch integration, without the need of extra words to explain that.
Figure 1.24 Four types of grafted switches: (a) T‐type, (b) inverse T‐type, (c) Π‐type, and (d) inverse Π‐type.
Note that for converter applications, the buck and the boost converters in cascade and with a simplified filter shown in Figure 1.25a are also feasible for their lower voltage stresses imposed on switches S1&S2 and diodes D1&D2, and Vi and Vo have a common voltage polarity, even though the cascaded converter requires two pairs of active–passive switches and more switch drivers.
Figure 1.25 Illustration of the buck‐boost converter derived with the graft switch technique.
The GST starts from dealing with two active switches, but in fact, it can be extended to any number of switches operated synchronously and with at least a common node. Moreover, with a transfer‐ratio decoding process, the GST can be applied to derive other PWM converters, including the converters shown in Figures 1.8–1.13.
Figure 1.26 Derivation of the buck‐boost converter with the converter layer technique.
Based on a transfer‐ratio decoding process, the converter layer technique (CLT) was proposed. With the CLT, the buck‐boost converter can be derived readily, as shown in Figure 1.26. Figure 1.26a shows a buck converter with its input‐to‐output voltage transfer ratio D. With a positive unity output feedback to the input, the input‐to‐output voltage transfer ratio can be determined as Vo/Vi = D/(1 − D), as shown in Figure 1.26b, which can be realized with the buck converter and a unity output feedback shown in Figure 1.26c. Redrawing the circuit in a well‐known form can be recognized as a buck‐boost converter, as shown in Figure 1.26d.
The GST can be equivalent to a converter feedforward approach, while the CLT is a feedback scheme. With these two techniques together, many new PWM converters can be derived. When further associated with the transfer‐ratio decoding process, more converters can be synthesized, and readers can understand the converter evolution mechanism or principle comprehensively.
1.5 Evolution
In “On the Origin of Species” by Charles Darwin, evolution is the primary principle of generating offsprings from the original species through natural selection. Evolution is the process of change in all forms of life over generations. Biological populations evolve through genetic changes that correspond to changes in the organisms' observable traits. Genetic changes include mutations, which are caused by damage or replication errors in organisms' DNA. As the genetic variation of a population drifts randomly over generations, natural selection gradually leads traits to become more or less common based on the relative reproductive success of organisms with those traits. The above statements are digested from “Introduction to Evolution” in Wikipedia. DNA carries codes for heredity through the mechanisms of replication, mutation, and natural selection in diverging species from the original one. Offsprings decode parental DNA for synthesizing all tissues and organs. In general, we are all a family and have the same ancestor.
Analogously, this book is entitled “Origin of Power Converters,” and we are searching for possible similar mechanisms for evolving power converters from the original converter. We will develop the decoding and synthesizing mechanisms for evolving power converters artificially. Additionally, we will make use of fundamental circuit theories to extend the converters with soft‐switching features and isolation.
1.6 About the Text
The objective of this book is to present approaches to decoding, synthesizing, and modeling PWM converters systematically and to provide readers a comprehensive understanding of converter evolution from the original converter. This book is divided into two parts.
1.6.1 Part I: Decoding and Synthesizing
Part I includes 11 chapters. They present an introduction, discovery of the original converter, some fundamentals related to power converter synthesis and evolution, illustration of converter synthesis approaches, synthesis of multistage/multilevel converters, extension of hard‐switching converters to soft‐switching ones, and determination of switch‐voltage stresses in the converters. Converters evolved from the original converter are the primary concept developed in this book.
Chapter 2 presents three approaches to creating the origin of power converters, including source–load, proton–neutron–meson, and resonant approaches. In addition, it reviews the properties and typical operation of three conventional PWM converters. Moreover, the conventional topological duality and current source
