(a) flyback, (b) forward, (c) push‐pull, and (d) quasi‐resonant flyback.
Converters with isolation transformers have many unique features, which include realizing multiple outputs, achieving galvanic isolation, providing one more degree of freedom in stepping down or stepping up voltage ratio, and protecting the components on the secondary side from damage by the high input voltage on the primary side. In literature, there are a big bunch of converters with isolation.
Each PWM converter has at least an inductor. With a coupled inductor, the converter can be modified to a new version. Figure 1.15 shows four PWM converters with coupled inductors, and they are derived from buck, boost, Ćuk, and buck‐flyback converters. In the converters shown in Figure 1.15a and b, they just simply introduce a secondary winding into the converter itself and place at a proper path where the magnetization and demagnetization of the inductor satisfies the volt‐second balance principle. Figure 1.15c shows the Ćuk converter in the form with a coupled inductor. Originally, the Ćuk converter has two separate inductors. Analyzing the operation of the converter will realize that the two inductors can be coupled with each other. Other examples are sepic and Zeta converters, of which there are two inductors in each converter and they can be coupled and wound on the same core. A converter with coupled inductors will reduce one degree of dynamic order. Can all of converters with two or more inductors be constructed with coupled inductors? How to place a secondary winding in a proper path in the converter is another issue, which needs to discuss further.
Figure 1.15 PWM converters with coupled inductors: (a) buck type, (b) boost type, (c) Ćuk type, and (d) buck‐flyback type.
Figure 1.15d shows a buck and a flyback combined converter with coupled inductors to form an isolated output VoA, which is the flyback‐type output, while output Vo is just the regular buck output without isolation. Comparing the converters shown in Figure 1.15a and d reveals that the coupled winding can be connected back to the converter itself or to a separate network, which can be isolated from the primary side. It is quite diversified when introducing coupled winding(s) to the converters. What is the mechanism behind in developing such kind of converters?
1.4 Approaches to Converter Development
In last section, many well‐known PWM converters were introduced, but many questions were also brought up. A general question is that how to develop the converters systematically. In this section, several typical approaches are described briefly for later discussion.
From the converters shown in Figures 1.7 and 1.8a, one can observe that the active and passive switches have a common node. Thus, a switching cell concept was introduced to explain the configurations of the converters. There are two types of switching cells, P cell and N cell, as shown in Figure 1.16, and each has two terminals for connecting to source or output and one current terminal for connecting to inductor to form a PWM converter. For examples, buck converter can be derived from P cell, while boost converter can be derived from N cell. This is a kind of intuitive approach with induction but without manipulation on the converters. In fact, with a little bit of manipulation by relocating capacitor C1 in the sepic and Zeta converters shown in Figure 1.8b and c from the forward path to the return path, a P cell and an N cell can be identified, and the converters can be derived accordingly. Typically, this approach is used to explain the existing converter configurations, but it is hard to develop new converters. It is based on a cell level.
Similarly, based on observation and induction, the converters shown in Figures 1.7 and 1.8 can be explained with two canonical switching cells, namely, Tee canonical cell and Pi canonical cell, as shown in Figure 1.17. By exhaustively enumerating all of possible combinations of Zin, Zout, and Zx, and including LC network, source, and load, the converters shown in Figures 1.7 and 1.8 can be derived. Additionally, the converters with extra LC filters and inverse versions of the converters can be derived. Figure 1.18 shows the buck and boost converters and their inverse versions, in which the input‐to‐output transfer ratios shown in the bottom of the converters are corresponding to continuous conduction mode, and D is the duty ratio of the active switch. Moreover, the converters with the same higher step‐up voltage transfer ratio but with different circuit configurations, as shown in Figure 1.19, can be developed. Although this approach can derive new PWM converters and is straightforward, it is still tedious and lacks of mechanism to explain the converters with identical transfer ratio but with different configurations. Again, it is based on a cell level.
Figure 1.16 (a) P cell and (b) N cell.
Figure 1.17 (a) Tee canonical cell and (b) Pi canonical cell.
Figure 1.18 (a) Buck and inverse buck and (b) boost and inverse boost.
Figure 1.19 With the same input‐to‐output transfer ratio of (2D − 1)/(1 − D) but with different configurations (a) and (b).
Based on a cell level, another approach to developing new converters with higher step‐up and step‐down voltage ratios by introducing switched‐capacitor or switched‐inductor cells to the PWM converters shown in Figures 1.7 and 1.8 was proposed. Typical switched‐capacitor and switched‐inductor cells are shown in
