shown in Figures 1.7 and 1.8, and how do the inventors know ahead they can achieve higher step‐down or step‐up voltage conversion? Moreover, can this concept be extended to all of other PWM converters, and what is the converter derivation mechanism behind?
Figure 1.10 Converters with a switched inductor/capacitor: (a) buck derived, (b) Ćuk derived, and (c) sepic derived.
In literature, there are several types of Z‐source converters, which have been widely applied to DC/DC and DC/AC power conversion. They are voltage‐fed, current‐fed, and quasi‐Z‐source converters, as shown in Figure 1.11, and each of which includes only one active–passive switch pair but has higher order LC network. The circuit configurations look quite different from the ones shown in Figures 1.7–1.10 and somehow look weird. For instance, a rectifier diode D1 is connected in series with a DC voltage source Vi, as shown in Figure 1.11a, and the inductor‐diode pair shown in Figure 1.10a is replaced with an LC network pair. Moreover, the output voltage of a Z‐source converter becomes negative under certain range of duty ratios, which will be discussed in Chapter 7. If the converter derivation is just based on trial and error, there are thousands of circuit combinations, and thus, it is almost impossible to derive a valid converter without a systematic mechanism.
Figure 1.11 (a) Voltage‐fed, (b) current‐fed, and (c) quasi‐Z‐source converters.
Quasi‐resonant converters were developed in the earlier 1980s by introducing LC resonant cells to PWM converters. Figure 1.12 shows quasi‐resonant buck, quasi‐resonant boost, and quasi‐resonant Zeta converters, which can achieve zero‐voltage switching at switch turn‐on transition. By following the same mechanism, the rest of PWM converters shown in Figures 1.7 and 1.8 can be transformed to their counterparts, quasi‐resonant converters. In Figure 1.12a and b, there are two LC pairs, LRCS and L1C1, in each converter, but their natural resonant frequencies are in different orders. They play different roles in the converter operation. Without the component values and without specifying the operational principle, it is hard to tell the difference between LRCS and L1C1 from the circuit configuration, although they are derived from the conventional PWM converters with L1C1 network only. It increases one more degree of difficulty in developing power converters.
For the quasi‐resonant converters, the power transfer from input to output is still based on LC network and active–passive switch pair, and it can be pulse‐width modulated. However, the current flow in LR can be bidirectional and has higher resonant frequency, while the one in inductor L1 is unidirectional only. How to construct this type of quasi‐resonant converters is worth further discussing. In literature, there are similar converters, such as zero‐current switching quasi‐resonant converters and multi‐resonant converters. Can they be developed with a systematical approach?
Figure 1.12 Quasi‐resonant converters: (a) buck type, (b) boost type, and (c) Zeta type.
PWM converters can have more pairs of active–passive switches, such as the half‐bridge and full‐bridge converters shown in Figure 1.13. Figure 1.13a shows a half‐bridge configuration, which has two pairs of switches. The two switches take turn conducting, and each one takes care of one‐half switching cycle. In each half cycle, the switch is pulse‐width modulated to control power flow from the input to the output. If the natural frequency of the L1C1 network is designed to be far below the switching frequency, the converter is just like a conventional PWM converter. On the other hand, if the frequency is close to the switching frequency, the current and voltage waveforms are sinusoidal‐like, and it is called a resonant converter. In fact, it is still belonged to a PWM converter but just with variable frequency operation. In general, it is also classified as a PWM converter, because its power transfer is still limited by an LC network. Figure 1.13b shows a full‐bridge converter, in which there are four switches and they form two pairs, S1&S4 and S2&S3. When these two pairs of switches take turn conducting or are in bipolar operation, the converter is the same as the half‐bridge one. Again, it can act as a conventional PWM or a resonant converter depending on the order of the LC network natural frequency. This is also classified as a PWM converter.
All of the converters discussed above are non‐isolated. By introducing transformers into the non‐isolated versions of PWM converters, they can be transformed to their isolated counterparts. Figure 1.14 shows four isolated converters, flyback, forward, push‐pull, and quasi‐resonant flyback. With a transformer, several secondary windings can be wound on the same core to form multiple outputs, such as the ones shown in Figure 1.14a and b. The one shown in Figure 1.14c is derived from a buck converter with a DC transformer, and Figure 1.14d shows a flyback with an LRCS resonant network to form a quasi‐resonant converter. Thus, it can be observed that combining the fundamental PWM converters with other components can yield new converters.
Figure 1.13 Converters with multiple pairs of active–passive switches: (a) half‐bridge and (b) full‐bridge configurations.
