Читать книгу Origin of Power Converters - Tsai-Fu Wu - Страница 19

Almost all people entering power electronics field know about buck, boost, and buck‐boost converters, as shown in Figure 1.7. To my best knowledge, it is unknown that who invented the buck converter and when it was invented. Since electricity started to be used frequently between the late nineteenth century and the early twentieth century, the invention of the buck converter was designated as year 1900. The boost converter was invented during World War II, which was used to boost voltage for transmitting radio signals across Atlantic Ocean. The buck‐boost converter was invented around 1950.

Analyzing their operational principles will realize that the buck, boost, and buck‐boost converters can achieve step‐down, step‐up, and step‐down/step‐up input‐to‐output voltage conversions, respectively. They all have a second‐order LC network and a pair of active–passive switches but have different circuit configurations.

If we explore further, there are another three famous converters, and each of which has a fourth‐order LC network and a pair of active–passive switches, as shown in Figure 1.8, in which they have different circuit configurations, but they all can fulfill the same step‐down/step‐up voltage conversion. Ćuk converter was invented by Prof. S. Ćuk in 1975. Sepic is an acronym of single‐ended primary inductor converter, which was invented in 1977. Zeta (dual sepic) converter was introduced in 1989.

Figure 1.7 Power converters with a second‐order LC network and a pair of active–passive switches: (a) buck converter, (b) boost converter, and (c) buck‐boost converter.

Figure 1.8 Power converters with a fourth‐order LC network and a pair of active–passive switches: (a) Ćuk converter, (b) sepic converter, and (c) Zeta converter.

Couples of questions come to our minds. Converter configurations are so diversified: thus, how to connect the components to become a converter, how to know ahead that the converter can achieve a step‐down or step‐up voltage conversion, why researchers spent around one century to develop these six PWM converters shown in Figures 1.7 and 1.8, does there exist an origin of power converters from which the rest of PWM converters can be evolved and derived systematically, and so on?

Based on the three PWM converters shown in Figure 1.7, three types of converters with a fourth‐order LC network can be derived, as shown in Figure 1.9. Again, some questions come to our minds: What is the difference between the converters shown in Figures 1.7 and 1.9, can we generate new converters by keeping on introducing extra LC networks into the old converters, what is the role of L₂C₂ network in Figure 1.9, how to verify a valid converter, etc.?

Figure 1.9 Converters with a fourth‐order network: (a) buck derived, (b) boost derived, and (c) buck‐boost derived.

With switched inductors or capacitors, some of the PWM converters shown in Figures 1.7 and 1.8 can be modified to the ones shown in Figure 1.10, which are called switched‐inductor/switched‐capacitor hybrid converters. They can achieve higher step‐down or step‐up voltage conversion than their original counterparts. In each of the converters, there are one active switch and two passive diodes with either a third‐order or a fifth‐order LC network. It looks like that a diode‐inductor or diode‐capacitor cell is inserted into a certain PWM converter to form a new one. It is curious to ask why the concept cannot be applied to all of the six PWM converters 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 V_i, 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, L_RC_S 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 L_RC_S 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 L_R 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 L_RC_S 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.

Figure 1.14 Isolated PWM converters: (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 V_oA, which is the flyback‐type output, while output V_o 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?