Origin of Power Converters. Tsai-Fu Wu
Additionally, EMI level can be also reduced significantly. Power converter configurations based on this type of network are called PWM power converters. Note that it requires an additional freewheeling path when switch S1 is turned off, which will be discussed in later section. For simplicity while without confusion in power electronics area, the short‐form PWM converters or converters will be used to represent the PWM power converters. They have been widely applied to various types of power conversion for their controllable power transfer, theoretically no loss, and finite component stresses.
The minimum‐order network of a PWM converter is a second‐order LC network, and it must at least include a switch to control power flow. The order of network can be increased to third, fourth, and even higher. For a valid PWM converter, the network must be always in resonant manner at either switch turn‐on or turn‐off.
Over the past century, PWM converters have been well developed and have diversified configurations, such as buck, boost, buck‐boost, Ćuk, sepic, Zeta, flyback, forward, push‐pull, half‐bridge, full‐bridge, Z‐source, neutral‐point clamped (NPC), modular multilevel, quasi‐resonant, and LLC resonant converters. They can be classified into non‐isolated and isolated configurations. Typically, the isolated versions can be derived from the non‐isolated ones by inserting a DC transformer or an AC transformer to a proper location of the converter. Thus, we will first introduce non‐isolated converters, which can lay out a firm foundation for later discussions on isolated converters.
1.3 Well‐Known PWM Converters
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