alone or capacitor–inductor pair. If it requires galvanic isolation, a transformer is introduced into the converter. Additionally, the transformer provides another degree of freedom in tuning input‐to‐output voltage ratio and can implement multiple outputs readily. To fulfill multiple functions or increase power capacity, converters can be connected in series or parallel, which will complicate analysis, design, and control.
Figure 1.3 Block diagram of a switching regulator.
As shown in Figure 1.4, connecting switch(es) and capacitors/inductors to form a power converter sounds simple. However, how to configure a power converter to achieve step‐up, step‐down, and step‐up/step‐down DC output, AC output, PWM control, variable frequency control, etc. is not an easy task. Even with the same step‐up/step‐down transfer ratio, there exist different converter topologies, and they might have different dynamic performances and different component stresses. Among the four types of power converter topologies is the DC to DC, simplified to DC/DC, converter type relatively popular. In the following, we will first present how to figure out the derivation of DC/DC converter topologies, on which the rest of converter types will be discussed. Exploring systematic approaches to developing power converter topologies is the unique feature of this book.
Figure 1.4 Possible components in a power converter.
1.2 Non‐PWM Converters Versus PWM Converters
In power converters, when switch turns on with infinite current through or infinite voltage across components, this is because there is no current‐limiting or voltage‐blocking components in the conduction path, resulting in severe electromagnetic interference (EMI) problems. This type of power converter cannot be controlled with PWM and is called a non‐PWM converter. On the contrary, there exist current‐limiting and voltage‐blocking components in the conduction path of a power converter, and it can be controlled with PWM, which is called a PWM converter. This claim will be presented and illustrated with some power converter examples, as follows.
1.2.1 Non‐PWM Converters
The major concern of a power converter is its input–output conversion efficiency. In practice, there is no resistor allowed in a converter configuration. A qualified converter includes only ideal switch(es) and capacitor(s)/inductor(s). However, even with these components only, there might still exist loss during power transfer, such as the converters shown in Figure 1.5a and b. Figure 1.5a shows power transfer between two capacitors, and it is controlled by switch S1. Assuming capacitor C1 is associated with an initial voltage of Vo and C2 is with zero voltage, and capacitance C1 = C2, it can be shown that there is an electrical energy loss,
Similarly, the conceptual inductor–inductor–switch configuration shown in Figure 1.5b has the same limitations. If, initially, inductor L1 carries a current of Io but there is no current in L2, after turning on switch S1, there will be an extremely high impulse voltage across the inductors and the switch, causing EMI problems and damage to the components. Again, there is half electrical energy loss
Figure 1.5 (a) Capacitor–capacitor–switch, (b) inductor–inductor–switch, and (c) capacitor–inductor–switch networks.
In summary, non‐PWM converters come out high inrush current or high impulse voltage, resulting in high EMI, as well as high component stress, and they could yield low conversion efficiency even with ideal components. In particular, under large initial voltage difference, the maximum electrical energy loss can be as high as 50%.
Other examples adopting the configuration shown in Figure 1.5a are shown in Figure 1.6. Figure 1.6a shows a two‐lift converter. When switches S1 and S2 are turned on, capacitor C1 will charge C2 directly. On the other hand, when switch S3 and S4 are turned on, capacitors C1 and C2 are connected in series to charge capacitor C3 and lift the output voltage Vo to be twice the input voltage Vi. It can be seen that during capacitor charging, there is no current limiter, resulting in high inrush current. Figure 1.6b shows the KY converter. When switch S2 is turned on, input voltage Vi will charge capacitor C1 through diode D1 but again without current limiter. When switch S2 is turned off and S1 is turned on, input voltage Vi together with capacitor voltage VC will magnetize inductor L1 through the output path. This path of power flow is with the current limiter of inductor L1. Figure 1.6c shows a re‐lift converter. When switch S1 is turned on, there are two capacitor charging paths without current limiter, Vi‐S1‐D2‐C3‐D3‐Vi and Vi‐S1‐D21‐C12‐D11‐Vi. When switch S1 is turned off, the energy stored in capacitors C3 and C12 will be released to the output through the inductors and capacitors, which are the current limiters.
