## 3 Resonant Converter Families
### Structure of Resonant Power Converter
#### 3.1.2 Resonant Tank Network (RTN)  
The second stage of the resonant converter is a resonant tank, also termed a resonant circuit. It is considered one of the most critical parts of a resonant power converter. This network contains an inductive and capacitor (LC) circuit (reactive element) that stores electricity, which oscillates with the frequency of the resonant circuit. The movement of energy between the capacitor yields resonance for the inductor in the LC circuit. The electromagnetic frequency is beneficial in various applications, which is generated by the repetitive back and forth electrical energy movement between a fully charged inductor and a capacitor (for instance, in telecommunications technology). In addition, the tank can be charged to a particular resonant frequency by arranging the values of the reactive component. There are various types of RTNs, all of which can be categorized by three major factors [87; 88]. The first classification is based on the correlation method of the tank elements, such as a series resonant converter (SRC), parallel resonant converter (PRC), and series-parallel resonant converter (SPRC) [89], as shown in Figure 19. The second type is based on the number of two components of the reactive elements (the number of the transfer function order). For the third type, the categories are based on the structure of a single element or resonant tank with multiple elements [86; 90].  
The third classification has two, three, and multi-element resonant tanks. There are several resonant power converter topologies with two elements (see Figure 19), but types a and b are regarded as second-order resonant tanks. As a result of their basic analysis, they are the most common and simplest topologies. Some RTNs are only suitable for the voltage sources, while others are good for the current source. In some cases, SRCs and PRCs are not ideal for use in certain applications, such as in contactless energy transfer devices [91] and high-voltage applications [16].  
The development of third-order resonant tanks was conceived as a solution to the problems of the two-component (two-element) RTN; a third element was added to the two-element resonant network to create a three-element resonant network. It can be viewed  
Figure 19: Several types of resonant tank networks: (**a**) Series resonant network; (**b**) Parallel tank network; (**c**) Series-Parallel tank network [10].  
as a combination of the strengths of the two most popular \(SRC\) and \(PRC\) resonant elements, ignoring their weaknesses. There are thirty-six separate third-order \(RTN\) tanks [80, 89, 92]; some of them consist of two capacitors and one inductor, while others are opposite shaped, as shown in Figure 20a-d. Three-element \(RTN\) resonant converters have been widely used in many industries, such as in \(LLC\), \(LCC\), \(CLL\), \(LCL\), and hybrid series-to-parallel resonant converters (\(RCs\)) [93, 94, 95, 96]. \(LLC\) and \(LCC\) are the commonest forms of third-order \(RTN\) converters. For the \(LLC\)\(RTN\), they are considered as three-element parallel \(RCs\) because they exhibit the features of the series resonant converter \(RC\) by integrating a parallel inductor that is placed before loading.  
Having four or more elements, as shown in Figure 20d, means the tank is a multi-element resonant tank. Generally, the resonant tank relation exists beyond the same number of \(RTN\) elements. The relationship between various tank orders indicates that a resonant tank network with multiple elements is equivalent to a lower number of tank elements [89].