## 3 Resonant Converter Families
### Classification of Resonant Converter
#### 3.2.1 Series Resonant Converters  
Load resonant converters (_LRCs_) have several distinct characteristics over conventional power converters. _LRCs_ are particularly appropriate for use in high-voltage applications because they allow operation at high frequency to reduce the equipment size without affecting the power conversion efficiency or further stressing the switches. There are three different combinations of _LRCs_; these are series resonant converters, series-parallel resonant converters, and parallel resonant converters [79]. The _DC-DC (SRC)_ series resonant converter has been utilized in a wide range of voltage and energy applications [101]. The tank is mounted in series with the load network and the rectifier in a series resonant converter (_SRC_); hence, the generation of the load and resonant tank voltages is dependent on the low voltage divider. The modification of the value of the resonant tank impedance is reliant on the moving voltage frequency to the tank [85]. The loads in _SRCs_ are connected in series with the resonant tank circuit, as designed by \(Lr\) and \(Cr\). Figure 24 shows a half-bridge configuration; when \(ILr\) is in the positive mode and \(T_{1}\) is active, the current of the resonant inductor flows via \(T_{1}\), else, it will flow via the \(D2\) diode. On the other hand, when \(ILr\) is in the negative mode and \(T_{2}\) is active, the current will flow through \(T_{2}\), else, it will flow through the \(D1\) diode. Both active switches work in a complementary mode in steady-state symmetrical service. The converter has several potential working modes based on the ratio of the switching frequency Fs to the converter resonant frequency \(Fr\)[102; 103; 104].  
Figure 23: Classification of resonant _DC-DC_ converter [79].  
The impedance value of the resonant tank, as shown in Figure 25, varies as a function of the driving voltage frequency to the tank. However, the impedance of the resonant tank can be modified through adjustment of the moving voltage frequency to the resonant network (see Figure 25); the proper resistive value to the \(RTN\), as provided by the rectifier, can be estimated using Equation (15), while the input voltage is fed to the tank impedance and the effective resistance [104]. It is expected that the voltage gain of the \(SRC\) should be \(<\)1 as the configuration framework of the \(SRC\) circuit can be estimated using Equation (16). In a light load scenario where the value of the load resistance is higher than the impedance of the resonant network \(Z_{O}\), all the input voltages will be fed to the load; hence, it will be hard to regulate the production at light load [87; 105].  
\[R_{ac}=\frac{8}{\pi^{2}}R_{L}\;\text{{Reflected load resistance}} \tag{15}\]  
\[M=\frac{V_{o}}{V_{in}}=\frac{1}{\sqrt{1+Q^{2}\left(1/F-F\right)^{2}}}\;\text{{ Voltage gain}} \tag{16}\]  
where \(Q\) is the quality load factor, which can be calculated using the relation:  
\[Q=\frac{Z_{O}}{R_{ac}} \tag{17}\]  
As mentioned above, there are several potential working modes of the \(SRC\) converter, which are conductive mode, frequency switching, and ranges of soft switching. These modes and their limit have been investigated in previous studies [106; 107]. The \(SRC\) modes can also be classified under discontinuous conduction (\(DCM\)) and continuous conduction (\(CCM\)) modes based on the ratio of the switching frequency, \(Fs\), to the converter resonant frequency, \(Fr\).