## 5 Structural Hybrid of Reviewed Converters
### Multilevel Bidirectional Resonant Converters  
The unique characteristics of multilevel inverters make them attractive to a variety of medium to high-power, high-voltage applications. Multilevel inverters have several benefits, such as reduced voltage stress, reduced loss of individual semiconductor chips, improved power efficiency, and reduced electromagnetic interference. The most significant challenges in multilevel inverters are voltage balancing, the increased number of semiconductors and capacitors, and more complicated control. The best structure can be chosen from a variety of classic and advanced multilevel inverter configurations. The most appropriate option is determined by the application, load, and requirement [151]. Although many papers have investigated different topologies, modulations, and control techniques of multilevel inverters with bidirectional or resonant converters, only a few studies have introduced the combination of these three converters; this section briefly reviews the existing literature on these structures. The study by [152] introduced the bidirectional multilevel resonant switched capacitor converter; this work presented a method that significantly improves the proposed converter's efficiency through topology modification. In a multilevel topology, the converter can achieve a high voltage ratio, which reduces voltage stress on switches and allows for high-power performance. The converter can be used as a link between _DC_ voltage systems that are used in several applications. Furthermore, the primary objective of the research was to verify the achieved topology improvement using silicon carbide (_SiC_) and silicon (_Si_) switches in a four-level bidirectional resonant switched capacitor with a _0.5/2 kV_ voltage conversion ratio. The proposed converter was composed of a basic switched-capacitor (_SC_) structure that makes energy transfer in both directions possible. The study concluded that the voltage gain was more stable, and the method was efficient with _Si MOSFET_; also, the system showed a negligible decrease in efficiency versus power with increases in power. A bidirectional modular multilevel resonant (_BMMR_) _DC-DC_ converter has been proposed for medium linkage with low-voltage grids; the desirable features of _BMMR_ converters include:  
* Bidirectional power conversion ability. * Modular structure with manufacturable, standardized submodules, and fault-tolerant operation that is simple to achieve. Owing to these characteristics, the _BMMR_ converter is a strong contender for _DC_ distribution applications. The authors presented an analysis of the converter's operation principles and submodule voltage balancing in a steady state. The proposed topology and control methods were checked using a 500-kW simulation model and experiments on a down-scaled prototype, where the employed structure was a half-bridge modular multilevel structure on the medium voltage (_MV_) part and a full-bridge structure applied on the low voltage (_LV_) side [153]. Bidirectional three-level _DC-DC_ converter soft-switching was presented by [154], which, when combined with a simple auxiliary circuit, can effectively minimize switching power losses. To reduce the turn-on switching power losses of main power switches, the  
Figure 36: Circuit diagram of three-level single-stage _LLC_ converter [149]. researchers utilized double \(LC\) resonant circuits. In comparison to previous converters that work without any auxiliary circuits and suffer from high switching losses, the proposed converter reduces switching power losses and improves power performance. The proposed converter outperformed conventional converters in terms of power efficiency. For the rated load state, it achieved power efficiencies of 97.7% in the step-up mode and 97.8% in the step-down mode. The study made the following conclusions:  
* The proposed converter can be used in a grid-connected battery energy storage device that needs high power density and performance.