## 5 Structural Hybrid of Reviewed Converters
### Multilevel Resonant Converters  
To meet the demands for high-voltage operation, more novel structures are needed in high-power applications to achieve better performances. This multilevel structure can reduce the voltage stress of power devices and this made them a good alternative [146]. In _DC-DC_ conversion, resonant converters are potential alternatives owing to their soft-switching attributes in a range of operations. In such situations, _LLC_ resonant converters are better options due to their advantages, such as _ZCS_ for rectifier diodes and _ZVS_ for power switches. Another multi-phase multilevel _LLC_ modular resonant converter that can realize _ZCS_ for rectifier diodes and _ZVS_ for primary switches, and can decrease voltage stress to one-nth of the input voltage under a range of load and input voltages, has been proposed to meet the demands for high-power and high-voltage operation. The system was also proposed to improve the power processing ability of the _LLC_ resonant converters in multilevel operations [147]. With this approach, the voltage stress on the power devices will be reduced, making it suitable for use in low-voltage rated _MOSFETs_. With low-voltage rated _MOSFETs_, conduction losses will be reduced for a given area and the switching losses due to ZVS will remain low [147]. Achieving _ZVS_ in multilevel operation also requires a reduction in the dead-time and magnetizing inductance current due to the storage of lower energy in the parasitic capacitances of the _MOSFETs_, thereby increasing the converter efficiency. The study by [148] presented a new two-phase interleaved flying capacitor _LLC_ resonant converter in which multilevel operation and multi-phase operation are combined. The need for the flying capacitor is to reduce the voltage stress on the primary side of the _MOSFETs_ to ensure a balance in the currents generated by the two phases, as well as to improve the power processing capabilities of the system. Hence, the converter exhibited all the positive attributes of conventional _LLC_ converters but retained the low sensitivity attribute to mismatches in the resonant tank parameters without requiring extra components. This made the system suitable for high-output current applications. A multilevel flying capacitor-based single-stage _AC-DC LLC_ topology has been proposed to address the dilemma of the voltage balancing of _DC_-bus capacitors and to lessen the voltage stress of the switching devices. The suggested three-level inverter configuration ensures zero voltage switching (_ZVS_) for all the switches, lower circulating currents, less switching stress, and lower losses. For improved efficiency, the converter employs a bridgeless rectification approach, and the power factor is practically unity by operating the source-side inductor in discontinuous current conduction mode. The output voltage of the converter is regulated using variable switching frequency control, while the _DC_-bus voltage is regulated via pulse width modulation. This control technique works well to maintain almost constant _DC_-bus voltage throughout a wide range of line and load variations. Figure 36 shows the three-level flying capacitor _LLC_ resonant converter [149]. The study by [150] presented a five-level cascaded _LLC_ resonant converter; the design and analysis of the system were presented for use in energy storage systems (_ESS_s) or grid-connected applications. The proposed converter operates in the same manner as a step-down converter; it also achieves _ZCS_.  
Figure 35: Circuit diagram of five-level _NPC DAB_ converter [145].