Publication: Cascaded multilevel llc resonant converter with bidirectional buck- boost stage for battery applications
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Date
2024-08
Authors
Salah Salem Assenni Alatai
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Abstract
Multilevel inverters (MLIs) are widely utilised in various power electronics applications. These converters have garnered significant attention in recent years in research and industry and come in various topologies with similar fundamental concepts. This thesis designed and evaluated an integrated cascaded pair of full-bridge LLC resonant bidirectional DC-DC converters usable in varied applications, including in energy storage systems, to function as an interface between two dc voltage buses in a variety of applications. The proposed converter combines an isolated five-level cascaded H-bridge LLC (IFCHB-LLC) resonant circuit with a buck/boost circuit (Bidirectional converter BID). In this converter, the inbuilt capabilities of an LLC
converter, which function as a current source and a voltage source, were exploited, resulting in the constant current (CC) and constant voltage (CV) charges while CV was implemented in the discharge stage (reverse flow). The modelling of the LLC converter was done following the first harmonic approximation (FHA) approach. Furthermore, to ensure improved efficiency of the proposed system, the passive elements of the resonance tank and isolation transformer ratio were programmed in a manner that the converter can be operated within the zero-voltage switching (ZVS) for the entire operation frequency range of (78 kHz < 𝑓𝑠<132 kHz) for all switches (S1- S8) and zero-current switching (ZCS) region for rectifier diodes. The feasibility and validity of the converter were simulated by MATLAB software and tested using a 500W prototype converter with an input voltage of 200V resulting in the highest efficiency level of 95.01%. The obtained experimental results are found in good agreement with the simulation results. Moreover, a detailed comparison between the proposed converter and other existing topologies of multi-level, resonant and bidirectional converters for battery charging in terms of control complexity, component counts, and soft-switching properties is presented. This comparison
showed that the proposed converter has better performance when compared to other existing converters.