Design Considerations and Performance Investigation of a Five-Level Cascaded Multilevel LLC Boost DC-DC Converter

This work proposes a cascaded five-level resonant boost ''DC-DC converter with variable frequency control that is aimed at achieving a wide load and input voltage variation, remarkable efficiency, and zero voltage switching (ZVS) to meet the strict demands of industry applications. The design consideration of this converter was also discussed in detail to confirm the achievement of each feature of the multilevel and LLC resonant converter (RC) combination. The selection of the voltage gain was set to achieve an output voltage that is 5 times the input voltage; the operation frequency range'' was selected within the range of 67kHz135kHz to ensure achievement of ZVS and the ability to work close to resonant frequency to achieve the estimated efficiency. Furthermore, a full-bridge LLC resonant frequency with the same assumptions and parameters was simulated using MATLAB/Simulink features to provide clear comparison and vision of the resonant behavior with the proposed topology. It was noticed that the LLC circuit with a five-level inverter differs from its typical behavior with full-bridge inverters; the switching frequency is always lower than the resonance frequency regardless of the input and load conditions. Also, the adopted variable frequency control proved its fast dynamic response to any changes in the input and load conditions to maintain 1000V output, and 5kW with 96.7% efficiency at full load conditions. Experiments were performed to validate the theoretical and simulation results using a laboratory prototype of the proposed converter; selected results were presented in this article to demonstrate the efficiency and viability of the proposed converter.

Automated summary

This work presents a cascaded five-level resonant boost DC-DC converter with variable frequency control to achieve wide load and input voltage variation, high efficiency, and zero voltage switching. The converter design and selection of voltage gain and operation frequency range were discussed in detail. Simulation results using MATLAB/Simulink showed the different behavior of the LLC circuit with a five-level inverter compared to full-bridge inverters. The adopted variable frequency control demonstrated fast dynamic response and high efficiency in various input and load conditions. Experimental validation with a laboratory prototype confirmed the effectiveness and viability of the proposed converter.