Efficiency Optimization in Parallel LLC Resonant Inverters with Current-Controlled Variable-Inductor and Phase Shift for Induction Heating

This paper presents a comprehensive analysis of a novel control approach to improve the efficiency of parallel LLC resonant inverters using a combination of a current controlled variable inductor (VI) and phase shift (PS). The proposed control aims to reduce the Root Mean Square (RMS) current, thereby reducing conduction and switching losses, and improving power transfer efficiency, while considering zero-voltage-switching (ZVS) operation, output power variations and load changes. Furthermore, a design methodology for the variable inductor is proposed, and design considerations relevant to this application are discussed. The effectiveness of the proposed approach is evaluated through mathematical modeling and experimental results. The mathematical modeling results show that the proposed approach, utilizing SiC MOSFETs, maintains a maximum efficiency of 98.5% for a 20 kW inverter over a wider range of output power, which is a significant improvement over existing approaches. The experimental results also confirm the effectiveness of the proposed control in improving the efficiency of parallel LLC resonant inverters.