High-performance resonant converters for battery chargers : efficiency and dynamics improvement

As the requirement for clean energy grows, the demand for high-performance power conversion for energy storage and battery charging applications has been soaring. Resonant converters, in particular, LLC or CLLC converters, have been broadly adopted for high-power battery chargers. The purpose of this work is to further improve the performance of the resonant converters from both the efficiency and dynamics aspects. In terms of improving the efficiency, this work focuses on reducing the conduction losses of the output rectifiers using Synchronous Rectification (SR). Conventional SR controllers detect the drain-source voltage during the SR turn-on phase (vds.on) as the control input. However, vds.on is low-magnitude and sensitive to the voltage noise caused by parasitic elements. The distorted vds.on causes SR mis-triggering, undermining the efficiency. With the focus on mainstream LLC resonant converters, this work first introduces a new SR driving strategy based on the resonant capacitor voltage (RCV). Next, a simplified SR method is proposed; it is based on the Volt-Second Product (VSP) of SR drain-source blocking voltage and rectifier current conduction time. Both methods employ large-magnitude voltages, which are insensitive to the noise generated by parasitic components, reducing SR on-time error. The proposed SR methods are compared with the conventional vds.on based SR to demonstrate the efficiency improvement. Regarding the dynamics aspect, this work focuses on improving the small-signal dynamic model for charge-controlled resonant converters. Charge mode control has been applied to resonant converters to improve the system dynamics, and yet the conventional small-signal model emphasizes only the low-frequency region, which is not suitable for high-bandwidth designs. This work establishes the small-signal modeling methodology based on Extended Describing Functions (EDF) and phasor analysis, which successfully predicts the system frequency response across low- to high-frequency regions, enabling high-bandwidth designs. As the proposed noise-tolerant SR methods improve the efficiency performance, the enhanced small-signal model assists to achieve wide loop bandwidth, improving the dynamic performance. This work provides solutions and insights to the design of high-performance resonant battery chargers.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat