Capacitor-Clamped LLC Resonant Converter for Constant Power EV Charging with Fixed Operation Frequency

LLC resonant converter is one of the most preferred topologies for electric vehicle (EV) battery charging. However, the wide voltage range of batteries requires a wide frequency excursion of LLC resonant converters, which is undesirable from the efficiency, power density and cost perspectives. This paper is focused on improving the operation of a type of LLC resonant converter, namely, the capacitor-clamped LLC resonant converter that was originally proposed for over-current protection purpose and which only operates in the zero-voltage-switching (ZVS) region of conventional LLC resonant converters. It is found that this converter, when operated in the zero-current-switching (ZCS) region, can actually achieve ZVS whilst demonstrating several new features as compared to the conventional LLC resonant converter, including constant frequency operation for constant power operation. This leads to a an elegant solution of improving system's efficiency and power density with very simple control and at a low cost. Both simulation and experiments are conducted to testify the viability of the proposed solution.