Improvements on capacitor-clamped LLC converters for electric vehicle charging

Abstract

Wide output DC-DC converters have become a research hotspot due to the promotion of renewable energy and electric vehicles (EVs). The LLC resonant converter has excellent characteristics of natural zero current and zero voltage switching, making it one of the preferred DC-DC topologies for small- and medium-power applications. Therefore, the LLC resonant converter has become one of the most adopted topologies for EV battery charging. However, due to the wide variation range of battery voltages, using conventional LLC resonant converters in EVs usually results in relatively larger magnetic size and lower power density. This thesis proposes novel approaches to improving the performance of LLC resonant converters for EV charging. The main contents are as follows: 1. The theoretical analysis of the characteristics of conventional LLC converters is provided, showing that design challenges arise when LLC resonant converters are used to provide a wide output voltage range. If pulse frequency modulation (PFM) is used to regulate the output voltage, the converter needs a wide operating frequency range, causing electromagnetic interference (EMI) problems and difficulties in the magnetics design. Alternatively, if phase shift modulation (PSM) is used, the converter needs a large phase shift range, causing loss of zero-voltage switching (ZVS) and reduced efficiency. 2. A novel method for the LLC-based converter to realize ZVS in the capacitive region for conventional LLC converters is explored. It leverages the existing capacitor-clamped LLC topology, which was originally proposed for applications needing overcurrent protection while operating the converter in the capacitive (rather than inductive) region of conventional LLC resonant converters. A design method is developed based on this operation, and it is validated that when working in the capacitive region, the capacitor-clamped LLC converter can realize not only ZVS in the MOSFETs and zero-current switching (ZCS) in the output diodes, but also significantly reduce the flux linkage requirement in the transformer compared to conventional LLC converters. The features of the capacitor-clamped LLC converter have helped to minimize the size of the magnetics and increased the power density. 3. A frequency folding technique is proposed, and its application to a half-bridge LLC resonant converter with an augmentation of three low-cost components (a relay and two diodes) to reduce the frequency variation range for EV charging applications, is discussed. The resultant converter utilizes both the clamping and non-clamping mode of operation of the conventional LLC resonant converter and can realize full-range ZVS to maintain high power efficiency. 4. Based on the full-bridge (FB) LLC converter structure, a novel converter for EV charging applications using the capacitor-clamped LLC converter topology and fixed-frequency PS modulation is proposed. Compared with traditional PS LLC converters, the proposed method can achieve a (1) smaller phase shift variation range, (2) lower turn-off current, and (3) wider soft-switching range, resulting in higher power efficiency of the converter. The merits of the capacitor-clamped LLC converters with the proposed design methods are validated by both simulation and experimental results, and comparative studies are conducted to verify the effectiveness and feasibility of the proposed solutions.