An Optional Series/Parallel Resonance-Based LLC Converter for On-Board Charger With Load-Independent Output Characteristics

In this article, an optional series/parallel LLC resonant converter with inherent load-independent output characteristics is introduced in the electric vehicles (EVs) on-board chargers. The converter operates at two different fixed resonant frequency points: the parallel-resonant frequency or series-resonant frequency as needed and performs as output current source or voltage source, respectively. A simplified open-loop modulation strategy is proposed during the whole charging process. It not only removes the need for feedback control to limit charging current to desired levels in the constant current (CC) charging mode but the batteries can also operate safely in the constant voltage (CV) charging mode. The converter operates at a fixed switching frequency during CC and CV charging thanks to this frequency-selective network characteristic and the dual-fixed-frequency point operation strategy. Moreover, a smooth transition between the two modes is achieved by employing a pulse-frequency modulation (PFM). In addition, an active branch for current injection is needed to help the switches realize zero-voltage switching (ZVS) in the CC charging mode. Finally, a 2 kW 250–430 V output experimental prototype is built. The experimental results show that the soft switch is realized and the high efficiency is maintained in the whole load range, and the maximum efficiency is 97.5%.