Design and control of a single-stage large air-gapped transformer isolated battery charger for wide-range output voltage for EV applications

In this paper, the design procedure for a single stage transformer isolated multi-element resonant converter for an Electric Vehicle (EV) battery charging application with a large air-gap of 8cm and wide-range output voltage is proposed. Instead of having the same resonant frequency on both primary and secondary tank circuits as have been proposed in the literature, different resonant frequencies are used to meet the system specifications. This gives a number of advantages for wide-range output voltage application including simple control and ensures ZVS on the primary side switches and ZCS on the rectifying diodes, which gives high efficiency. A systematic procedure is proposed to design controllers for both current loop and voltage loop to meet the transient performance requirements. To validate the design procedure and controller design, a 1.8 kW system was built and tested. The efficiency was found to be 96.6% for a large air-gap of 8 cm during the constant current mode.