A Control-Free Series Resonant Converter for Battery Charging With Automatic CC-to-CV Profile and Whole-Process High Efficiency

Abstract

Constant current (CC) and constant voltage (CV) outputs are required by electric vehicle chargers to comply with the battery profile, which are challenges to the wide operating range, accurate current and voltage control, and high efficiency of the battery chargers. This paper proposes a control-free battery charger based on a series resonant converter (SRC) that features an inherent automatic CC-to-CV profile and whole-process high efficiency. Both the CC and CV modes enjoy identical switching sequences and operating waveforms, and the transition of CC-to-CV depends on the load condition. Automatic and smooth CC-CV transition can also be obtained after the CC charging process, thus avoiding the overvoltage charging risk. Detection of battery state and active control for the charging profile can therefore be eliminated, making the proposed battery charger rugged and reliable. The CC and CV outputs are flexibly configurable by setting the operating frequency and designing the turn ratio of the transformer respectively. Moreover, with an assistive circuit, soft switching can be realized for all switches regardless of the operating conditions to achieve a high efficiency throughout the whole charging process. Finally, a 1 kVA prototype is presented to validate the control-free and automatic CC-to-CV profile. High efficiency up to 98.4% is measured during the whole CC and CV charging periods.