Wide-Load-Range Double-T Resonant Converter for CC/CV Battery Charging

Abstract

This article proposes a load-independent constant current (CC) or constant voltage (CV) output Double-T circuit (DT) for electrical vehicles (EVs) or electrical bikes (EBs) charging systems to improve the conversion efficiency over a wide-load range during battery charging processes. Among available studies, the LLC converter is a widely adopted resonant topology for EV or EB charging. However, in CC-CV charging, the wide output voltage caused by the wide-load range requires a wide switching frequency range to achieve, which decreases the efficiency in the wide-load range. To address such issues, in this article, two T-circuits are cascaded to form an output load-independent DT with fixed duty cycle and frequency, which can implement CC-CV modes and zero phase angle at the resonant frequency simultaneously, which not only significantly reduces reactive power in energy storage elements but also eliminates the adverse effect of efficiency reduction owing to switching frequency variation. Finally, based on experimental results, the variation of current in CC mode is within 4.18%, and that of voltage is within 4.44% in CV mode, which demonstrates the inherent load-independent capability of the DT converter. During the battery pack charging experiment, the peak dc-dc conversion efficiency reached 96.70% and the average conversion efficiency was higher than 94.01%.