Unified Bidirectional Resonant Frequency Tracking for CLLC Converters

Unregulated CLLC converters have significant potential for dc transformers and two-stage battery chargers due to their high efficiency and bidirectional capability. In these applications, unregulated CLLC converters are usually designed to have their switching frequency fixed at resonance for maximum efficiency. However, the resonant frequency drifts from the designed value due to resonant tank parameter deviations, which leads to a decrease in efficiency for unregulated converters. To address this issue, this article proposes the bidirectional resonant frequency and a unified tracking algorithm. By analyzing the generalized resonant frequency equations in depth, an interesting feature is discovered for CLLC converters: forward mode and backward mode achieve maximum efficiency at an identical (but not constant) frequency. Thus, this unique frequency is defined as bidirectional resonant frequency (BRF). As the BRF varies with parameter deviations, a unified bidirectional tracking algorithm is proposed, which unifies the tracking of bidirectional maximum efficiency points into the tracking of a BRF. The introduction of the BRF combined with the unified tracking algorithm provides a simple, accurate, and low-cost solution for bidirectional resonance tracking of unregulated CLLC converters. Finally, experimental results show that unified bidirectional tracking and efficiency improvement are achieved under parameter deviations.