Bidirectional Resonant Frequency Tracking for CLLC Converters Based On Voltage Falling Edges

Due to high efficiency and bidirectional capability, unregulated CLLC converters have significant potential for the DC transformer and chargers with two stages, where the switching frequency is designed to be fixed at resonance. However, the actual resonant frequency may drift from the designed value because of resonant tank component tolerances, temperature, and aging. This leads to a decrease in efficiency for unregulated CLLC converters with a fixed frequency. To solve these issues, a bidirectional resonant frequency tracking method is proposed for CLLC converters in this paper. Based on the analysis of input and output impedance, a unique feature is discovered that no matter how resonant tank parameters deviate, the switching frequencies harvesting maximum efficiency are identical for bidirectional operations. To track this global optimal switching frequency, the proposed method adopts a voltage edges-based sensing technique, so the bidirectional tracking is realized and costly current sensors of conventional methods are eliminated. It is lower cost and robust against noise. Experimental results show that bidirectional fast-tracking and efficiency improvement are achieved for parameter deviation conditions.