A Digital Sensor-less Synchronous Rectification Algorithm for Symmetrical Bidirectional CLLC Resonant Converters

For bidirectional CLLC resonant converters, synchronous rectification (SR) is a key technique that can reduce the conduction losses by replacing the diodes to bring an improvement of the performance. Unfortunately, existing methods suffer from high cost, complex hardware and imprecise control effect. Aiming at these problems, this paper proposes a digital sensor-less SR algorithm based on time domain model for symmetrical CLLC converters under pulse frequency modulation (PFM). The developed approach can enable the SR switching action more precisely within almost full load range without any additional hardware, e.g., anti-parallel diode, SR IC and high bandwidth sensor. Its implementation only needs to measure the DC signal (i.e., the output current), rather than to sense the zero-crossing point of the high frequency resonant current. Additionally, a simplification of the algorithm has been made with no accuracy compromise, thus the requirements of the digital controller decrease greatly. Finally, experiments have been performed on a prototype to verify the validity and applicability of the presented SR algorithm, and the experimental results show that the SR effects are substantially improved.