A Simplified Sensorless Synchronous Rectification Method for Inner Phase Shift Control in CLLC Converters

The
CLLC
converter is a promising topology for bidirectional power conversion applications, such as vehicle-to-grid and battery energy storage systems. In these applications, wide output voltage range is needed. However, traditional pulse-frequency modulation method is not suitable for wide output range because of wide switching frequency range. Inner phase shift (IPS) control is a good solution for this issue. In
CLLC
converters, to further increase the efficiency, synchronous rectification (SR) is required. In this article, a simplified sensorless SR method is proposed for IPS control. A comprehensive operation mode analysis and an accurate operation mode model are established for IPS control in
CLLC
converters. With the operation mode model, waveform of secondary-side current is simplified. As a result, only a standard quadratic-root formula needs to be calculated to generate SR drive signal, and no extra sensors are needed. The method is low cost and is suitable to implement in embedded systems. A prototype is built to verify the proposed simplified sensorless SR method. The proposed method achieves an obvious efficiency improvement (up to 1.88%) over uncontrolled rectification, and the effect of the proposed method is nearly the same as that of complete SR (maximum efficiency difference is 0.40%).