A Switching Delay Strategy for Sensorless Synchronous Rectification in CLLC Converters

Sensorless synchronous rectification (SR) is often used in CLLC converters to achieve higher efficiency without extra system cost. Conventionally, sensorless SR methods turn on secondary-side switches at the same instant as the primary side. In such conditions, secondary-side switches usually operate under hard turn- on conditions due to the existence of output capacitors {C}_{\text{oss}}. In this article, the mechanism of {C}_{\text{oss}} causing secondary-side hard switching under conventional sensorless SR is analyzed comprehensively. Actual operating process considering the effect of {C}_{\text{oss}} is analyzed and modeled using the state trajectory method. To realize soft turn- on on the secondary side, a switching delay strategy is proposed. The proposed strategy delays the turn- on instant of secondary-side switches to help realize secondary-side zero voltage switching (ZVS) conditions. A quantitative state trajectory model is also established for the proposed strategy, and a searching method for suitable switching delay time is also provided. With searched switching delay time, the proposed strategy can guarantee secondary-side ZVS condition for sensorless SR in full load range, improving efficiency. A lab-level prototype was built to verify the proposed switching delay strategy. The experimental results prove the correctness of the proposed state trajectory analysis and show that sensorless SR with the proposed switching delay strategy can achieve an obvious efficiency improvement (up to 3.61%) over conventional sensorless SR method in full load range, especially when the load is light.