Light-load efficiency improvement for LLC converter with synchronous rectification in solid-state transformer application

Synchronous rectification technique can reduce secondary-side conduction loss of the LLC resonant converter. Typically, the control of synchronous rectifier (SR) relies on either voltage or current information; however, the detection circuit is sensitive to parasitic effects and high frequency noises. Since reliability is among top priorities in solid-state transformer application, open-loop controlled scheme becomes advantageous. Unfortunately, secondary-side current of LLC converter reaches zero-crossing-point (ZCP) earlier at light-load condition and the SR signal could turn off after ZCP. In that case, high circulating current appears in the secondary side and dramatically deteriorate efficiency. Therefore, a tuning method utilizing external primary-side output capacitor and dead-time extension is proposed to avoid late turn-off issue of open-loop controlled scheme. In this paper, the cause of ZCP shifting and late turn-off issue are explained first. Then a model for dead-time transient of LLC converter is derived as the theoretical basis of proposed tuning method. Finally, hardware testing results of a 4-kW LLC converter module are presented. With the proposed tuning method, the open-loop controlled synchronous rectification can improve the efficiency of the LLC converter module even at light-load condition.