The ZVS Transition Analysis and Optimization for CLLC-Type Resonant DC Transformer
Yuliang Cao, Minh Ngo, Dong Dong, Rolando Burgos
Abstract
The isolated bidirectional dc transformer (DCX) has been widely used in the energy storage systems, data center power supplies and transportation electrification. Due to zero voltage switching (ZVS) and small rms current, CLLC resonant converter is considered a good candidate for DCX when it works at resonant frequency with an open-loop constant voltage gain. To address the problem of unsmooth bidirectional power flow and current distortion in the CLLC-DCX with traditional synchronization rectification (SR) modulation, dual-active synchronization (DAS) modulation is adopted, with which the gate signals on primary and secondary side are the same. First of all, after considering the large output capacitance of the device, the switching transition of DAS modulation is fully studied. With the comparison between seven different switching transition types, a so-called “Sync-ZVS” transition is preferred due to ZVS, no deadtime conduction loss, and load-independent voltage transfer ratio. To realize “Sync-ZVS” transition, this paper proposes an “Axis and Center Symmetric” (ACS) method. Finally, an 18 kW 500kHz open-loop CLLC-DCX was built to verify both the “Sync-ZVS” transition and the proposed “ACS” method.