Variable Frequency Phase-Shift Modulation Symmetrical Series-Resonant Bidirectional DC-DC Converter-Analysis and Verification of ZVS Performance and Reactive Power Minimization-

Conventional dual active bridge (DAB) converters based on the non-resonant circuit topology have the sever limitation of soft switching under light load conditions without complex multi-pulse modulations such as dual or triple phase shift. To address this technical problem, this paper explores a symmetrical series-resonant bidirectional dc-dc converter (SSR-BDC). The SSR-BDC features the variable frequency phase shift modulation (VF-PSM) for the minimization of reactive power in a wide range of soft switching operations. The CLLC series resonant tanks provide symmetrical steady-state characteristics of the load power and voltage regulations in forward and reverse power flows, which is advantageous for the seamless exchange of power flows in power electronics applications based on a simple control logic. The principle of the power regulation is described with the frequency-domain analysis of SSR-BDC, whereby the theory for determining the most suitable phase shift angle is clarified. The experimental verifications on a 500W prototype are presented, and the practical effectiveness is revealed; the actual efficiency improves by 1.4% and reactive power reduces by 75% after adopting the VF-PSM.