Gain Modeling and Parameter Optimization of Symmetric Bidirectional CLLC Resonant Converters in LVDC Distribution Systems

Low-voltage DC (LVDC) distribution systems are increasingly adopted in data centers, electric vehicle charging, and industrial power supply due to their ability to reduce multi-stage energy conversion, enhance efficiency, and integrate distributed energy sources. At their core, high-performance DC-DC converters enable efficient power transfer and provide electrical isolation across different voltage levels. Beyond these fundamental roles, they regulate voltage, improve power quality, and enhance system reliability under varying load and source conditions. Their versatility makes them indispensable in applications ranging from renewable energy integration to low-voltage DC distribution networks. Among various topologies, the CLLC resonant converter, with its high efficiency, wide operating range, and inherent bidirectional power flow capability, is particularly suitable for isolated energy conversion and system-level energy management in LVDC applications. However, In the process of analyzing and designing the CLLC. the conventional fundamental harmonic approximation (FHA) method exhibits significant discrepancies when analyzing the actual operating behavior of the CLLC converter. This paper first introduces the basic topology of the CLLC resonant converter and subsequently demonstrates the limitations of the FHA-based analysis. A time-domain analytical (TDA) model investigates the high-efficiency operating modes and derives their gain expressions from intrinsic time-domain characteristics. Boundary conditions further constrain the operation to avoid transition into loss-dominant modes. Furthermore, we designed and implemented a 135W laboratory prototype, which ensures reliable soft-switching operation across the intended load range and achieves a peak efficiency of 98.1%.