Analysis of Output Voltage Characteristics and Voltage Limiting Methods for LLC-DCX Converters

The LLC converter, as an isolated DC-DC converter, features simple topology, soft-switching capability, and high power conversion efficiency. The LLC-DCX variant, operating at a fixed resonant frequency to maximize efficiency, is widely adopted in two-stage power conversion systems. While conventional Fundamental Harmonic Analysis (FHA) suggests minimal load dependence of the output voltage gain, practical noload and light-load output voltages significantly exceed FHA predictions. Research group analysis attributes this discrepancy to distributed parasitic capacitances as the primary cause of no-load voltage elevation. This paper systematically investigates the impact of parasitic capacitances on output voltage characteristics, derives analytical expressions for no-load voltage gain, and reveals its parametric dependence on parasitic components. Corresponding voltage-limiting strategies are proposed from both parasitic parameter optimization and control strategy perspectives. Simulation results validate the theoretical framework and the proposed voltage-limiting methods.