Soft Start-up of Three Phase CLLC Converter Based on State Trajectory Control

The bi-directional five-element CLLC resonant converter is attracting great attention for battery charger applications. The three-phase interleaved CLLC converter can deliver more power with lower conduction losses as compared to the single-phase CLLC. Resonant converters, including the three-phase interleaved CLLC, suffer from high levels of inrush current during startup as well as short circuiting. A proper soft startup needs to be implemented to limit the resonant current during startup. This paper discusses the soft start-up of single-phase and three-phase interleaved CLLC converters based on state trajectory control. The state trajectory of the CLLC primary resonant tank is simplified and treated like an LLC. Optimal trajectory analysis during startup is implemented to limit the resonant tank and to achieve a fast startup. The three-phase interleaved CLLC is reduced to the full-bridge CLLC for easier analysis and control, then is switched back to three-phase interleaved operation after a successful soft startup. Experimental results are provided for a 12kW 500kHz 850V CLLC converter using digital MCU F28379D.