Increasing Light Load Efficiency in Phase-Shifted, Variable Frequency Multiport Series Resonant Converters

Multiport power conversion topologies provide the capability of multiple independent converters with a single transformer having multiple windings (i.e., ports) potentially increasing power densities and enabling flexible (and bidirectional) power routing. In automotive onboard charger (OBC), the multiport approach combined with symmetrical series resonant circuits, the so-called multiport series resonant converter (MSRC), allows for a galvanic isolated connection between all ports: the grid-side converter (i.e., usually an AC/DC power factor correction (PFC) stage), vehicle's main and the auxiliary low-voltage (LV) battery. The variation of the battery voltage significantly affects the MSRC operation, particularly for light loads at a low state-of-charge, and high losses can be experienced since zero-voltage-switching (ZVS) conditions are lost. In addition to the conventional control approach of the MSRC, where the power flow is set with a phase-shift between the individual full bridges or by changing the switching frequency, this paper proposes a novel and coordinated approach, including the manipulation of both and the additional modulation of the duty cycle as a function of the DC-link voltages, aiming to introduce a zero-voltage interval on the full bridge output voltages. A full mathematical description of the adopted converter topology is provided, including accurate simulation models that allow a comparison between the proposed duty cycle mode and the conventional control strategy. A detailed description of achieving ZVS within the connected full bridges is also included. Experimental results validate the proposal and demonstrate significant efficiency improvements compared to standard control approaches.