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

Multiport power conversion topologies provide the functionality of more single (and independent) converters but with just one transformer having multiple windings (i.e., ports). In automotive on-board chargers, the multiport approach combined with symmetrical series resonant circuits, the so-called multiport series resonant converter (MSRC), enable the required galvanic isolated connection from the grid-side converter (i.e., usually an AC-DC power factor correction (PFC) stage) to the vehicle’s batteries, i.e., the main high-voltage (HV) traction and the auxiliary low-voltage (LV), theoretically allowing to achieve increased power densities and enabling a flexible (and bidirectional) power routing among the different ports. High efficiency figures are normally specified in the considered application field for a very wide voltage ratios among the different ports, due to the variability of battery voltages, thus requiring proper and optimized control strategies of the underlying converter. The variation of the battery voltage is in fact highly affecting the MSRC operation, especially for light loads at a low state-of-charge, and high losses can be experienced since zero voltage switching conditions are lost. Beside 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 grid-side DC-link voltage (i.e., the output of the PFC stage), aiming at introducing a zero-voltage interval on the full bridge output voltages. A full mathematical description of the adopted converter topology is reported, also including accurate simulation models for losses estimation allowing a fair comparison between the proposed duty-cycle mode control and the conventional control strategy. Detailed operation guideline for achieving zero-voltage switching within the connected full bridges is also reported. Experimental results validate the proposal and highlight a significant peak and average efficiency improvements with respect to standard control approaches.