Design and control methods to enhance the efficiency of two-port and three-port DC-DC resonant converters in electric vehicle applications

Abstract

DC-DC resonant converters benefit from soft switching and reduced peak currents over other topologies. However, the design and control of resonant converters are challenging due to non-linearities in the resonant tanks. This research focuses on design and control methods for two-port and three-port resonant converters in EV applications. The two-port LLC resonant converter is attractive for on-board charger applications. However, if not appropriately designed, the frequency-modulated LLC converters will have a wide range of switching frequencies and lose efficiency in wide voltage range OBC applications. Hence, practicing proper converter design and control methods is essential to maximize efficiency. This work proposes a design framework for a wide-voltage range LLC converter to enhance efficiency. Topology morphing is used to reduce the effective voltage gain, and an online topology morphing method, along with a cascaded closed-loop control system, is also proposed. Three-port DC-DC converters can facilitate power transfer among three sources/ sinks. With the emerging trend of dual auxiliary voltages in EVs, the three-port resonant converter topology is an ideal candidate to interface the high voltage battery with low to medium voltages. This work proposes an optimal control method for a TPRC based on duty-ratio and phase-shift control to maximize its efficiency. The control method is optimized using a novel harmonic approximation-based model. A 300 – 700 V input, 250 – 450 V output, 3.3 kW LLC converter is designed and tested to validate the proposed design and control methods of the LLC converter. The time-weighted averaged efficiency above 96.7% is observed over the entire input voltage range. A 400 – 800 V/ 46 – 50 V/ 10 – 14 V, 6kW rated power TPRC is also designed and tested to validate the proposed optimal control method. Peak efficiency of 96.34% is observed, with a maximum efficiency improvement of 12.4% compared to the conventional phase-shift control.