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A Soft-Switched Back-to-Back Bi-directional DC/DC Converter with a FPGA based Digital Control for Automotive applications

The paper proposes a soft-switch back-to-back bi-directional DC/DC converter. There is no input inductor in this topology in both the step-down and step-up mode, as a result, there is no RHP zero in the system model and hence the converter always functions as a minimum phase system. This makes the converter a first-order system, and the close loop control design becomes easy. The removal of the inductor reduces power loss during the operation. Moreover, soft switching using zero-voltage-switching (ZVS) reduces switching power losses. The proposed topology achieves high efficiency and high power density. The paper describes the analysis and design for the converter system under non-ideal conditions. The possible ZVS range of the load variation is also given. A large signal average model and a small signal model for the bi-directional converter are derived. Based on the system model, a digital PI controller is designed and is implemented by using an FPGA. Closed loop control and variable load operating performance are experimentally validated on a 100 W prototype converter. A maximum efficiency of 92.3% is achieved.