Multi-mode tri-state operation of a 4-switch bidirectional DC-DC converter for interfacing a supercapacitor to a DC grid

Energy storage devices such as supercapacitors and batteries are frequently used to support DC nanogrids that employ renewable resources. They make the system more stable and reliable, by contributing to power balance with high charge/discharge rates. The 2-switch class C DC-DC converter with dual-state logic is widely used as the interface for supercapacitors to a DC nano grid. It presents slow dynamic response due to the Right-Half Plane (RHP) zero in the transfer function of Vo/D in Boost mode, when supercapacitors supply power to the DC grid. This converter also faces an issue of losing current control when the voltage of the supercapacitor is higher than that of the DC nanogrid. A 4-switch bidirectional DC-DC converter in the Buck-Boost mode that consists of two half-bridges and an intermediate inductor can be used as the interface. By using tri-state logic with fixed Doff, it can eliminate the RHP zero in the transfer function Io/ Don and control the current flow all the time. However, the voltage gain of the output voltage over the supercapacitor’s voltage decreases because of the fixed Doff, which is an issue when the converter only operates in one mode and the supercapacitor voltage changes between half voltage (24 V) to rated voltage (48 V). Thus, the mode must be changed between Boost and Buck-Boost modes as the voltage in the supercapacitor varies. The tri-state logic can present different sequences with the three states Don, Doff, and Dfw. Thus the traditional pulse width modulation (PWM) technique using one modulating signal Don cannot be used in the tri-state logic. This thesis proposes a flexible space vector modulation scheme, which concerns the modes of operation, the sequences of the tri-states, and the state of each switch. A smooth mode transition logic is presented to reduce the output current variation when the mode changes between the Boost and Buck-Boost modes. The main goal of this work is to regulate the output current at a set value, while the converter changes between the Boost and Buck-Boost modes as a function of the voltage gain (Vo/Vin) with the input voltage varying in a wide range. The analysis of the system and design of controllers are presented and verified. Simulation results iv with the proposed modulation scheme and smooth mode transition logic, as well as experimental implementation, are presented and discussed