Research On Topology Improvementof A Bidirectional DC-DC Converter And Its Efficiency Increasement For UPS

A bidirectional DC-DC converter can exchange energy in two-way flow in a single module. It has the advantages of high power density and cost, so it has been widely used in Uninterruptible Power System(UPS), electric vehicles, photovoltaic power generation, multi-port power supply and other systems which can achieve bi-directional energy transfer. The bidirectional DC-DC technology has become a research hotspots in the world in recent years. Aiming at the application of low cost Online UPS, this paper focuses on the design of a bidirectional DC-DC converter and its efficiency improvement.Traditional current type push-pull full bridge topology can’t meet the soft starting for UPS and stable operation under no-load or light load. In this thesis, a secondary winding of inductance and two fly-wheel diodes are proposed and added to the traditional topology. This improvement can make push-pull switches work in arbitrary duty cycle state, which satisfies the design requirements of the bidirectional DC-DC for UPS.The working process of the improved design is simulated in order to verify its feasibility. Then the design of feedback compensation is carried out to improve performance of the converter. The state space averaging method is used to establish a small signal model for system and the compensation parameters of the voltage outer loop and current inner loop are obtained through the transfer function. The stability of the system is verified by the frequency domain and time domain simulation, too.The magnetic component loss and semiconductor switch loss are the main factors that affect efficiency of the converter. In order to improve the bidirectional DC-DC’s efficiency, this paper starts from two aspects, the high frequency transformer design and the novel switching device. Maxwell 2D software is firstly used to simulate the loss of transformers with different winding arrangements. A better winding arrangement mode then can be choiced to reduce the transformer winding loss and improve the efficiency of the converter. Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistor(SIC MOSFET) is used to replace the traditional SI MOSFET or Insulated Gate Bipolar Transistor(IGBT) to reduce the loss of the switches and improve the efficiency of the converter.Finally, a prototype is made and measured in order to test the feasibility of the design. The experimental results show that the improved topology can realize soft starting and stable operation of the system under different loads with higher efficiency.