Research On Push-pull Forward Three-level DC-DC Converter For Wide Input-voltage Range

With the increasing concern about the fossil energy shortage and environmental pollution,the renewable energy sources,such as solar energy and fuel cells,have the characteristics of clean,no pollution and flexible.Therefore,they get more and more attention by people in the distributed generation system.The output of fuel cell or solar cell is direct current.In addition,the output voltage of them is often low and varies widely.The input current ripple is high in the traditional voltage-source DC-DC converter.When the converter works in the wide input-voltage range and high step-up ratio,the voltage stress of the rectifier diode is high.The output current ripple and voltage spike of the switches are high in the traditional current-source DC-DC converter.It limits the converter operating in wide input-voltage range application.Therefore,the research of the DC-DC converter for low and wide input-voltage range application is important and practical.A push-pull forward three-level(PPF TL)DC-DC converter for wide input-voltage range application is proposed.The proposed converter can operate in three-level and two-level modes.The input and output current ripples are low,and the voltage spike of the switches is low.Therefore,it is fit for low and wide input-voltage range applications.The operating principle of the PPF TL DC-DC converter is illustrated,and the waveform of the operating mode for switch tube is analyzed.According to the equivalent circuits of 0 level,1/2 level,and 1 level,a small signal model for the converter is established.The transformer,output filter inductor,output filter capacitor,input divided capacitors,MOSFETs,and the second-side rectifier diode are designed and selected.The control strategy of the converter is described in detail.The specific design schemes for the control circuit is given,such as the output voltage control circuit,current limit control circuit,and the input voltage feed-forward loop.According to the loss calculation of transformer,output filter inductor,MOSFETs,and the second-side rectifier diode,the efficiency curve of the converter is given under different input voltage and output power,the performance of the converter is analyzed and the improvement measures are put forward.The proposed converter is simulated using the Saber software.A 500 W PPF TL DC-DC converter principle prototype was built.The simulation and experimental results from the PPF TL DC-DC converter confirm the theoretical analysis.