Research On Non-isolated High Step-Up DC-DC Converters

The output voltage of photovoltaic(PV)source and fuel cell is generally low.In the gridconnected power system,high voltage step-up and high-efficiency dc-dc converters are needed due to the relatively high bus voltage of the grid-connected inverter.In the case where galvanic isolation is not required,the non-isolated converters are preferred.This dissertation is dedicated to non-isolated high step-up dc-dc converters.This dissertation proposes three methods to increase the voltage gain of the non-isolated dc-dc converters.The first method is to adopt switched capacitors to increase the voltage gain.The basic principle is that the switched capacitors are charged in parallel and discharged in series,and thus a high voltage gain is achieved.An inductor-type energy storage cell is proposed to charge the switched capacitors,and the voltages of the switched capacitors can be regulated by regulating the duty cycle of the power switch.As a consequence,the output voltage could be regulated.Based on this mehod,a family of high step-up switched-capacitor dc-dc converters adopting inductor-type energy storage cells is proposed.And the operating principle and high efficiency of the proposed converter are verified by the experimental results from a 25 V ~ 45 V input,380 V output,100 W prototype.The second method is to adopt switched capacitors and coupled inductor to increase the voltage gain.Based on the high step-up switched-capacitor dc-dc converter,a coupled inductor is introduced to further increase the voltage ratio.As a result,the voltage stress of the power switch is reduced,leading to reduced conduction loss and switching loss,and thus the efficiency of the proposed converters is increased.Based on this method,a family of high step-up dc-dc converter adopting switched capacitors and coupled inductor is proposed,and the operating principle of the proposed converters is verified by the experimental results from a 25 V ~ 45 V input,380 V output,100 W prototype.By adopting the coupled inductor,the efficiency is higher.The third method is to adopt auxiliary capacitor and coupled inductor to increase the voltage gain.An auxiliary capacitor is introduced into the input side of the boost converter.The capacitor serves as a voltage source in series with the input voltage source,and thus the input voltage is equivalently increased,which leads to high voltage gain.Meanwhile,the input current remains continuous and the input current ripple is low.To reduce the voltage stresses of the power switch and diodes,a coupled inductor is further introduced to charge multiple output capacitors.These output capacitors are connected in series,and the voltage of each output capacitor can be reduced.Thus,the voltage stress of the power switch can be reduced,and the conduction loss and switching loss can be reduced.Based on this method,a high step-up dc-dc converter adopting auxiliary capacitor and coupled inductor is proposed.The operating principle of the proposed converter is verified by the experimental results from a 25 V ~ 45 V input,380 V output,300 W prototype.Due to the continuous input current and small current ripple,the proposed converter is suitable for the medium or high power application.