Abstract
High step-up voltage ratio DC converters are widely used in a variety of high voltage applications such as the front-end stages of uninterruptible power systems and distributed energy systems. Also, for safety reasons, galvanic isolation is essential. Although one can use high turns ratio transformer and/or coupling inductors to achieve high step-up voltage ratio, however, the resulting high voltage stress and decreased efficiency are the major drawbacks. Hence, in this thesis, a different approach is employed to achieve high step-up voltage ratio without relying on a high turns ratio transformer and/or coupling inductors. Basically, the major contributions of this thesis may be summarized as follows. Frist, a series of novel isolated two-phase high step-up voltage ratio DC converters are proposed to achieve a voltage gain of, where is isolated transformer turns ratio, is the duty ratio of the converter and . Special features of the proposed converters include less voltage stress of all active switches and diodes, low input current ripples, automatic current sharing among input phases, and higher efficiency. Second, modeling and characteristic analysis of a six times voltage multiplier converter are made in the context. In addition, optimal design of output capactiors is investigated to achieve minimum output voltage ripples. Third, a 400W prototype with 25V input and 400V output is constructed to verify the feasibility of the proposed converter. It is seen that the voltage stress of active switches and diodes are and respectively, and the corresponding European and maximum efficiencies are 93.49% and 95.56% respectively. Also, by adding an active clamped circuit, the resulting efficiencies are raised to 94.73% and 96.04% respectively. Finally, for even higher power levels, the corresponding three-phase and four-phase DC converters with voltage gain are also presented for reference. Keywords：Muti-Phase DC Converter, Isolated Converter, High Step-Up Voltage Ratio, High Efficiency.