The fuel cell systems are considered as current intensive sources with low output voltage, so power electronic converters are essential part of fuel cell applications. A dc/dc converter with high voltage ratio and low stress is developed in this paper. The inductors of the output series interleaved boost converter (OS-IBC) are interleaved in parallel at input which can reduce the input current ripple. Besides, the structure of output series can provide a high voltage ratio. A small signal ac model and the corresponding transfer function of the OS-IBC is derived as well as controller design is carried out in this paper. A series of simulation and experimental results validate the small signal ac model and the controller of the OS-IBC.