Sepic and Flyback topology were analyzed in this paper. To meet the requirements of DC/DC module power,auxiliary supply voltage was designed based on Sepic and Flyback mixed topology. The auxiliary supply voltage had multiple outputs and could realize the isolation of outputs. The model was derived by the state space averaging method. A closed loop control for Sepic converter was discussed in detail. The closed loop control had two control loops, voltage loop and current loop. Simulation were carried out to confirm the effectiveness of the topology by Matlab. The auxiliary supply voltage based on Sepic and Flyback mixed topology with three isolated 12 V and one non-isolated 6 V outputs was applied in a DC/DC converter that has wide input range and wide output range. Experimental verifications confirm the validity of the auxiliary supply voltage based on Sepic and Flyback mixed topology.