Bi-directional full bridge DC/DC converter is a nonlinear time-varying system. To design the closed-loop control system, it needed to be transformed into a continuum mathematical model. This paper proposed a modeling method by using state space average method and the small signal analysis method to build the model. Firstly, a nonlinear mathematical model of the converter was established by using the state space average method, and on this basis, using small signal analysis method to convert the nonlinear mathematical model to a continuum mathematical model. Then the stability criterion of the system is proposed and the PID parameters were adjusted by combining with the automatic control theory. Furthermore the digital closed-loop network of each working mode is designed. Finally by simulation and a series prototype experiments, the results verified the feasibility of closed-loop control system and the DC/DC converter. (C) 2016 Elsevier Ltd. All rights reserved.