The continuous time state-space models of DC/DC converters such as Boost, Buck-boost and Flyback DC/DC can be expressed by a series of bilinear differential equations. The main difficulties in controlling these kinds of DC/DC converters arise from dealing with the bilinear term and the inherent hybrid nature of a DC/DC converter. To solve the problems, this paper proposes a hybrid modeling and optimal control approach. The switch period is first divided into multiple sub-periods so that the discrete-time state-update equation of the converter can be derived. Then the bilinear term which contains the whole state-update equation is linearized in a piecewise fashion. In this way the hybrid model of converter can be established. By taking a Boost converter and a non-inverting Buck-boost converter as research objects, their hybrid models are established, and then the optimal controls are carried out. Through the comparison of the control effects of the two DC/DC converters, the commonality, effectiveness and superiority of the proposed method are proved. This work is supported by National Natural Science Foundation of China (No. 61561008), Basic Scientific Research Ability Improvement Project for Middle-young Teachers of Gunangxi Universities (No. 2020KY04027), Xiangsi Lake Young Scholars Innovation Team of Guangxi University of Nationalities (No. 2018RSCXSHQN03), and Introduction of Talent Project of Guangxi University of Nationalities (No. 2018KJQD18).