This article proposes a modified interconnection and damping assignment passivity-based control (IDA-PBC) for dc/dc converter cascaded with an LC filter. The plant is modeled using the port-controlled Hamiltonian (PCH) form. The main objective is to stabilize the cascaded system in case the system supplies constant power load (CPL). To solve the instability issues caused by tightly controlled cascaded systems, the IDA-PBC based on an overall PCH model, including LC input filter and dc/dc converter, is established. Moreover, to ensure that the proposed IDA-PBC admits one unique solution, an adaptive interconnection matrix is designed to build the internal links in the PCH model. Furthermore, in order to improve the implementation on an onboard dc microgrid application with time-varying CPLs, a modified IDA-PBC algorithm is proposed based on the error between the state vector and the desired operating point, which might be variable. The closed-loop Hamiltonian function is chosen as the Lyapunov candidate function to guarantee that the system operates in a stable manner. The virtual damping assignment technique is addressed to tune the dynamic characteristic of the closed-loop system. Simulation and experimental results are carried out to illustrate the proposed method’s effectiveness.