This study presents a comprehensive approach for the design optimisation of passive components and heatsinks in power electronic converters. The design and selection of passive components are posed as a multi-objective optimisation problem, which can be simplified to a single-objective optimisation based on preferences and weights assigned by the designer. The mathematical equations describing the efficiency, volume, and reliability are laid out to describe the feasible space for optimisation in this study. In addition to the proposed optimisation, a substantial amount of passive component comparison data has been consolidated and presented in this study. The results from the optimisation are demonstrated on the target power electronic circuit of a regulated transformer rectifier unit for more-electric aircrafts. The optimisation framework is developed in a generic manner that would lend itself to the optimisation of any choice of converter topology, circuit components, or objective functions.