In this thesis a computer aided design of air-gapped ferrite cores for forward, push-pull and flyback SMPSs is presented. The design includes minimum core size selection from given core series, minimum acceptable number of turns, and minimum acceptable spacer thickness which are achieved to minimize weight, size, and cost selection and maximize efficiency and reliability of the SMPS converters . The design is based on maximizing the time constant of primary winding for transformer and winding for output inductor, as maximum as possible to increase the calculated power-handling capability of the core for a given spacer thickness. From the results obtained it is found that computer aided design of air-gapped ferrite core for SMPS is an efficient tool which can be used to save time, effort and cost of the practical implementation.