A flyback step-up dc-to-dc power converter is modeled utilizing a linear magnetic core, transistor switch, diode, filter capacitance, and load resistance. Assuming constant voltage drops across the semiconductor elements when conducting, the circuit is analyzed to obtain an equation for the required number of turns, subject to the constraints of specified output voltage, operating frequency, maximum winding factor, maximum and minimum input voltage, output power, and flux density. Using an interpretive language, a program has been written to run on a minicomputer which searches a data base of magnetic core characteristics obtained from commercial core catalogs and prints a list of windable cores. In an evaluation section of the program, a core from the windable core list is selected by the designer and the losses in the various elements, along with maximum and minimum transistor current are computed. Another program is used to display on a computer graphics device the loci of maximum and minimum flux density in a selected core as functions of input voltage and output power in a perspective plot. Converter circuits derived from this computer-aided design procedure when tested in the laboratory have yielded data in close agreement with that predicted by the program.