The design of an active-unity power factor correction circuit with variable-hysteresis control for off-line dc-to-dc switching power supplies is described. Design equations relating the boost inductor current ripple to the circuit components selection and circuit performance arc discussed. A computer-aided design program (CADO) is developed to give the optimal circuit components selection. A 500 watt, 300 volt experimental circuit is built to verify the simulation and analysis results. The control-to-output response of the power factor circuit is verified with the experimental results. Design guidelines for the low-frequency feedback network are presented. Small-signal closed-loop responses are measured with an experimental power factor circuit. / Master of Science