The analysis of a switching power supply for functional specification verification with a fly-back converter operated discontinuously in voltage control mode is demonstrated by two methods; 1) behavioral and 2) pulse-by-pulse. The behavioral method employs state-space-time averaged elements and equivalent averaged sources. The pulse-by-pulse model contains a mixed analog and digital signal model of the controller integrated circuit chip. The behavioral model gave useful swept frequency information such as Bode diagrams. The model was very efficient in computational time. The pulse-by-pulse model is useful for parts stress calculations and high frequency effects. Because of the “brute force” calculation of each switch, the pulse-by-pulse model required a lot of time on the computer. Because of the long simulation times, the pulse-by-pulse model was used only after the desired results were obtained using the state averaging model. However, as computers become more powerful, the pulse-by-pulse model will become the preferred model for most analysis.