At SEPIC converter there are no equations which summarize the overall behaviour of this circuit converter. Determining factors like the throughput power of the circuit and peak voltages and currents, means developing a computer model. In this model the operation of the circuit is broken down into its separated modes and the appropriate equations derived for each of theme. For all modes the time function for the currents and voltages can be derived by circuit analysis. Using Laplace transformation it is possible to derive the time functions for currents in each of its modes. The method allows the initial values of currents and voltage to be easily introduced into equation.