In this paper a dynamic analysis of the nonisolated and isolated dc-dc SEPIC (Single Ended Primary Inductance Converter) based on the substitution of the switches (transistor and diode) by PWM averaged switch model equivalent circuit is presented. The SEPIC converter is used when the wide range of input voltages is required. Besides the switches the SEPIC converter circuit consists of two inductors and two capacitors. Due to this, its dynamic model is of the fourth order differential equations system. The duty cycle and transformer turn ratio have great impact on the converter dynamics. The system parameters design guidelines are given to assure that the dynamic system has well separated resonance frequencies. The derived dynamic model was verified by simulation toolbox SimPowerSystems.