High step-up converters are required and used in photovoltaic applications, due to low voltage of photovoltaic modules. In this paper, an isolated dc-dc high step-up SEPIC with a Greinacher voltage quadrupler cell is presented. It has the advantage of continuous input current, high efficiency, high voltage gain, isolation and demands a single switch, being suitable for low power grid-tie photovoltaic systems. The operating principles and steady-state analysis are presented, including the detailed analysis of resonant stage, where the value of primary side capacitor is taken into account and plays an important role in the design of the converter, since it directly affects the resonance frequency and RMS current values. Simulation results are presented to validate the analysis and design. https://doi.org/10.53316/sepoc2021.049