In this paper, a new three-port DC-DC converter with high voltage conversion ratio is proposed. The proposed converter is useful for photovoltaic (PV) systems, which have a PV source integrated with an extra battery source to supply the output load. The voltage conversion ratio of the high dc voltage port over the low voltage dc port can be more increased by increasing the turns ratio of the coupled inductors for the whole range of duty cycles. The main advantage of the proposed converter comparing to other conventional three-port converters, is achieving higher voltage gain and higher ratio of voltage gain over components’ number. The proposed converter has also the leased normalized voltage stresses on switches for duty cycles higher than 0.3 among the conventional converters of the same type. Moreover, two output voltages of the proposed converter can be simultaneously regulated on different constant levels with a proper precision. In this study, the proposed converter is analyzed and the voltage conversion ratios, the inductors’ average currents, the voltage and current stress on switches are calculated theoretically. Finally, to verify the operation of the proposed converter, the experimental and simulation results of 20V/40V/400V prototype are extracted.