This paper proposes a novel concept for a loop thermosyphon cooling system, designed for the transistor module of a transformer. The thermosyphon uses a free convection cooled condenser, making the system completely passive, i.e. no energy consumption is needed for it to function. Furthermore, the novel configuration uses the separation of the liquid and vapor phases at the evaporator outlet in such a way that the liquid is recirculated at the evaporator inlet while the vapor goes up in the riser. The frictional pressure drops in the riser and the condenser are thus reduced, and the overall gravitational force on the working fluid is increased, leading to an increase in the mass flow rates. Two low-GWP working fluids are tested in both the single and recirculation loops: R1234ze and R1234yf. The comparison between the two loop configurations is carried out discussing the temperature readings of the heaters used to emulate the transistors, the overall thermal resistance and the mass flow rates. It is demonstrated that the recirculation loop is a desirable solution for low-pressure working fluids, such as the R1234ze, because of the higher reduction of the liquid phase in the riser. For high-pressure fluids, such as the R1234yf, the improvements regarding mass flow rates are minor, but still present. At the maximum heat load tested for each configuration (65 - 70 W), the overall thermal resistance is within the range 0.44 - 0.48 K/W, and the temperature of the heaters is always lower than 55 degrees C. According to these data, the thermosyphon system proposed here seems to be an optimal solution for the cooling of high-heat flux electronic devices.