In the present paper, a closed-loop thermosyphon cooling system for power electronics has been developed and tested. The evaporator is a multi-microchannel heat sink designed for high heat fluxes. The power electronics module is emulated by four independent electrical heaters, which can generate a non-uniform heat flux on the evaporator. The condenser is air-cooled and is tested in both natural convection and forced convection modes. R1234ze has been selected as the working fluid. Although in natural convection mode the cooling system experiences a maximum thermal resistance of approximately 0.6 K/W, the complete passiveness makes such a system very attractive. With values of heat fluxes of 107 W/cm(2) and 25 W/cm(2) equally distributed on the four heaters, the maximum heater temperature measured is 53 degrees C at a saturation temperature of 45 degrees C. In forced convection mode, two different values of air flow have been tested. A reduction of 50% on the overall thermal resistance has been measured with an imposed air flow rate of 87.8 m(3)/h, consuming only 5.29 W to drive the fan for dissipating 70.2 W. At this condition, the maximum heater temperature is reduced to 36 degrees C. In summary, it has been demonstrated that a thermosyphon coupled with air cooling can dissipate high heat fluxes of power electronics with no or little energy consumption. (C) 2017 Elsevier Ltd. All rights reserved.