Two-phase micro-scale cooling implemented with passive, gravity-driven closed-loop thermosyphons represents a highly-reliable solution to increase heat dissipation and to maximize energy efficiency of the next-generation electronics cooling technologies. The current paper presents an updated description of the general simulation software presented at ITHERM 2017, which is able to analyze and design thermosyphon-based cooling systems with high accuracy. In particular, the present simulator is mainly composed of two nested sub-routines: (i) an internal routine that incorporates the best literature methods to evaluate local flow boiling and local condensation heat transfer coefficients and pressure drops in micro-scale evaporator cold plates (at the heat source location) and condensers (at the heat rejection location); and (ii) an external routine that includes the methods to simulate all components of a thermosyphon (condenser, riser, downcomer, evaporator and, optionally, liquid accumulator) with their operational characteristics and thermal performance. In addition, a new extensive experimental validation of the thermosyphon code is performed, in which the simulation results are compared against a comprehensive thermosyphon database, including several types of micro-scale evaporators, different types of condensers (air-cooled and liquid-cooled), various riser/downcomer diameters, a range of thermosyphon heights and numerous refrigerants as working fluids. In fact, the thermosyphon simulator has been validated over a wide range of thermosyphon sizes, going from the smallest height of 15 cm for server cooling applications up to the largest height of 50 cm to cool high-power telecommunications electronics. Finally, the paper discusses the effects of different parameters on the thermosyphon thermal-hydraulic performance, such as working fluid, riser/downcomer diameters, secondary side coolant inlet temperature and mass flow rates, filling ratio and heat load in order to give guidelines and recommendations for an accurate and robust system design.