Abstract: An UPS is an equipment that provides continuous power supply, with high levels of quality and reliability. This equipment is usually employed for supplying mission-critical systems, in which disturbances or power interruptions can cause financial losses or damages (e.g., hospitals, bank offices, datacenters, etc.). In the literature, several topologies have been proposed as an alternative to the classical configuration with low-frequency transformers, as a solution for enhancing efficiency and power density and decreasing overall costs. Among these alternatives, transformerless solutions (i.e., without transformers) become promising, as they do not require low-frequency transformers which comprise an expressive part of overall weight and volume of a conventional UPS. In this context, a three-arm UPS is often reported as a suitable transformerless topology for low- and medium-power single-phase UPS, with high efficiency and power density. This thesis aimed to assess the performance of the single-phase three-arm UPS, concerning efficiency and power density. An alternative connection for battery set was also evaluated. Furthermore, a prototype of a direct double-jacketed calorimeter was built in order to measure the losses of power converters. The adopted methodology was based on the comparison of several three-arm UPS designs with devices made by different materials (silicon, silicon carbide and gallium nitride) and of various technologies (IGBTs, MOSFETs and cascode configuration with silicon MOSFET and GaN HFET) and packages (TO-247-3, TO-247-4, SMT top and HSOF-8). The UPS was also simulated in MATLAB/Simulink, in order to validate the modulation strategies and the techniques adopted for controlling, synchronizing and commutating between the normal state and the stored energy mode, as well as the alternative connection of the battery set. These simulations demonstrated the operation in asynchronous and synchronous modes, i.e., with an output frequency that can be decoupled or not to the means frequency. All details related to construction, dimensioning and calibration of the double-jacketed calorimeter were also presented. Preliminary experimental results were also discussed in this thesis.