In this diploma work, something will be said about converters, and converters, if the direction of the energy flow is not taken into account, are the broadest type of power electronics circuits, because they exist in every part of electronic equipment that is powered from the electrical network. The basic property of these converter circuits is the bidirectional flow of energy, which means that an alternating current system can be converted to direct current and opposite. The AC-DC converter through which energy passes from the AC system to the DC system is called a rectifier. The DC-AC converter through which energy passes from the DC system to the AC system is called an inverter. We use PWM comparators in uninterruptible power supply systems, in other words UPS. Uninterruptible power supply systems are used to power important and specific devices with appropriate direct or alternating voltage in the event of a primary power source failure. Uninterruptible power supply systems must meet two basic criteria: power supply continuity and voltage quality. Depending on the UPS structure itself, we distinguish: off-line, interactive and on-line systems. This type of system is modeled in the MATLAB/PLECS program package, and after modeling, a simulation is performed on the system. After the analysis of the simulation, results were obtained that can be used for a better implementation of the real system.