This UPS reference design was built employing off-line topology with AVR boost and buck regulation of the mains. The inverter module contains the push-pull DC-DC converter and the DC-AC output full bridge generating a quasi-sine waveform. A microcontroller controls all UPS functions. It monitors the mains parameters (input/output mains voltage and phase, output inverter voltage, battery voltage, output current and output power) in order to ensure the proper level of the output voltage. The UPS was designed with RS-232 joint signal and USB interfaces (USB for RS-232 converter). It is possible to connect the UPS to a PC using an RS-232 or USB cable and control the UPS status through the "UPS Monitor 3.0x" software that must be installed on the same PC.