A converter to supply an AC voltage and current from three DC voltages comprising two switching units provided with first switching means connected between an input and a switching output, said converter comprising, for each switching unit, second switching means connected between said switching unit and a modulated signal output, and a switching aid circuit, said converter comprising control means acting on the second switching means associated with the switching unit that is connected to the voltage input of opposite sign to the sign of said AC voltage to establish turn-off of said second switching means when said AC voltage and said AC current are of opposite signs.An uninterruptible power supply comprising the converter described above.