An uninterruptible power supply apparatus (100, 100'), comprising: a switch configured to be coupled to an alternating current, AC, power source for receiving an AC power (Vac); a first inductor coupled to the switch; a direct current, DC, controlling unit (20, 20') coupled to the switch and configured to be coupled to a DC power source for receiving a DC power (Vdc); wherein the direct current controlling unit (20, 20') comprising: a second inductor coupled to the switch and configured to be coupled the DC power source; a power diode coupled to the second inductor; and a power switch coupled to the power diode and configured to be coupled to the DC power source; wherein the uninterruptible power supply apparatus further comprising: a first bridge arm coupled to the first inductor; a second bridge arm connected in parallel with the first bridge arm; a bus capacitor (Cbus) connected in parallel with the second bridge arm; and a third bridge arm connected in parallel with the second bridge arm, wherein the AC power (Vac) is configured to be converted to a bus voltage (Vbus) across the bus capacitor (Cbus) through the first inductor, the first bridge arm and the second bridge arm, and then converted to an output power (Vo) through the second bridge arm and the third bridge arm when the switch is switched to connect the first inductor to the AC power source, and the DC power (Vdc) is configured to be converted to the bus voltage (Vbus) through the DC controlling unit (20, 20') and the first inductor, and then converted to the output power (Vo) through the second bridge arm and the third bridge arm when the switch is switched to connect the first inductor to the DC controlling unit (20, 20'); wherei n the second inductor is coupled to the first inductor such that a voltage across the first inductor is obtained by multiplying a voltage across the second inductor by a turn ratio of the first inductor to the second inductor. a switch configured to be coupled to an alternating current, AC, power source for receiving an AC power (Vac); a first inductor coupled to the switch; a direct current, DC, controlling unit (20, 20') coupled to the switch and configured to be coupled to a DC power source for receiving a DC power (Vdc); wherein the direct current controlling unit (20, 20') comprising: a second inductor coupled to the switch and configured to be coupled the DC power source; a power diode coupled to the second inductor; and a power switch coupled to the power diode and configured to be coupled to the DC power source; wherein the uninterruptible power supply apparatus further comprising: a first bridge arm coupled to the first inductor; a second bridge arm connected in parallel with the first bridge arm; a bus capacitor (Cbus) connected in parallel with the second bridge arm; and a third bridge arm connected in parallel with the second bridge arm, a first bridge arm coupled to the first inductor; a second bridge arm connected in parallel with the first bridge arm; a bus capacitor (Cbus) connected in parallel with the second bridge arm; and a third bridge arm connected in parallel with the second bridge arm, wherein the AC power (Vac) is configured to be converted to a bus voltage (Vbus) across the bus capacitor (Cbus) through the first inductor, the first bridge arm and the second bridge arm, and then converted to an output power (Vo) through the second bridge arm and the third bridge arm when the switch is switched to connect the first inductor to the AC power source, and the DC power (Vdc) is configured to be converted to the bus voltage (Vbus) through the DC controlling unit (20, 20') and the first inductor, and then converted to the output power (Vo) through the second bridge arm and the third bridge arm when the switch is switched to connect the first inductor to the DC controlling unit (20, 20'); wherei n the second inductor is coupled to the first inductor such that a voltage across the first inductor is obtained by multiplying a voltage across the second inductor by a turn ratio of the first inductor to the second inductor.