A power supply covering both power sharing and power backup functions run in a more efficient and flexible way. The power supply adopts a power sharing converter coupled between a first bus terminal and a second bus terminal, so that if one of the bus terminals provides insufficient power, the other bus terminal kicks in by way of the power sharing converter to provide power support. In addition, a storage capacitor may also kick in to provide power support if one of the bus terminals provide insufficient power via or not via the power sharing converter.