Power supply

The power source comprises an output port for outputting output direct current (DC) voltage and a circuitry for uninterruptible power supply. An intermediate circuit (10) is arranged between an input side DC-DC converter (8) and an output-side output switching regulator (9-1-9-n), to control the output DC voltage. An energy storage module (11) is attached at the intermediate circuit. An independent claim is included for method for operating power source.