A power device includes a first conversion module and a second conversion module. The first conversion module includes a first conversion unit and receives a first divided voltage of a bus voltage. The second conversion module includes a cascade conversion circuit with a second conversion unit and a third conversion unit. The input terminal of the second conversion module and the input terminal of the first conversion module are connected with each other in series. The input terminal of the second conversion module receives a second divided voltage of the bus voltage. The output terminal of the second conversion module and the output terminal of the first conversion module are electrically connected with each other in parallel so as to provide an output voltage to a load. The third conversion unit detects the output voltage to control the output voltage stable.