A DC power supply device includes transformers that operate with an enhanced efficiency and can reduce the factors that generate ripples in the output
voltage to make it possible to downsize the transformers and the
smoothing filters of the device, reduce the loss attributable to the switching elements during its
constant power output operation and also downsize the
heat sink fins so that the overall dimensions of the power supply device may be reduced. The DC power supply device also comprises a pair of switching changers, two output transformers and two rectifying /
smoothing sections for rectifying the respective outputs of the switching changers. Each of the two switching changers has the same
switching cycle period and the striking phases of the switching elements of one of the switching changers and those of the switching elements of the other switching changer are made variable. Then, a
constant power output level is achieved by utilizing the difference of the striking phases of the two switching changers in order to isolate the switching changers from each other for operation and connect them in series or in parallel for operation.