A power supply device includes a main unit and a power switching module. The main unit includes a primary circuit board, a transformer including a primary and a secondary coil, a primary-side circuit and a secondary-side circuit. The power switching module includes a separate PCB formed with at least two connection pads and two conductive tracks, and at least one power switching element disposed on the PCB and having two terminals respectively connected to the two connection pads through the two conductive tracks. The power switching module is in the form of a separate PCB that is electrically connected to the primary- or secondary-side circuits through the two connection pads.