A switch-mode power supply device (1) is disclosed. The switch-mode power supply device (1) has a main circuit (6) configured to receive a DC input voltage and to provide a DC output voltage. The main circuit (6) comprises: an inductor element (12) generating the DC output voltage, a switching element (9) connected to the inductor element (12), and a controller (7) configured to switch the switching element (9) between a conducting state and a non-conducting state, wherein the switching element (9) is configured to feed a pulsed direct current to a ground potential (10). The switch-mode power supply (1) also has an auxiliary circuit (16) configured to provide an auxiliary voltage. The auxiliary circuit (16) comprises an auxiliary inductor (18) connected to receive the pulsed direct current and magnetically isolated from the inductor element (12).