A switching mode power supply SMPS (100, 300, 400, 500, 600) usable with an image forming apparatus wherein the SMPS is configured to receive an alternating current AC power from an external electric power supply source and to output a direct current DC power, the SMPS comprising: a transformer (350, 450) to transform an electric power input to a primary coil and to output the transformed electric power to a secondary coil; a switching unit, the switching unit comprising a main switch (362, 462) connected to the primary coil to adjust the output power of the transformer (350, 450) and a switch controller (364, 464) connected to the main switch (362, 462) to control a switching operation of the main switch (362, 462); a blocking unit arranged to block an electric power input to the switch controller (364, 464) when a supply state of an electric power to the image forming apparatus is a turned-off state; and an electric power supplying terminal to supply the electric power input to the switching unit, wherein when the supply state of the electric power to the image forming apparatus is the turned-off state, the blocking unit is configured to discharge a potential remaining between the electric power supplying terminal and the switch controller (364, 464) so that an output voltage of the SPMS is lowered below a reset voltage of the image forming apparatus to enable a rebooting of the image forming apparatus, wherein the blocking unit comprises: a signal detector (374, 474, 574, 674) configured to detect the supply state of the electric power to the image forming apparatus; and an electric power blocking unit (372, 472, 572, 672) configured to block the electric power input to the switch controller (364, 464) according to the detected supply state, wherein the electric power blocking unit (372, 472, 572, 672) comprises a discharging circuit located between the electric power supplying terminal and the switching unit to discharge electric charges that are charged in the electric power supplying terminal, and wherein the electric power blocking unit (372, 472, 572, 672) is configured to block the electric power input to the switch controller (364, 464) by discharging the electric charges of the electric power supplying terminal when the supply state of the electric power to the image forming apparatus is the turned-off state. a transformer (350, 450) to transform an electric power input to a primary coil and to output the transformed electric power to a secondary coil; a switching unit, the switching unit comprising a main switch (362, 462) connected to the primary coil to adjust the output power of the transformer (350, 450) and a switch controller (364, 464) connected to the main switch (362, 462) to control a switching operation of the main switch (362, 462); a blocking unit arranged to block an electric power input to the switch controller (364, 464) when a supply state of an electric power to the image forming apparatus is a turned-off state; and an electric power supplying terminal to supply the electric power input to the switching unit, wherein when the supply state of the electric power to the image forming apparatus is the turned-off state, the blocking unit is configured to discharge a potential remaining between the electric power supplying terminal and the switch controller (364, 464) so that an output voltage of the SPMS is lowered below a reset voltage of the image forming apparatus to enable a rebooting of the image forming apparatus, wherein the blocking unit comprises: a signal detector (374, 474, 574, 674) configured to detect the supply state of the electric power to the image forming apparatus; and an electric power blocking unit (372, 472, 572, 672) configured to block the electric power input to the switch controller (364, 464) according to the detected supply state, wherein the electric power blocking unit (372, 472, 572, 672) comprises a discharging circuit located between the electric power supplying terminal and the switching unit to discharge electric charges that are charged in the electric power supplying terminal, and wherein the electric power blocking unit (372, 472, 572, 672) is configured to block the electric power input to the switch controller (364, 464) by discharging the electric charges of the electric power supplying terminal when the supply state of the electric power to the image forming apparatus is the turned-off state. a signal detector (374, 474, 574, 674) configured to detect the supply state of the electric power to the image forming apparatus; and an electric power blocking unit (372, 472, 572, 672) configured to block the electric power input to the switch controller (364, 464) according to the detected supply state, wherein the electric power blocking unit (372, 472, 572, 672) comprises a discharging circuit located between the electric power supplying terminal and the switching unit to discharge electric charges that are charged in the electric power supplying terminal, and wherein the electric power blocking unit (372, 472, 572, 672) is configured to block the electric power input to the switch controller (364, 464) by discharging the electric charges of the electric power supplying terminal when the supply state of the electric power to the image forming apparatus is the turned-off state.