Input circuit for a power supply at least having: - a switch element (SE) arranged on the input side; - an inductance (L), which is arranged in series with the switch element (SE); and also a freewheeling diode (FD); wherein, through periodic switching of the switch element (SE) between a conductive state and a blocked state, an input voltage (Ue) is able to be converted to an output voltage (Ua); wherein the inductance (L), at least partially during a switching period of the switch element (SE), has a current (I L ) flowing through it, which charges a capacitance (Ca) arranged on the output side at which the output voltage (Ua) is able to be made available; wherein the freewheeling diode (FD) absorbs the current (I L ) in the blocked state of the switch element (SE) and wherein, in the blocked state of the switch element (SE) and if a predefined breakthrough voltage (Ud) at the switch element (SE) is exceeded, a current flow (I AVAL ) via the switch element (SE) is permitted, characterised in that , on occurrence and recognition of an overvoltage on the input side of the input circuit (ES), the switch element (SE) is put into the blocked state, that the current flow (I AVAL ) via the switch element (SE) in the blocked state is relayed to the inductance (L) and the freewheeling diode (FD) is blocked, and that the switch element (SE) and the inductance (L) are dimensioned in such a way that, on occurrence of the overvoltage, what is known as avalanche energy arises at the switch element (SE), at which the switch element (SE) withstands a current (I L ) flowing through the inductance (L) for a duration of the overvoltage. - a switch element (SE) arranged on the input side; - an inductance (L), which is arranged in series with the switch element (SE); and also a freewheeling diode (FD); wherein, through periodic switching of the switch element (SE) between a conductive state and a blocked state, an input voltage (Ue) is able to be converted to an output voltage (Ua); wherein the inductance (L), at least partially during a switching period of the switch element (SE), has a current (I L ) flowing through it, which charges a capacitance (Ca) arranged on the output side at which the output voltage (Ua) is able to be made available; wherein the freewheeling diode (FD) absorbs the current (I L ) in the blocked state of the switch element (SE) and wherein, in the blocked state of the switch element (SE) and if a predefined breakthrough voltage (Ud) at the switch element (SE) is exceeded, a current flow (I AVAL ) via the switch element (SE) is permitted, characterised in that , on occurrence and recognition of an overvoltage on the input side of the input circuit (ES), the switch element (SE) is put into the blocked state, that the current flow (I AVAL ) via the switch element (SE) in the blocked state is relayed to the inductance (L) and the freewheeling diode (FD) is blocked, and that the switch element (SE) and the inductance (L) are dimensioned in such a way that, on occurrence of the overvoltage, what is known as avalanche energy arises at the switch element (SE), at which the switch element (SE) withstands a current (I L ) flowing through the inductance (L) for a duration of the overvoltage. <EOS>