Clocked flyback converter circuit for operating one or more illuminants, comprising: a controllable switch, a transformer having a primary winding coupled to the controllable switch and a secondary winding to which the illuminant can be coupled, a control device for controlling the controllable switch, and means (8a,..., 8c) for detecting a temporal mean value of the current flowing through the switch and for supplying a signal representing this mean value to the control device, wherein the control device is designed to determine, on the basis of this signal, the respective time at which the switch is switched off and back on again, the transformer has a primary-side auxiliary winding, to which is coupled the control device for detecting the voltage emitted by the auxiliary winding, and the control device is designed to derive the voltage U LED dropping across the illuminant from the voltage detected by the auxiliary winding, characterized in that the control device is designed to detect at least one operating state of the flyback converter circuit; to determine a power loss P loss of the flyback converter circuit associated with this operating state and stored in the control device; to determine the current I LED flowing through the illuminant, taking into account the determined power loss P loss according to the formula I LED = (U in ·I in -P loss)/U LED; and to determine the point in time at which the switch is switched off and back on again, on the basis of a deviation of the determined current I LED from a prespecified value, wherein U in is the voltage applied to the primary winding, I in is the temporal mean value of the current flowing through the primary winding and indicated by the signal, and P loss is the power loss of the flyback converter circuit. a controllable switch, a transformer having a primary winding coupled to the controllable switch and a secondary winding to which the illuminant can be coupled, a control device for controlling the controllable switch, and means (8a,..., 8c) for detecting a temporal mean value of the current flowing through the switch and for supplying a signal representing this mean value to the control device, wherein the control device is designed to determine, on the basis of this signal, the respective time at which the switch is switched off and back on again, the transformer has a primary-side auxiliary winding, to which is coupled the control device for detecting the voltage emitted by the auxiliary winding, and the control device is designed to derive the voltage U LED dropping across the illuminant from the voltage detected by the auxiliary winding, characterized in that the control device is designed to detect at least one operating state of the flyback converter circuit; to determine a power loss P loss of the flyback converter circuit associated with this operating state and stored in the control device; to determine the current I LED flowing through the illuminant, taking into account the determined power loss P loss according to the formula I LED = (U in ·I in -P loss)/U LED; and to determine the point in time at which the switch is switched off and back on again, on the basis of a deviation of the determined current I LED from a prespecified value, wherein U in is the voltage applied to the primary winding, I in is the temporal mean value of the current flowing through the primary winding and indicated by the signal, and P loss is the power loss of the flyback converter circuit.