Synchronous flyback converter circuit for operating an illuminant line with at least one illuminant, in particular with at least one light-emitting diode, wherein the synchronous flyback converter circuit comprises: - a transformer having a primary winding (Np) which is electrically connected to a first switch, and a secondary winding (Ns) which is electrically connected to an output (A1, A1') of the synchronous flyback converter circuit via a second switch, the illuminant line being connectable to the output (A1, A1'), characterized by - a control unit designed to actuate the first switch and to actuate the second switch with electrical isolation, and - a detection circuit; - wherein the transformer galvanically isolates a primary side of the synchronous flyback converter circuit on which the primary winding (Np), the first switch, the control unit and the detection circuit are arranged, from a secondary side of the synchronous flyback converter circuit on which the secondary winding (Ns), the second switch and the output (A1, A1') are arranged; and - wherein the detection circuit is designed to - capture separately a temporal mean value of the positive component of the switch current (I s1) flowing through the first switch and a temporal mean value of the negative component of the switch current (I s1) flowing through the first switch; - feed to the control unit a first signal representing the temporal mean value of the negative component of the switch current (I s1) and a second signal representing the temporal mean value of the positive component of the switch current (I S1). - alternatingly actuate the first switch and the second switch on the basis of the first signal and the second signal fed to the control unit. - a transformer having a primary winding (Np) which is electrically connected to a first switch, and a secondary winding (Ns) which is electrically connected to an output (A1, A1') of the synchronous flyback converter circuit via a second switch, the illuminant line being connectable to the output (A1, A1'), - a control unit designed to actuate the first switch and to actuate the second switch with electrical isolation, and - a detection circuit; - wherein the transformer galvanically isolates a primary side of the synchronous flyback converter circuit on which the primary winding (Np), the first switch, the control unit and the detection circuit are arranged, from a secondary side of the synchronous flyback converter circuit on which the secondary winding (Ns), the second switch and the output (A1, A1') are arranged; and - wherein the detection circuit is designed to - capture separately a temporal mean value of the positive component of the switch current (I s1) flowing through the first switch and a temporal mean value of the negative component of the switch current (I s1) flowing through the first switch; - feed to the control unit a first signal representing the temporal mean value of the negative component of the switch current (I s1) and a second signal representing the temporal mean value of the positive component of the switch current (I S1). - alternatingly actuate the first switch and the second switch on the basis of the first signal and the second signal fed to the control unit.