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 (T) having a primary winding (Np) which is electrically connected to a first switch (S1), and a secondary winding (Ns) which is electrically connected to an output (A1, A1') of the synchronous flyback converter circuit via a second switch (S2), the illuminant line being connectable to the output (A1, A1'), characterized by - a control unit designed to actuate the first switch (S1) and to actuate the second switch (S2) with electrical isolation, and - a detection circuit ; - wherein the transformer (T) galvanically isolates a primary side (PS) of the synchronous flyback converter circuit on which the primary winding (Np), the first switch (S1), the control unit and the detection circuit are arranged, from a secondary side (SS) of the synchronous flyback converter circuit on which the secondary winding (Ns), the second switch (S2) 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 (S1) and a temporal mean value of the negative component of the switch current (I s1 ) flowing through the first switch (S1); - feed to the control unit a first signal (Sg1) representing the temporal mean value of the negative component of the switch current (I s1 ) and a second signal (Sg2) representing the temporal mean value of the positive component of the switch current (I S1 ). - alternatingly actuate the first switch (S1) and the second switch (S2) on the basis of the first signal (Sg1) and the second signal (Sg2) fed to the control unit . - a transformer (T) having a primary winding (Np) which is electrically connected to a first switch (S1), and a secondary winding (Ns) which is electrically connected to an output (A1, A1') of the synchronous flyback converter circuit via a second switch (S2), the illuminant line being connectable to the output (A1, A1'), - a control unit designed to actuate the first switch (S1) and to actuate the second switch (S2) with electrical isolation, and - a detection circuit ; - wherein the transformer (T) galvanically isolates a primary side (PS) of the synchronous flyback converter circuit on which the primary winding (Np), the first switch (S1), the control unit and the detection circuit are arranged, from a secondary side (SS) of the synchronous flyback converter circuit on which the secondary winding (Ns), the second switch (S2) 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 (S1) and a temporal mean value of the negative component of the switch current (I s1 ) flowing through the first switch (S1); - feed to the control unit a first signal (Sg1) representing the temporal mean value of the negative component of the switch current (I s1 ) and a second signal (Sg2) representing the temporal mean value of the positive component of the switch current (I S1 ). - alternatingly actuate the first switch (S1) and the second switch (S2) on the basis of the first signal (Sg1) and the second signal (Sg2) fed to the control unit . <EOS>