Synchronous flyback converter having terminals for supplying current to an LED load, comprising: - a sensing winding (L w)coupled to a primary winding (L p) of a transformer T of the flyback converter, - a control unit configured to control a primary side switch in series to the primary winding (L p) of the flyback converter using a feedback signal for a closed loop control of a secondary side voltage of the flyback converter by controlling the frequency and/or duty cycle of the switching of the primary side switch, wherein the feedback signal is derived from a sensing voltage (V w) across the sensing winding (L w) sampled by the control unit once during a switch-on cycle of a secondary side switch of the flyback converter or at every n -th switch-on cycle thereof, n being an integer, and - a comparator configured to compare a primary side current (i p) to a reference value (i ref) to check when a zero-cross occurs, characterized in that the control unit is configured to control the secondary side switch such that there are time periods with negative current flow on the secondary side, the control unit is configured to measure a time T p the primary side switch is closed, a time Ts the secondary side switch is closed, and a time interval Δ in which the primary side current (i p) is greater than the reference value (i ref), the control unit is configured to calculate a ratio: ratio = Δ / T p, and the control unit is configured to sample the sensing voltage (V w) at a time point t s: t s = T s Δ / T p. - a sensing winding (L w)coupled to a primary winding (L p) of a transformer T of the flyback converter, - a control unit configured to control a primary side switch in series to the primary winding (L p) of the flyback converter using a feedback signal for a closed loop control of a secondary side voltage of the flyback converter by controlling the frequency and/or duty cycle of the switching of the primary side switch, wherein the feedback signal is derived from a sensing voltage (V w) across the sensing winding (L w) sampled by the control unit once during a switch-on cycle of a secondary side switch of the flyback converter or at every n -th switch-on cycle thereof, n being an integer, and - a comparator configured to compare a primary side current (i p) to a reference value (i ref) to check when a zero-cross occurs, characterized in that the control unit is configured to control the secondary side switch such that there are time periods with negative current flow on the secondary side, the control unit is configured to measure a time T p the primary side switch is closed, a time Ts the secondary side switch is closed, and a time interval Δ in which the primary side current (i p) is greater than the reference value (i ref), the control unit is configured to calculate a ratio: ratio = Δ / T p, and the control unit is configured to sample the sensing voltage (V w) at a time point t s: t s = T s Δ / T p. the control unit is configured to control the secondary side switch such that there are time periods with negative current flow on the secondary side, the control unit is configured to measure a time T p the primary side switch is closed, a time Ts the secondary side switch is closed, and a time interval Δ in which the primary side current (i p) is greater than the reference value (i ref), the control unit is configured to calculate a ratio: ratio = Δ / T p, and the control unit is configured to sample the sensing voltage (V w) at a time point t s: t s = T s Δ / T p.