Patent ID: 7295449

Claim:
Switched-mode power supply with a primary side and a secondary side and with a transformer with a primary-side winding, a secondary-side winding and at least one auxiliary winding, wherein the primary-side winding and the auxiliary winding are connected to the primary side and the secondary-side winding is connected to the secondary side, a primary-side switch, which is connected to the primary-side winding in order to interrupt a flow of current through the primary-side winding, a freely oscillating control circuit for generating switching pulses which drive the primary-side switch, a circuit for generating an image voltage between the terminals of the auxiliary winding, in order to generate an image voltage, which on the primary side replicates a voltage to be controlled on the secondary side, wherein the switched-mode power supply further comprises a timing unit, which is coupled to the primary-side switch such that the duration of a turn-off period of the primary-side switch is variable within a switching cycle, wherein the timing unit comprises a control capacitor, and wherein the turn-off time of the primary-side switch can be adjusted by the charging current of said control capacitor, and wherein the timing unit comprises a diode, which is arranged between the primary-side switch and an input terminal of the switched-mode power supply such that the charging current of the control capacitor can be limited during the turn-off time of the primary-side switch.