A control circuit generates a control signal to control a duty cycle of a switched mode power supply such that the magnetic flux density in the transformer is balanced, thereby preventing saturation of the transformer core. This permits the use of unsymmetrical duty cycles within the switch cycle. The control circuit comprises a flux density calculator and a regulator. The flux density regulator receives a signal indicative of the input voltage of the switched mode power supply and a feedback signal comprising the generated control signal, and it generates therefrom an average flux density signal. The regulator receives the generated average flux density signal and a signal indicative of the output voltage of the switched mode power supply, and generates the control signal in dependence upon the average flux density signal, the reference flux density signal, the signal indicative of the output voltage and a reference voltage signal.