To calculate a control signal for the duty cycle of a switched mode power supply (SMPS), a voltage feed forward compensator is integrated into a feedback unit. A control unit has an adder with a first input to receive a signal dependent upon an output voltage. A second input receives a signal from a feedback circuit, and an output outputs a signal comprising the sum of the signals applied to the inputs; another output outputs a control signal for controlling the duty cycle, the control signal is dependent upon the output signal from the adder; a voltage feed forward compensation signal generator calculates a compensation signal to adjust the control signal depending upon an input voltage of the SMPS; and a feedback circuit between the output of the adder and the second input of the adder calculates a compensated feedback signal and to feed the compensated feedback signal to the second input of the adder.