Abstract: Negative feedback control of a power supply is improved with respect to accuracy of control in that the controlled value is monitored by parallel analog to digital conversion. Additionally, within the negative feedback control loop adjustment of the controlled value to follow a rated value is performed by pulse-width modulation. So as not to be bound with respect to accuracy of adjustment by pulse-width modulation to a minimum pulse-width adjustment increment, fine adjustment is additionally done by superimposing to the pulse-width modulation a pulse-frequency modulated signal.