A device for the correction of the power factor in power supply units with forced switching operating in transition mode is described. The device comprises a converter and a control device coupled to said converter so as to obtain from an alternating network input voltage a regulated output voltage on the output terminal. The converter comprises a power transistor and the control device comprises a pilot circuit suitable for determining the period of switched-on time and the period of switched-off time of said power transistor and control means coupled to said pilot circuits and with said converter and which are capable of prolonging said period of switched-on time of the power transistor at the instants of time in which the alternating network voltage substantially assumes the value zero.