A power supply in which control of the switch-on instant of a power device of a boost-circuit (operating in a discontinuous mode) is implemented by monitoring the current which actually flows through the "fly-back" inductance. The current is monitored on a sensing resistance, and, by the use of a comparator, a signal is produced for enabling/disabling the transfer to an input of a PWM driving circuit of a switch-on signal produced by a null detector. This configuration provide good noise immunity, which is particularly useful in power-factor-correction applications, and reduced power dissipation.