A power supply system comprising: an input line configured to receive input AC power; a rectifier having an input coupled to the input line and an output, the rectifier configured to rectify the input AC power to generate rectified AC power; a switch having a first end coupled to the output of the rectifier and a second end selectively coupled to an inductor, the switch configured to receive the rectified AC power from the rectifier; a capacitor coupled to the inductor; and control circuitry (107, 116) coupled to the inductor and the capacitor, comprising a processor having a power supply input coupled to the inductor and the capacitor, characterized in that the control circuitry is configured to: control the switch to repeatedly and selectively couple the output of the rectifier to the inductor, thereby generating a first DC power level supplied to said capacitor and derived from the rectified AC power during an active mode of the control circuitry, and a second DC power level supplied to said capacitor and derived from the rectified AC power during a standby mode of the control circuitry, monitor the rectified AC power received by the switch, so as to detect a phase angle of an approaching zero crossing window of the rectified AC power; transition from the active mode into the standby mode in response to detection of the approaching zero crossing window, said processor being configured to place selected processor tasks other than a task for controlling said switch on hold in the standby mode; transition from the standby mode to the active mode once having determined that the zero-crossing window has passed. an input line configured to receive input AC power; a rectifier having an input coupled to the input line and an output, the rectifier configured to rectify the input AC power to generate rectified AC power; a switch having a first end coupled to the output of the rectifier and a second end selectively coupled to an inductor, the switch configured to receive the rectified AC power from the rectifier; a capacitor coupled to the inductor; and control circuitry (107, 116) coupled to the inductor and the capacitor, comprising a processor having a power supply input coupled to the inductor and the capacitor, characterized in that the control circuitry is configured to: control the switch to repeatedly and selectively couple the output of the rectifier to the inductor, thereby generating a first DC power level supplied to said capacitor and derived from the rectified AC power during an active mode of the control circuitry, and a second DC power level supplied to said capacitor and derived from the rectified AC power during a standby mode of the control circuitry, monitor the rectified AC power received by the switch, so as to detect a phase angle of an approaching zero crossing window of the rectified AC power; transition from the active mode into the standby mode in response to detection of the approaching zero crossing window, said processor being configured to place selected processor tasks other than a task for controlling said switch on hold in the standby mode; transition from the standby mode to the active mode once having determined that the zero-crossing window has passed. monitor the rectified AC power received by the switch, so as to detect a phase angle of an approaching zero crossing window of the rectified AC power; transition from the active mode into the standby mode in response to detection of the approaching zero crossing window, said processor being configured to place selected processor tasks other than a task for controlling said switch on hold in the standby mode; transition from the standby mode to the active mode once having determined that the zero-crossing window has passed.