Switching power source device

摘要

The present invention provides a switching power supply which includes a main rectifying and smoothing circuit (40) and a secondary rectifying and smoothing circuit (50) for rectifying and smoothing an output obtained at a secondary coil (22) of a converter transformer (20) whose primary coil (21) is supplied with a switching output from a switching element (30) which switches a direct input. A error detecting circuit (60), which is driven by a secondary rectified and smoothed output from the secondary rectifying and smoothing circuit (50), detects a main rectified and smoothed output from the main rectifying and smoothing circuit (40) to be supplied to loads so as to control the switching operation of the switching element (30) by the PWM controlling circuit (80) so that the main rectified and smoothed output is caused to be a predetermined state, and compares the voltage of the secondary rectified and smoothed output with the voltage of the main rectified and smoothed output so as to change the operation state of the PWM controlling circuit (80) to the standby mode in case the voltage of the secondary rectified and smoothed output declines to come below the voltage of the main rectified and smoothed output by a predetermined value.