Switching-power supply unit for reducing power consumption in a stand-by-mode

A pulse-width modulation switching power-supply unit having a rectifier circuit for converting an alternating current from an alternating power supply to a direct current, a switching element for switching a current rectified by this rectifier circuit, and a primary coil for supplying a current to be switched by this switching element, said switching power-supply unit further comprising:

• a transformer having a secondary coil for inducing power corresponding to current supplied to this primary coil;
• DC output means for rectifying and smoothing a power induced at this transformer's secondary coil to output direct current power;
• voltage control means for generating an error voltage based on a comparison between said direct current power output from said DC output means and a reference voltage;
• pulse-width modulation controller for controlling a switching operation of said switching element according to said error voltage;
• and
• control means for controlling operations of said pulse-width modulation controller,
• wherein said control means, in a normal operation mode, continuously operates said pulse-width modulation controller so as to operate said switching element at a basic oscillation frequency, wherein said control means, in a standby mode, intermittently operates said pulse-width modulation controller so as to turn on said switching element at a given repetition interval and for a given time period, wherein that repetition interval is shorter than a time for decreasing an output voltage from said DC output means down to a guaranteed load operating voltage due to power consumption of the load and said DC output means in the standby mode and is longer than a time for entering a sonic frequency band, and wherein that time period is longer than a time for stabilizing an output from said DC output means and is longer than a time for entering a sonic frequency band, and wherein said control means further comprises a microcontroller and uses program software to variably set a control time such as a repetition interval in said standby mode using a watchdog timer and to supply a wakeup signal to a starter circuit of the pulse-width modulation controller at the repetition interval so as to stabilize a secondary output voltage.