Switched-mode power supply having a standby state

Claim 1

A switched-mode power supply circuit having an operating state and a standby state comprising: a first controllable switch connected in series arrangement with a primary winding of a transformer, the series arrangement being coupled to the terminals of a d.c. input voltage, said transformer having a first secondary winding for providing a first d.c. output voltage in the operating state and a second secondary winding for providing a second d.c. output voltage, a second switch for bringing the supply circuit into the stand-by state in which the value of the first output voltage is considerably lower than in the operating state, and a third switch controllable by means of the second switch and coupled to the second secondary winding for maintaining the second output voltage in the stand-by state at substantially the same value as in the operating state, a comparison stage for comparing an output voltage with a reference voltage and for generating a control signal for controlling the duration of periodically occurring drive pulses applied to the first switch, characterized in that a duration-determining element is coupled to the second secondary winding and to the comparison stage for maintaining an oscillation mode in the stand-by state, whereby the first switch conducts a number of consecutive times and subsequently is non-conductive during a given period, under the influence of the control signal, the frequency of said oscillation being many times lower than the repetition frequency of the drive pulses applied to the first switch.