Switched-mode power supply having a standby state

Claim 1

A self-oscillating power supply circuit having an operating state and a stand-by state comprising: a pair of terminals for connection to a source of DC voltage, a transformer having a primary winding and at least first and second secondary windings, a first controllable switch connected in series arrangement with the primary winding of the transformer, means coupling said series arrangement to said pair of terminals, said first and second secondary windings supplying first and second DC output voltages, respectively, in the operating state of the power supply circuit, a second controllable switch for controlling the power supply circuit into the stand-by state wherein the first secondary winding supplies a lower value of the first DC output voltage than in the operating state, a third controllable switch coupled to the second secondary winding and controlled by the second switch so that in the stand-by state the second secondary winding maintains the second output voltage at approximately 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, and a duration-determining circuit controlled by the second switch in the stand-by state so as to maintain a burst oscillation mode in which the first switch conducts a number of consecutive times and subsequently is non-conductive for a given time period and with an oscillation frequency much lower than the repetition frequency of said drive pulses.