Patent ID: 7400063

Claim:
A switched-mode power supply comprising: a primary circuit including a primary winding and a switching device, the primary winding constituting a part of a transformer; a plurality of secondary circuits each including a secondary winding and a load, the secondary windings constituting a part of the transformer, the switching device turning on/off an electric power supply to the primary winding to produce secondary electric powers applied from the plurality of secondary windings to the loads, a specified-voltage sensor section for detecting an output voltage from only a specified one of the secondary circuits; and an output limiting section for limiting an electric power output to at least one non-specified secondary circuit of the plurality of secondary circuits when the specified-voltage sensor section has detected a voltage being more than or equal to a first predefined value in the specified secondary circuit, wherein the output limiting section includes: a total power sensor section for detecting a value in accordance with a total secondary power which is a sum of power outputs to all the secondary circuits; a control section for controlling turning on/off of the switching device in accordance with a detection result from the total power sensor section; a short-circuit section, including a thyristor, provided between output lines for the specified secondary circuit, for quasi-shorting the output lines together when the specified-voltage sensor section has detected the voltage being more than or equal to the first predefined value, wherein the specified secondary circuit includes a resistor between the specified-voltage sensor section and the load, and wherein a holding current required for the thyristor to be quasi-shorted is retained flowing through the thyristor even when the load for the specified secondary circuit is shorted out.