Patent ID: 9065342
Filing Date: 2015-06-23
Classification: H02M

Abstract:
1. A switching power supply comprising: a transformer including a primary winding and a secondary winding, so that an AC voltage based on a current supplied to the primary winding is output through the secondary winding; a switching element which controls current supply to the primary winding; a rectifying/smoothing circuit which converts the AC voltage output from the secondary winding to a DC voltage; a voltage detecting circuit which detects the DC voltage converted by said rectifying/smoothing circuit; a feedback amplifier which is supplied with the DC voltage detected by said voltage detecting circuit as one input and which supplied with a reference voltage as the other input, so as to output a difference between values of these input voltages; and a control circuit which controls said switching element, so as to eliminate the difference detected at said feedback amplifier, wherein upon receipt of an instruction signal from an external apparatus, said voltage detecting circuit supplies a voltage higher than a voltage output before receipt of the instruction signal, to said feedback amplifier, and wherein said voltage detecting circuit includes a series circuit in which a variable-resistance circuit and a resistive element whose resistance value is fixed are connected in series, the series circuit is connected in parallel between output lines connected to the secondary winding, a divided voltage according to a ratio between the respective resistance values of the variable-resistance circuit and the resistive element is supplied to one input of said feedback amplifier, and the ratio between the respective resistance values varies upon receipt of the instruction signal.