Patent ID: 8085007

Claim:
A switching power supply circuit, comprising: a switch connected to one of two input terminals to which an input voltage is applied; an energy transmitter which transmits, as energy, the input voltage applied through said switch; an output generation circuit which smoothes the energy transmitted by said energy transmitter and outputs the smoothed energy through two output terminals as an output voltage; an output voltage detection circuit which detects the output voltage of said output generation circuit and generates a detected current signal according to the output voltage; a transmission circuit which is connected to one of the two output terminals and outputs transmission signal according to a value of the detected current signal generated by said output voltage detection circuit; a control circuit which generates a drive signal that controls a switching of said switch according to the transmission signal, using a potential at a node between said switch and said energy transmitter as an operating reference voltage; a rectifier, is connected between said transmission circuit and said control circuit, which outputs the transmission signal only in a direction from said transmission circuit to said control circuit; and a capacitor connected to a control terminal of said control circuit, wherein an other one of the two output terminals is connected to a potential ground (GND), said output voltage detection circuit includes: a first resistor and a second resistor inserted in series between the two output terminals; and a current signal output circuit which outputs, to said transmission circuit, a current signal according to a divided voltage at a node between said first resistor and said second resistor, as the detected current signal, said current signal output circuit includes: a shunt regulator having a terminal for detecting a reference voltage connected to the node between said first resistor and said second resistor; a cathode connected to said transmission circuit; and an anode connected to the other one of the two output terminals, wherein said current signal output circuit generates the detected current signal according to an error between (i) a voltage of the node between said first resistor and said second resistor and (ii) a reference voltage that is pre-set in said shunt regulator, said rectifier is configured to apply the transmission signal in a direction to said control circuit only when said switch is off, and to prevent current from flowing into said transmission circuit from said control circuit which has a higher potential than said transmission circuit when said switch is on, said capacitor converts the transmission signal into a feedback signal with a constant current and voltage, and applies the feedback signal to said control terminal, and said control circuit generates, based on the feedback signal, the drive signal for maintaining a voltage of said output generation circuit at a constant value.