Patent ID: 7236381

Claim:
A current balancing circuit for use in a switching power supply unit that operates in parallel with one or more other switching power supply units with current balancing terminals connected to each other so as to apply a stable voltage to a load, adapted to control a balance between output currents by comparing output current detection voltages of the switching power supply units, comprising: a first resistor that receives a detection signal of the output current input via a diode and outputs the signal as an output current detection voltage; a second resistor connected between the current balancing terminal and a connection point between the first resistor and the diode; an operational amplifier that receives an output current detection voltage, which is applied to the current balancing terminal from a balancing circuit of the other one or more switching power supply units, via a connection point between the second resistor and the current balancing terminal and an input resistor, and receives the output current detection voltage from the first resistor via the connection point between the first resistor and the diode and an input resistor, so as to output a signal corresponding to a difference between the output current detection voltage applied from the balancing circuit of the other one or more switching power supply units and the output current detection voltage from the first resistor; and a switching circuit configured to increase an inside impedance as viewed from the current balancing terminal by disconnecting the second resistor from the current balancing terminal in response to a failure detection signal or by disconnecting the first resistor from ground in response to the failure detection signal.