PURPOSE: To reduce the loss of the elective breaking circuit of a parallel running DC power supply, by eliminating the voltage drop of its current sensing circuit through the direct sensing of the current flowing in a MOSFET.

CONSTITUTION: Normally, in the drain-source voltage of a MOSFET, the source and drain voltages are brought respectively to a plus and a minus voltages. In this case, when the noninverting and inverting inputs of a voltage discriminator CV are connected respectively with the source and drain of the MOSFET and the sensed drain-source voltage of the MOSFET is fed to the inputs, the MOSFET is turned on by the holding of the output of the voltage discriminator CV in a high voltage. Thereby, the voltage drop of the MOSFET is reduced. Also, when a DC power supply fails, in the drain-source voltage of the MOSFET, the source and drain voltages are brought respectively into minus and plus voltages. Therefore, by the holding of the output of the voltage discriminator CV in a low voltage, the MOSFET is turned off, and thereby, the faulty DC power supply is separated from another DC power supply.