On-board power supply devices employ a DC/DC converter that converts, by stepping-up or stepping-down, a DC voltage into a desired DC voltage. Such a DC/DC converter is required to be able to accurately detect the magnitudes of electric currents flowing through input-side and output-side conduction paths. For example, JP H09-119949A discloses an overcurrent detection circuit having a configuration in which an input voltage for a load device is divided at a diode and a voltage-dividing resistor, and a resultant voltage is applied as a base bias to the base of a transistor. If an overcurrent flows through a power supply path and a voltage across terminals of a current-detecting resistor is larger than a voltage across terminals of the voltage-dividing resistor, the transistor is biased in a forward direction and is turned on, thereby transmitting an overcurrent detection signal to a power supply device.
JP H09-119949A is an example of related art.
However, in the configuration of JP H09-119949A, if detecting currents at a plurality of positions within the circuit is desired, a plurality of current detection circuits having the same configuration above need to be provided. Specifically, if detecting currents flowing through an input-side conduction path and an output-side conduction path of a DC/DC converter is desired, one current detection circuit for each conduction path, namely, two current detection circuits, are required. For example, if, as shown in FIG. 5, it is desired that currents be detected based on signals output from sensors respectively provided on different conduction paths, one current detection circuit (a circuit including a resistor, a comparator, and the like) that corresponds to each of the sensors is required. This leads to the problem of an increase in the number of constituent components.
To solve this problem, it is conceivable to provide a current detection circuit, as shown in FIG. 6, that is partially shared by conduction paths, with a configuration having an OR circuit using diodes. However, due to the use of the diodes, a voltage drop will occur, and the diodes will have temperature characteristics. Accordingly, in a configuration in which the magnitudes (thresholds) of voltages to be detected are set based on voltage drops of the diodes, the thresholds may vary and the current detection accuracy may deteriorate.