A conventional shunt resistor (12) for detecting a current value includes a resistance element and electrodes (12a) and (12b) at both ends of the resistance element, as illustrated in FIG. 11. When such a shunt resistor (12) is surface-mounted to a substrate to detect a current value, patterns, of which the width is adjusted to that of the electrodes at both ends of the shunt resistor, are recommended as patterns (50) and (51) on which the shunt resistor (12) is surface-mounted, as illustrated in FIG. 12. In such recommended patterns, the current generated at a current-generating point (50a) flows perpendicularly to the electrodes (12a) and (12b) of the shunt resistor (12), passes laterally through the shunt resistor (12), and then flows toward a current inflow point (51a). A current detection circuit (40) is connected to the shunt resistor (12) through detection patterns (12C) connected to both ends of the shunt resistor (12), and detects the value of the flowing current based on a voltage value across the shunt resistor (12) detected through the detection pattern (12C) and a resistance value of the resistance element.
However, when the value of current flowing from a motor to a capacitor is detected by the shunt resistor (12), for example, the recommended patterns as illustrated in FIG. 12 are not adopted in many cases due to some limitations on the layout of components such as a power module for driving a motor and a capacitor. In that case, those elements are not arranged along the line that runs straight through the shunt resistor (12). For instance, a current-generating point (52a) of a pattern (52) connected to a power module for driving a motor and a current inflow point (53a) of a pattern (53) connected to a capacitor may be arranged diagonally to each other with the shunt resistor (12) interposed therebetween as illustrated in FIG. 13. In such cases, as illustrated in FIGS. 13 and 14, the current generated will be distributed all over the patterns, and will pass through the shunt resistor (12) not only laterally through it but also vertically through it, obliquely across it, and in various other directions. As a result, a potential difference is created in the direction in which the electrodes (12a) and (12b) of the shunt resistor (12) extend (i.e., in the longitudinal direction), resulting in occurrence of an error of the current value detected by the shunt resistor (12). Moreover, as illustrated in FIG. 15, if the output terminals of a three-phase motor are independently provided for the three phases U, V, and W and if the respective phases have their own current-generating points (52u), (52v), and (52w), the paths of the current flowing into the shunt resistor (12) will be different from each other between the three phases, resulting in a variation in current detection value from one phase to another, which is a problem with the related art.
In order to overcome such a problem, according to a conventional technique, the pattern (52) including the current-generating points and the pattern (53) including the current inflow point may be each provided with slits (60) around both of the electrodes of the shunt resistor (12) as illustrated in FIG. 16 to prevent the current paths from spreading right around the shunt resistor (12). On the other hand, according to another conventional technique, the pattern (54) including the current-generating points and the pattern (55) including the current inflow point may have their width that are narrowed toward the shunt resistor (12) and have their width matched to that of the shunt resistor (12) right around the shunt resistor (12) as illustrated in FIG. 17 to prevent the current paths from spreading too much and regulate the current paths into a desired shape, thereby reducing a dispersion in current detection value.
However, according to both of these conventional countermeasures illustrated in FIGS. 16 and 17, the current path of the patterns is narrowed in the vicinity of the shunt resistor (12), and the heat generated by the shunt resistor (12) is not transferred easily to these patterns to result in poor heat dissipation performance and causing the problem of heat generation by the shunt resistor (12).
The heat generated by the shunt resistor may be reduced by improving the heat dissipation performance of the patterns with their thickness increased, for example. However, the larger the amount of the current flowing through the shunt resistor is, the larger the quantity of heat generated by the shunt resistor is. For that reason, there is a limit to such a measure of adjusting the thickness of the patterns.
As for a conventional technique for improving the heat dissipation performance of a shunt resistor, Patent Document 1 discloses a configuration in which a radiator having a U-shaped cross-section is stacked on a resistance element (shunt resistor), including an electrode on a substrate, with a resistive base member interposed between them and in which the resistive base member and the radiator are bonded to each other with an adhesive having thermal conductivity and the radiator is crewed onto the substrate in order to improve the heat dissipation performance of the shunt resistor by utilizing the heat dissipation ability of the radiator.