An electronic control unit in which high-heat-generating devices, such as switching devices, are mounted on a circuit board is known. The high-heat-generating device generates a lot of heat when the electronic control unit operates. For example, in an electronic control unit disclosed in JP-A-2011-23459 corresponding to US 2011/0013370A1, switching devices as high-heat-generating devices are mounted on a surface of a circuit board, and a surface of a heat-dissipating member is located facing the surface of the board.
In the electronic control unit disclosed in JP-A-2011-23459, the switching device is held in a room formed in the heat-dissipating member. However, the room is not shaped to match the shape of the switching device, and the capacity of the room is a few times larger than the volume of the switching device. For this reason, in the assembled condition of the electronic control unit, there is a large distance between the inner wall of the room and the outer wall of the switching device. Accordingly, it may be difficult to efficiently transfer heat of the switching device to the heat-dissipating member. As a result, the heat of the switching device is not efficiently dissipated through the heat-dissipating member.
Further, in the assembled condition of the electronic control unit disclosed in JP-A-2011-23459, there is a large distance between the facing surfaces of the heat-dissipating member and the board. Accordingly, it may be difficult to transfer the heat of the switching device to the heat-dissipating member through the board.
Furthermore, in the assembled condition of the electronic control unit disclosed in JP-A-2011-23459, a heat-conducting member is provided in each room. Accordingly, the number of man-hours needed to place the heat-conducting member may be increased.
In particular, the electronic control unit disclosed in JP-A-2011-23459 is used to control a motor of an electric power steering apparatus for a vehicle. When the motor of the electric power steering apparatus is driven and produces assisting-torque to help a driver to steer the vehicle, a large current flows through the motor and the high-heat-generating devices of the electronic control unit. Therefore, when the motor is driven, the high-heat-generating devices generate a lot of heat. For this reason, when an electronic control unit is used to control a motor of an electric power steering apparatus, it is preferable that heat of high-heat-generating devices of the electronic control unit should be dissipated efficiently.