In the example, the at least one rib includes a single rib having two ends respectively connected to two opposite sides of the lateral wall. Thus, the structure is simple and the manufacturing cost is lowered.

In the example, the at least one rib includes a plurality of ribs connected to each other in sequence. A first one of the plurality of ribs has a free end connected to the lateral wall. A last one of the plurality of ribs has a free end connected to the lateral wall. Thus, the structure is simple and the manufacturing cost is lowered.

In the example, the at least one rib includes two ribs respectively connected to the bottom and the cover of the case. Thus, the structure is simple and the convenience in assembly is improved.

In the example, one of the two ribs which is connected to the cover includes an extension portion, and the extension portion is connected to a lateral face of said rib at a free end of said rib. Thus, a reliable connection between the two ribs can be provided while reducing the amount of the air that leaks out of the gap at the interconnected ribs.

In the example, the cover includes a lateral wall connected to an edge of a back plate, the air inlet is located on the lateral wall, and the heat outlet is located on the cover. Thus, the structure is simple and the convenience in assembly is improved.

In the example, the cooling fan is spaced from the bottom of the case at a gap. The air entrance of the cooling fan faces the bottom of the case. The air exit of the cooling fan faces the heat outlet of the case. The heat source is located between the air inlet of the case and the cooling fan. Thus, the cooling fan can draw the heat and then expel the heat through the heat outlet of the case.