Centrifugal fan

A centrifugal fan, comprising:

• a housing, wherein a top side of the housing has a main air inlet, a bottom side of the housing has a holder, a board, and a plurality of supporting ribs connecting the board and the holder, and the holder, the board, and the supporting ribs define a first secondary air inlet and a second secondary air inlet;
• and
• an impeller having a central axis and a hub portion, and installed on the holder, wherein a top surface and a bottom surface of the hub portion respectively face toward the top side and the bottom side, the first secondary air inlet does not shield an outer edge of the impeller, the second secondary air inlet shields a portion of the outer edge of the impeller, and a ratio of an area of the first secondary air inlet not shielding the impeller to an area of the second secondary air inlet not shielding the impeller is in a range from 1.4 to 1.6;
• wherein in an extending direction of the central axis, the impeller projects an upper projection to the top side, and projects a lower projection to the bottom side;
• wherein a size of the upper projection is greater than a size of the main air inlet, and at least one portion of an outer edge of the first secondary air inlet, an outer edge of the lower projection, and the central axis are arranged in sequence.