Impeller and centrifugal fan using the same

A centrifugal fan comprising:

• a casing including a square shaped lower casing;
• and a square shaped upper casing having an air inlet opening;
• and four pillars disposed between the upper casing and the lower casing, wherein air outlet openings are provided between the pillars;
• an impeller housed in the casing, wherein the impeller includes an annular upper shroud, a main plate, and a plurality of blades disposed between the annular upper shroud and the main plate;
• and
• a fan motor for rotating the impeller,
• wherein an upper surface of the annular upper shroud faces the upper casing includes a curved surface,
• wherein each of the pillars is disposed in a vicinity of a corner portion of the casing, and
• wherein a center of rotation of the impeller is the same with a center of the casing and a plurality of angles formed by a plurality of straight lines connecting between the center of rotation of the impeller and each of the pillars are different from each other.