[Hube]

If you have proper air coming IN from the soffits and proper air going OUT at the ridge, then that is all that is necessary.
A fan is normally not required IF soffit and ridge vents are sized properly
For each 150 sq ft of attic floor area; 1 sq ft of "free" vent area.
1 sq ft = 144 sq in.
Example; a 12 x 12 opening has an area of 144 sq in ( 1 sq ft)....but if this 12 x 12 opening is screened with a 1/4" mesh, then the "free" area is cut down approx 20% to only 116 sq in.
Example; An attic floor area of 900 sq ft would require a MINIMUM of 6 sq ft of "free" vent area
(divided 50/50 between soffit and ridge)
Thus 3 sq ft of "free vent area would be the MINIMUM amount required for the soffit and also the ridge.
Make sure you have enough area, and keep in mind as long as you have good insulation within the attic you can never have too much venting.