The DC bias current flowing through the inductor causes the inductance to drop from the initial value. It is due to the magnetic properties of the core. The core, and some of the space around it, can only store a given amount of magnetic flux density. Beyond the maximum flux density point, the permeability of the core is reduced. Thus, the inductance is caused to drop. The drop affects the original filtering characteristics of the inductors, so for limiting the workable range, generally, the DC bias current causing the inductance drop 10% or 20% is defined as the “saturation current” of the inductor.