Analysis On Variable Inductances Of Planar Coils Sandwiched By Soft Magnetic Layers

Filmed planar coils have obvious advantages for miniaturization, micro-fabrication, and circuit integration. Amorphous and nanocrystalline films exhibit extremely large permeabilities and are extensively used to film planar coils for inductance enhancement. In this paper, we analyze aspects in designing a soft magnetic film/planar coil multilayer structure. It is pointed out that when the inductance of a filmed coil varies with the film permeability, it may get saturated before the permeability reaches its maximum. The saturation and variable tendency are closely related to the filmed inductor geometry. It is the ratios of the film dimensions and gap to the coil conductor width that determine the variation rate and saturated value. In addition, the film aspect ratio must be considered to produce suitable effective permeability as the inductance actually responds to the effective permeability instead of the nominal material permeability. Experimental and simulation data demonstrate the validity of the analysis.