Power electronic inductors, with values of several μH, have been integrated into thermally stable ceramic multilayer circuit boards by the use of NiZnCu and MnZn ferrite tapes in low temperature cofired ceramic (LTCC) technology. These ferrites are particularly attractive for switched mode power supplies in automation, drives, and consumer applications, where the miniaturization of modules is triggered by advances in transistor technology and switching frequencies. The small signal analysis of embedded individual inductors and coupled transformer coils reveals the generic design rules for these components and additional materials properties beyond those accessible by ring core measurements. In the process of adapting the materials to LTCC, the distinct differences between the two materials become blurred, for example, they can be engineered to exhibit similar cutoff frequencies. NiZnCu ferrite, which is sinterable in air, may even achieve higher permeability than MnZn ferrite. The latter, however, shows higher saturation flux density and current capacity of buried inductors for power line filters. The coupled inductor design in a transformer is particularly ruled by the shunt capacitance inside the coils and by the fact that Maxwell equations preclude strong magnetic coupling between ferrite-embedded conductor lines. While parasitic capacitances remain tolerable for standard dielectric layer material up to several MHz, the need for magnetic coupling requires a fabrication process for magnetic vias.
Special issue/Three-dimensional molded circuit boards. Molded interconnection devices.