Fabrication and Evaluation of PCB-Embedded Broadband Signal Transformers With Custom Machined Racetrack-Shaped Ferrite Cores for Ethernet Applications

Abstract

To reduce the manufacturing variability in broadband ferrite-core magnetic components such as ethernet transformers, there is great industry interest in the embedding of the core materials within a planar substrate and fabricating the windings using high-precision photolithography. An embedding process has been previously developed and found to impose design restrictions on embedded devices that can be feasibly fabricated. It was previously found that a racetrack core shape (toroidal halves joined by straight legs) allows the most efficient use of area to embed multiple magnetic devices with maximum inductance. In this paper, three different sizes of racetrack-shaped cores were fabricated from bulk ferrite. The racetrack cores were then embedded within printed circuit board substrate, wound as transformers, and measured to evaluate bandwidth and inductance. It was found that the transformer mutual inductance was significantly reduced through the embedding process. However, the devices were found to exhibit excellent broadband coupling behavior. Certain designs of transformers met the inductance and bandwidth requirements of 100 and 1000Base-T ethernet.