Analysis of Nested Winding Dielectric-Core Transformers for Ethernet Applications

The current design of wide-band Ethernet isolation transformers uses interleaved windings to minimize leakage inductance. A superior alternative to interleaved windings is a nested winding geometry, where the primary winding toroid is nested entirely within the secondary winding toroid. Nesting windings will reduce leakage inductance, enable the control of cross-winding capacitance, and allow the turn-ratio to be adjusted away from unity. In this paper, nested winding transformers are built and measured to verify the expected improvement in flux confinement and cross-winding capacitance control over interleaved winding transformers. Measurements verified the improved flux confinement over comparable interleaved winding devices, as well as the ability to reduce cross-winding capacitance by increasing the nested separation without impacting inductive behavior. The bandwidth limiting issue of intra-winding capacitance is also discussed.