Astrid Lewalter

Analytical modelling of winding capacitances and dielectric losses for planar transformers

Efficient and sufficiently accurate analytical models are valuable tools for the design of planar magnetic components. This paper describes a new analytical model of winding capacitances and dielectric losses for planar transformers. It is compared qualitatively with a more simple model valid for wire wound components and validated against measured capacitances of some prototypes. The new model can be used to optimize the design of planar magnetic components. Optimizing the thickness of the insulation layers between the copper layers of a planar winding is shown as an example.

Growth of uniform carbon-nanotube arrays with sandwich technology

— In this work, a novel approach to grow structured, highly oriented carbon nanotubes (CNTs), which are vertically aligned to the substrate and show large field emission is reported. Growth is performed on lithographically defined dots of catalysts, which can be deposited on metallic, semiconducting, and glass substrates. A sandwiched catalyst structure and microwave plasma chemical vapor deposition enables the formation of uniform CNT arrays of 1.6 × 1.6 μm2. The method is easily scalable to large areas. The CNT arrays exhibit a stable field emission of 20 mA and a macroscopic current density of 50 mA/cm2 at a rather low electric field of 5.33 V/μm. Modeling of space charge indicates that space charge reduces the magnitude of the CNT emission at high field strength: this agrees satisfactorily with the measurements.