Higher frequency in SMPS power conversion is often limited by magnetic elements in the circuit. While planar transformers provide high power density, limitations include high capacitive coupling between windings, and in providing a high level of isolation between the windings isolation. Wire windings provide a solution but are limited by winding losses. Copper foil and litz wire have lower AC winding loss at SMPS frequencies and inherently low capacitive coupling. The suitability of litz wire and copper foil however depend on a number of variables including voltage and frequency. In this presentation we will explore tradeoffs and present a roadmap for when a litz wire winding is preferred, or copper foil is a better choice.