Characterizations of High Frequency Planar Transformer With a Novel Comb-Shaped Shield

Abstract

This paper presents a newly developed comb-shaped Faraday shield for high frequency (HF) planar transformers. High power density and small size planar transformers are widely used in switching mode power supplies (SMPS) as magnetically coupled devices. However, the intra-winding capacitance couples HF noise from the primary winding to the secondary winding. The coupling capacitance can cause serious common mode problems, and thus the design may not be electromagnetically compatible with the surrounding environment. The tradeoff between the leakage inductance and the coupling capacitance becomes problematic for high frequency applications. The coupling capacitance can be reduced either by increasing the separation distance between the primary and the secondary windings or by inserting a conventional Faraday shield. The former may cause the leakage inductance to increase and further increase the insertion losses, whereas the latter would result in a serious demagnetizing problem in a HF planar transformer. These problems can be resolved by using a novel comb-shaped Faraday shield.