Effects of stray capacitances between transformer windings on the noise characteristics in switching power converters

In switching-mode power converters, transformers are frequently used for the dc isolation between input and output terminals. The transformer has parasitic effects such as leakage inductances and stray capacitances of windings. These parasitic effects cause the complexity of noise problems in power converters. Particularly in the transformer with nonunity turns ration of windings, the description of the noise characteristics becomes more complicated due to the stray capacitances between windings. This paper presents a high-frequency equivalent circuit of the two-winding transformer with nonunity turns ration, and describes the noise transmission characteristics of a dc-to-dc converter with a transformer. Furthermore the extension of this equivalent circuit to the three-winding transformer and the cross-noise characteristics are discussed. The above discussion are followed by the experimental results.