The passive EMI cancellation effects of Y capacitor and CM model of transformers used in switching mode power supplies (SMPS)

Connecting a Y-safety capacitor between the primary return line terminal and the secondary return line terminal is a very common measure in EMI design for an isolated switching mode power supplies (SMPS). Why this Y-safety capacitor can improve EMI performance of SMPS is not so well known. Analysis of the passive EMI cancellation effect of this Y-safety capacitor has been carried out and experimentally evaluated. And an accurate and simple CM (common model) model of transformer has been discussed.

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