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Due to the interaction between human and the power supply equipment, more and more Switch-mode power supplies (SMPS) require lower leakage current. In the AC-DC SMPS, the leakage current is mainly caused by the Y-capacitors. It is better to remove the Y-caps in this situation. But connecting Y-caps between the primary return line terminal and the secondary return line terminal is a very common method in EMI design. Certain works should be done to acquire good EMI design without the help of Y-cap. In this paper, a novel balanced winding topology is proposed to mitigate EMI noise without the need for Y-cap. To have a better understanding of the operational principle, the EMI noise transmission path with and without Y-cap are analyzed. The design procedure of the proposed topology is discussed in detail. The experimental results demonstrated the prototype has excellent EMI performance without the help of Y-cap. It also can simplify the transformer design and be easy to implement mass production.

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