Effects of Mix-Mode Noise on the Filtering Mechanism and the Design Procedure of Off-line Power Supply EMI Filters

Conducted electromagnetic interference (EMI) noise was traditionally classified into two coupling modes, differential-mode (DM) and common-mode (CM). But there were phenomena unexplainable with this traditional theory. An additional noise-coupling mode, the mix-mode (MM), was then proposed to fill this inadequacy. The main focus of this dissertation is to explore the mix-mode noise coupling mechanism even further and use the new understanding to improve practical EMI filter designs.
Specifically, three areas were explored. One was to look at the new role in one differential-mode capacitor (X capacitor) and two-serial common-mode capacitors (Y capacitors) play in filtering mechanism. Besides the conventional impedance mismatching concept, a “noise current balancing” concept was introduced. Second area of exploration was about a common-mode capacitor (Z capacitor) connection in a two-wire system which is common in many hand-held device applications. The effectiveness of such common-mode capacitor was not clear before. An explanation with the mix-mode phenomenon was proposed in this dissertation. From this proposed theory, several practical common-mode capacitor connecting means were proposed. Experiments were conducted to prove the theory. The third area of exploration was to incorporate the mix-mode considerations into a practical EMI filter design. It’s concluded that with this new approach, smaller EMI filter can be achieved.