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Single-stage Electromagnetic Interference Filter Design for Server Power Supply

DM noise testing results
Fig.1. DM noise testing result under different conditions
Electromagnetic interference (EMI) filters are required in switching power converters to attenuate conductive noise. With low frequency noise, the current path is easier to identify and therefore easier to block. However, with high frequency noise, the current path will flow through parasitics, such as equivalent parallel capacitance (EPC) of inductors and equivalent series inductance (ESL) of capacitors. Therefore, the current path is hard to identify. Also, the near field coupling between the inductor and capacitor (M1, M2 coupling) and the coupling between the capacitors (M3 coupling) will induce another mutual inductor in the capacitor branch and deteriorate the high frequency performance of the filter. Accordingly, the general practice in conventional server power supply is the two-stage or multi-stage EMI fil-ter, with one of the stages intentionally designed to attenuate the high frequency noise. However, with more stages added to the EMI filter the size and cost are increased and the power density is reduced. Several approaches for self-parasitics and mutual-parasitics cancellation have been proposed but not demonstrated in converters. The reason is that the near field effect has submerged in the strong magnetic field of inductors in conventional two-stage or multi-stage EMI filters. Therefore, a single-stage filter is desired in order to have better self- and mutual-parasitics cancellation. In this paper, with PCB winding magnetics, balance and shielding technique demonstrates good performance for common mode (CM) noise reduction. At the same time, phase interleaving for power factor correction (PFC) reduces differential mode (DM) noise. As a result, a single-stage EMI filter will easily meets the low frequency EMI noise standard limit. In order to improve the high frequency performance for the single stage EMI filter, High Frequency Structure Simulator software from ANSYS Inc. is used to analyze the flux distribution for the DM inductor from low frequency to high frequency. Then flux distribution is further verified by near field measurement with near field probe. By drawing the flux distribution see the change of the flux pattern from low frequency to high frequency is observed, and this provides us useful information for adjusting the positions of the inductor and capacitors to re-duce M1 and M2 coupling. Next the X-cap is built up to reduce the ESL of the capacitor branch. Finally a flux cancellation loop is introduced to reduce the M3 coupling. A single-stage EMI filter is demonstrated based on a 1kW server power supply. The proposed single-stage filter demonstrates better high frequency performance compared to a conventional single-stage EMI filter.
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