Multiple Feedback Architecture to Improve the Low-Frequency Immunity of an Active EMI Filter

Abstract:
A compact design of the electromagnetic interference (EMI) filter is of paramount importance in high-density AC/DC applications, such as server-rack power supplies for enterprise and on-board chargers (OBCs) for automotive. Fortunately, an active EMI filter (AEF) circuit for common-mode (CM) noise attenuation, through miniaturization of the toroidal-cored CM chokes in the equivalent passive filter, can considerably reduce the size, weight and cost of the overall design. However, the frequency response of the AEF circuit below 150 kHz may cause large-signal voltage saturation at the output of the AEF amplifier when low-frequency (LF) CM conducted disturbances exist at the filter input port. This paper examines a multiple feedback (MFB) technique to improve the immunity behavior below 150 kHz while preserving the filter-attenuation performance for high-frequency (HF) emissions above 150 kHz. Simu-lation and experimental results of a single-phase AEF circuit with MFB confirm the theoretical analysis.