Active Electromagnetic Interference (EMI) Filter for Conducted Noise Reduction

The extensive use of switched-mode power supplies (SMPSs) induces severe conducted electromagnetic interference (EMI) noise that affects the neighbor circuit and devices. EMI filters have been widely used to reduce the noise. Passive EMI filters (PEFs) are the traditional solutions; however the size and volume are inevitably big which will decrease the power density of the whole system. Hence over the last few years researchers have studied and developed various types of active EMI filters (AEFs). This paper proposed a novel AEF circuit. This active circuit is transformerless and can be designed simply from the impedance point of view based on mathematical calculations. The proposed circuit is then designed and built into a prototype hybrid EMI filter (HEF) module. Experiment shows using the AEF circuit could achieve more than 10dB improvement on noise reduction performance comparing with only using PEF circuit in the full load condition for a 50W DC-DC 1/8 brick converter.