Investigation of Differential Mode Electromagnetic Interference Reduction in Switched Mode AC/DC Power Converters with Hybrid Filter

Electromagnetic Interference (EMI) is a major problem for electronic circuits in modern days. The widely used switched mode power conversion systems rely on pulse width modulation (PWM) techniques with high frequency switching and generate conducted EMI noise. Due to the increasing EMI problem different standards were developed to limit the emission from the electronic circuits. These standards define the frequency range and the maximum magnitude of the EMI noise that is permissible. To combat the EMI issue and meet the EMI standards, passive EMI filters are commonly used. But they have huge size and weight due to the big size of the passive components like capacitors and inductors. Hence, with a view to reducing the size of the EMI filter, active EMI filters have been investigated in available literature which can be combined with smaller passive filter components. The overall size of the hybrid (active + small passive) filter becomes much smaller than the originally used passive filter. However, most of the active and hybrid filter techniques, as found in the available literature, dealt with common mode (CM) noise. Few literatures dealt with differential mode (DM) noise, but either for DC/DC converters or the EMI suppression is on the DC side of DC/AC inverters. The active DM EMI filter design on AC side is more challenging than the active DM filter design on DC side, due to stability and other issues. Hence, these issues need to be addressed to design active DM EMI filters for AC/DC converters.;In this work, issues and challenges related to active DM EMI filters for AC/DC converters have been discussed and solved. At first, DM noise of AC/DC converters has been analyzed and model for hybrid EMI filter has been developed, including noise source and noise receiving end impedances, passive EMI filter elements and active filter's functional blocks. Based on control theory, using the developed closed loop model, method for stability analysis of the active filter has been introduced and the active filter was compensated to ensure stability. Measurements were done to verify the developed model. Next, a hybrid filter design method has been discussed for a commercial boost PFC AC/DC converter to reduce its existing passive filter size by half. Optimal topology for the active filter has been discussed and different issues for the active filter circuit design were solved. Then, stability analysis and compensation design were done based on the developed models. The designed hybrid filter prototype which reduces the passive filter size by half, has been tested with the converter and it met the CISPR-22 standard's requirements. Next, with a view to improve the performance of the single FB active filter, the effects of using multiple active filters simultaneously with different control schemes (Feedback, Feed-forward), have been investigated. Finally, scopes for future development have been discussed.