Systematic design approach for coupled inductor filters with improved stability and high-frequency attenuation

This paper introduces a systematic design approach for the coupled inductor filter, a well-established topology known for effectively suppressing high-frequency ripples. The coupled inductor filter is particularly suitable for mitigating switching ripples in switching converters because of its specific frequency notch characteristic. Its transfer function comprises two double poles and one double zero, with one double pole defining the filter’s bandwidth frequency and the double zero determining the notch frequency. However, the presence of an additional double pole may lead to stability issues, depending on its frequency domain placement, making precise positioning critical. Despite the coupled inductor filter’s robust performance, a systematic design approach for this filter has not been previously established, primarily due to its complexity. To address this gap, this paper details a method for accurately positioning the problematic double pole using the pole-zero placement technique, thereby reducing the risk of instability. Moreover, this method enhances the filter’s high-frequency attenuation capabilities, as confirmed by transfer function analysis. Experimental results are presented to validate the implementation and effectiveness of the proposed design method.