Design of EMI filters having low harmonic distortion in high-power-factor converters

This paper studies how electromagnetic interference (EMI) filters affect the harmonic distortion in high-power-factor converters. The EMI filter presents a tradeoff: higher noise attenuation results in higher harmonic distortion. In many modern converters, the EMI filter is designed to meet certain noise requirements. However, when tested in practice, the filter, while attenuating the EMI noise well, is found to generate high harmonic distortion, and should be redesigned. The cause of this distortion is a nonlinear interaction of the filter and the bridge rectifier. The current understanding of this process is incomplete, especially in modern converters that use high-order LC filters, and modern low-cost inverters. As a result, harmonic distortion is typically tuned by repeated trial-and-errors. In this paper, we provide a simple model for evaluating the harmonic distortion. The harmonic distortion is shown to be dominated by a single parameter: the filter total capacitance. The total harmonic distortion and capacitance are shown to be to be related by a simple power-law function. This relation is proved to be accurate if the total filter inductance is smaller than a given threshold. Experimental results show that total harmonic distortion can be estimated by the capacitance, with a normalized error of 0.09%.