Optimization of EMI Filter with Consideration of the Noise Source Impedance for DC/DC Converter

Due to limited space and the focus on cheap and effective power electronics in automotive vehicles, the demand for fast switching devices has increased drastically. Such fast switching devices produce high electromagnetic interference (EMI) emissions which result in a gain of size and weight of EMI filters to comply with the electromagnetic compatibility (EMC) requirements. Due to uncertainties, the EMI filters used are often oversized. The goal of this paper is to show how an EMI filter is designed that fulfills the requirements in a way that smaller components can be chosen. Therefore, a passive EMI filter for a 400 V to 12 V DC/DC converter is created. A common method used to calculate the L and C filter components is to find the minimum required corner frequency and determine appropriate parts. However, this is a simplification of the filter design. In reality the source and load impedances also influence the design. Those impedances are often defined as 50 Ω. But the real source and load impedances of the system deviate from 50 Ω. In order to achieve effective filtering of the emissions, the knowledge of the noise source impedance is a very important factor in calculating the exact insertion loss of the filter. In this paper, the value of the impedance is estimated by analyzing the emission paths of the DC/DC converter. Then the influence of the impedance on the filter design is shown. With that knowledge, an improved EMI filter is created and demonstrated with measurements.