Reduction of differential-mode to common-mode conversion by means of analytic description in common-mode chokes used for an automotive communication bus system

The following study describes the attempt to reduce differential-mode to common-mode conversion of data-line common-mode chokes (CMCs) which are used in unshielded-twisted-pair automotive communication bus systems. Conversion of a choke leads directly to emissions of the data line. Furthermore, the emission within the information signal's frequency range varies depending on the signal conversion. For high speed bus systems like for example 100 Mbit/s Ethernet, this leads to a significant emission in case of state-of-the-art CMCs used nowadays. The study is based on measurement results, simulations and an analytic description of different CMCs. In the analytic description all important parameters of a choke contributing to the mode conversion are shown. These parameters are the length of connection lines (connection from the windings on the coil to the package) and the length difference between the connection lines, as well as the characteristic impedance of connection lines and the characteristic impedance difference between the connection lines. The study indicates that the conversion factor and the emission level decrease by 10 dB when the length difference between the connection lines reduces from 4.8 to 0.2 mm. The overall result shows that the reduction of conversion factor is 25 dB when all parameters introducing in this study are optimised. The theory described in this study can point out the mutual influence of parameters and the weighting or prioritisation of parameters. This means that it is possible to design CMCs with small conversion and hereby reduce the emissions of an unshielded-twisted-pair automotive communication bus system.