Analysis of CM-Signal Suppression Effect from DM Signal by Common-Mode Choke Coil Using Chain-Parameter Matrix

Common-mode choke coil (CMC) is used in vehicles and electronic devices to improve electromagnetic compatibility (EMC), and the analysis of the CMC effect is essential to improve the design quality of EMC. In this paper, the suppression effect of CMC for the common-mode (CM) signal converted from the differential-mode (DM) signal is analyzed using a chain-parameter matrix. The chain-parameter matrix of the CMC was obtained from the SPICE model and the measured data. And then, the amount of conversion level from DM signal to CM signal was calculated using the chain-parameter matrix of the signal source, the termination, the cable, and the CMC. The CM signal level was calculated for the shielded twisted pair cable with one pair considering imbalances of the signal source, the termination, the cable, and the CMC. The calculation value was compared to the measurement value due to confirm the validity of the method, and the calculated results were in close agreement with the measurement value. The investigation results showed that we should pay attention to the CMC imbalance when the cable is well balanced. In addition, the results suggested that the CMC should be prepared at both ends of the cable to reduce the current outside the shield.