Characterization of Common Mode Chokes at High Frequencies With Simple Measurements

In this paper, we present a technique to characterize common mode chokes at high frequencies that only requires a measurement, which can be performed with a spectrum analyzer with the tracking generator. This technique is based upon a theoretical modal analysis of the common mode choke as a four-ports device. This analysis demonstrates that the transmission coefficient measured for one of the windings of the common mode choke, while the other winding is open circuited will always show two minimums, which are associated with resonances involving currents flowing, respectively, in common mode and in differential mode in the common mode choke. Therefore, the response at high frequencies of the common mode choke to both a common mode and a differential mode stimulus can be foreseen from the measurement of this transmission coefficient. Moreover, from the analytical expressions obtained for the frequencies of resonance of the common mode choke in that configuration, we develop a method for obtaining the capacitive, resistive, and inductive parameters of a circuit model of the common mode choke. To validate the proposed technique different commercial common mode chokes have been measured and the predicted performance of the model has been compared with measured responses. We have verified that in all the cases the measured transmission coefficient exhibits the resonant behavior predicted by the theoretical analysis. We have checked that in most cases the method designed for extracting the high-frequency parameters of the circuit model of the common mode choke yields an accurate model of the device up to frequencies as high as 30-50 MHz. An exception are common mode chokes made of materials with an extreme variation of their properties with regards to frequency, such as nanocrystalline materials.