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This letter presents a method for modeling of the differential-mode (DM) current-excited core saturation effect in a common-mode (CM) choke. This is performed by finite-element analysis (FEA) and computation of 3-D magnetic field distribution in the magnetic core of CM choke. The simulation result shows that the DM-current-excited flux is not canceled completely in the core, and some leakage flux remains, which increases the flux density in the core and gives rise to partial saturation of the core. In order to determine the CM inductance of the CM choke when its core is partially saturated by DM current excitation, the letter then presents a method to calculate the dynamic inductance at the operation point of the CM choke with assistance of FEA. Finally, an experiment for CM inductance measurement of a prototype of CM choke under DM DC bias is carried out to validate the method of dynamic inductance calculation.