Modelling and minimization of the parasitic capacitance of ring core inductors

High-frequency static converters requires inductors, common and differential mode chokes with a low equivalent parallel capacitance. To reduce it, this paper proposes first a semi-analytical model of this capacitance for inductors made with coated ring cores of nano-crystalline material. This model is based on 2D finite element method to compute the turn-turn and turn-core parasitic capacitances, taking into account loose windings. Then the total capacitance is computed thanks to an energetic approach. The model is validated against measurements performed on one core for several numbers of turns. Since the model shows that the capacitance is almost linear with the parasitic turn-core capacitance, specific devices are proposed to increase the distance between turns and core and thus decrease the capacitance. It is demonstrated that a small increase in overall volume allows to divide the equivalent parallel capacitance of a common mode choke by almost a factor of three while increasing its overall volume by only 37%.