The Parasitic Capacitance of Magnetic Components with Ferrite Cores Due to Time-Varying Electromagnetic (EM) Field

Abstract

The parasitic capacitance of a magnetic component, such as an inductor and a transformer, is critical because it can determine component's high frequency impedance, leading to voltage / current spikes, and causing EMI issue. The simulation-based techniques, the measurement-based techniques and the calculation-based techniques are popularly employed to model the parasitic capacitance. Measurement-based techniques are accurate in calculating the parasitic capacitance. The measurement-based techniques are black-box techniques and cannot provide guidelines in reducing the parasitic capacitance during manufacturing. The calculation-based techniques extract the parasitic capacitance based on calculated electric energy. Choosing the winding structure with reduced electric energy can also reduce the parasitic capacitance. However, when calculating the electric energy, the existing techniques are based on the quasi static approximation in which the electric field excited by the time-varying magnetic field is ignored. Simulation issues such as convergence issue exist in the simulation-based techniques. This paper focuses on the solution of comprehensive calculation of all the parasitic capacitance, including both the electrostatic capacitance and the time-varying EM capacitance. It proves that for the magnetic components with ferrite cores, the time-varying EM capacitance can be the dominant capacitance for high frequency magnetic components and it cannot be ignored. Therefore, it explained why the calculated capacitance is not accurate for certain type magnetic components. Furthermore, the paper proposes certain structures to achieve small parasitic capacitance.