Electric field breakdown in Wireless Power Transfer systems due to ferrite dielectric polarizability

Magnetic Field Wireless Power Transfer (MF-WPT) systems typically employ ferrite cores in order to improve magnetic coupling and minimize the extraneous field. The size of the couplers and hence the ferrite cores for an MF-WPT system are determined to a large part by the separation. The shape of the core is important in minimizing the extraneous magnetic flux. However, large, custom ferrite parts are expensive. Moreover, large monolithic pieces of ferrite are quite prone to breakage and thus not very suitable to a vehicular environment. Thus, it can be advantageous to employ a multi-piece ferrite core consisting of simple standardized shapes. The use of smaller ferrite cores separated by air gaps also mitigates dimensional resonances. Mn-Zn ferrite provides high permeability and high saturation flux density but intrinsically exhibits very large dielectric polarizability. The dielectric behavior of the ferrite tends to guide electric flux lines such that the normal flux is continuous across a dielectric boundary. The electric field of an MF-WPT system can be decomposed into conservative (irrotational) and solenoidal (divergenceless) components. The conservative electric field is contrasted with the solenoidal electric field predicted by Faraday's law and can be thought of as arising from the voltage drop across the windings. Typically, the conservative component of the electric field is much greater in magnitude than is the solenoidal component. If the permittivity is sufficiently large and an air gap exists which is normal to the natural direction of the conservative electric field, nearly the entire terminal voltage will appear across the air gap resulting in very intense electric field in the gap. If the terminal voltage is large and the air gaps are sufficiently small the field intensity can exceed breakdown levels. Even with dielectric material in the air gap, partial discharge is possible over time. Discussion on the mitigation of this effect is provided.