The effects of rectification on RFI in low-frequency inductive power transfer systems

Inductive power transfer (IPT) systems typically employ a relatively pure sinusoidal primary current. The frequency of operation is chosen to not interfere with existing LF systems such as RFID transponders. Rectification combined with filtering on the secondary side of an IPT system necessarily introduces significant harmonic content into the secondary current as well as ringing, in some cases. Although the IPT system is typically so small that radiation can be neglected at the fundamental frequency, radiation at harmonic frequencies is much more significant due to the strong frequency dependence of the radiative properties of the electrically-small magnetic dipole and quadrupole radiators comprised by the couplers. We examine the primary and secondary current waveforms of a typical IPT system in the time and frequency domains using nonlinear circuit simulations. We then perform full-wave frequency domain simulation of the linear magnetic components (i. e., the inductive couplers) at the harmonic frequencies including the full dispersion model of the ferrite. The topology of the secondary-side matching network is shown to have a strong influence on the harmonic content of the secondary current and hence the extraneous electromagnetic field. Because the harmonic currents in the secondary are typically much greater in magnitude than those in the primary, the fields at the harmonic frequencies tend to be predominantly dipolar in nature.