Design of Compensation Capacitor in S/P Topology of Inductive Power Transfer System with Buck or Boost Converter on Secondary Side

When an inductive power transfer system is applied to a battery charger for electric vehicles, a diode bridge rectifier with a dc–dc converter, called a secondary-side converter in this paper, is connected to the secondary side of the resonant circuit in order to regulate the current and voltage of the battery. A compensation capacitor is typically used to improve the input power factor in an inductive power transfer system, and a resonant circuit is configured. This paper presents a design method for the primary compensation capacitor in an inductive power transfer system with series compensation on the primary side and parallel compensation on the secondary side (S/P topology) to connect a boost or buck converter via a rectifier circuit on the receiving side. For the S/P topology, the capacitance of the primary-side compensation capacitor influences the duty ratio of the switch used in the secondary-side converter because it affects the input-to-output voltage ratio of the resonant circuit. Further, the duty ratio of the secondary-side converter affects the resonant-circuit efficiency. In addition, the primary compensation capacitance affects the output power factor of the inverter, which is connected to the primary side of the resonant circuit. Therefore, the capacitance of the primary-side compensation capacitor also affects the inverter efficiency and resonant-circuit efficiency. In this paper, a primary-side capacitor design method is examined. The results show that the optimum capacitance using a buck converter differs from that using a boost converter.