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.

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SUMMARY The switching frequency affects the efficiency of a secondary‐side converter that is configured with a diode bridge rectifier connected to a DC‐DC converter in an inductive power transfer (IPT) system. The efficiency characteristics for the switching frequency of a secondary‐side converter were analyzed theoretically in order to establish a highly efficient control scheme the controls the switching frequency of the converter. A control scheme is proposed in which the switching frequency of the secondary‐side converter is controlled according to the efficiency characteristics. The validity of the proposed control scheme was clarified through experiments. The results showed that the proposed control scheme improved the efficiency of the IPT system by 3.4 points compared to the conventional control scheme.

An inductive power transfer (IPT) system is sometimes applied to the battery charger of electric vehicles. A DC-DC converter with a diode bridge rectifier (secondary-side converter) is connected to the secondary side of the resonant circuit in order to regulate the current and voltage of a battery. In an IPT system, the secondary-side voltage of the resonant circuit changes significantly depending on the coupling coefficient between the primary coil and the secondary coil. In addition, the voltage changes depending on the load condition. Therefore, a buck-boost converter is suitable for the secondary-side converter because the output voltage range is wider than the other converters, and the IPT system with the buck-boost converter is more efficient than the IPT system with the buck converter. In order to develop a high-efficiency IPT system, a theoretical analysis that considers the characteristics of the IPT system is required. Thus, in this paper, the relationship between the coupling coefficient of the coils and the loss of the secondary-side converter is investigated in order to develop a high-efficiency IPT system with theoretical analysis and experiments that consider the coupling coefficient of the coils. In addition, the circuit parameter selection guideline that makes the secondaryside converter more efficient is shown on the basis of the results of theoretical analysis and experiments. As a result, it was confirmed that the tendency of the characteristics obtained with the experiments is similar as that of theoretical analysis. Thus, it was indicated that theoretical analysis in this paper is useful in analysis of the efficiency and loss of the whole IPT system including secondary-side converter.

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