By using bi-directional inductive power transfer (IPT) systems as battery chargers for electric vehicles (EVs), battery charging operations become convenient and safe. However, IPT systems have problems such as occurrences of much electromagnetic noise and power loss because the converters of IPT systems are driven in high frequency by tens of kHz. To solve these problems, there is a case where the soft-switching technique needs to be applied to the converters of IPT systems. However, in soft-switching operation, the power factor of the resonant circuit becomes lower, resulting in a lower resonant circuit efficiency. In previous works, when the soft-switching technique was applied to the converters, the resonant circuit had not always been able to be operated with high efficiency because the influence caused by soft-switching operation had not been considered. For this reason, there was a case where the efficiency of the overall system with soft-switching operation became lower than the efficiency in hard-switching operation. Therefore, in this paper, the influence on the efficiency of the resonant circuit caused by the soft-switching operation is clarified by the theoretical analysis and experiments; then, the guideline for improving the efficiency of IPT systems is shown. As a result, in the experiments, it could be understood that the efficiency of the overall system with soft-switching operation becomes higher than the efficiency in hard-switching operation when the operating point of the resonant circuit was close to the requirement guideline, which is shown by using the primary-side voltage and the secondary-side voltage of the resonant circuit. Therefore, it is suggested that the efficiency of IPT systems could be improved by properly regulating the primary-side direct current (DC) voltage.
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