Experimental verification and analysis of AC-DC converter with an input impedance matching for wireless power transfer systems

This paper discusses the performance of an AC-DC converter which converts power from 13.56MHz AC to DC in a receiving side of wireless power transfer systems. The wireless power transfer systems are required to operate in high-frequency such as 13.56 MHz in order to achieve a high power density of transmission coils. Thus the AC-DC converter in the receiving side is demanded to operate at high-frequency. In such high-frequency region, the reflected power occurs when the input impedance is not matched to the characteristic impedance of the transmission line. In other words, the input impedance of the AC-DC converter needs to have the same impedance to the characteristic impedance of the transmission line. In order to overcome the problem, the AC-DC converter with an input impedance matching is proposed in this paper. The proposed AD-DC converter achieves the input impedance matching with a simple circuit configuration. It means that the converter can obtain sinusoidal input current and unity input power factor without a high-frequency switching except the diodes. In this paper, the impedance matching characteristics and the analysis of the operational modes of the proposed circuit are presented. The experimental results confirmed that the proposed converter enables a conversion from 13.56-MHz AC to DC with the sinusoidal input current. In this operation condition, the input impedance is 29.6 + j0.51 Ω. Because the design value of the input impedance is 50 + j0 Ω, there is a non-negligible error on the real part. This is attributed to the parasitic capacitances on the diodes. In order to solve this problem, an improved AC-DC converter is proposed newly. From experimental results, it achieves the input impedance of 52.7-j0.02 Ω. Besides, the reflection coefficient is suppressed by up to 94.5% compared with that of the conventional capacitor input-type diode bridge rectifier (CI-DBR).