Abstract
An efficient wireless power transfer (WPT) system is proposed using two self-resonant coils with a high-quality factor (Q-factor) over medium distance via an adaptive impedance matching network using ferrite core transformers. An equivalent circuit of the proposed WPT system is presented, and the system is analyzed based on circuit theory. The design and characterization methods for the transformer are also provided. Using the equivalent circuit, the appropriate relation between turn ratio and optimal impedance matching conditions for maximum power transfer efficiency is derived. The optimal impedance matching conditions for maximum power transfer efficiency according to distance are satisfied simply by changing the turn ratio of the transformers. The proposed WPT system maintains effective power transfer efficiency with little Q-factor degradation because of the ferrite core transformer. The proposed system is verified through experiments at 257 kHz. Two WPT systems with coupling efficiencies higher than 50% at 1 m are made. One uses transformers at both Tx and Rx; the other uses a transformer at Tx only while a low-loss coupling coil is applied at Rx. Using the system with transformers at both Tx and Rx, a wireless power transfer of 100 watts (100-watt light bulb) is achieved.
Highlights
Accepted: 14 December 2021Wireless power transfer (WPT) using a magnetic field in a near-field region is a topic that has drawn much attention recently
In [11], a wireless power transfer (WPT) system that is based on magnetically coupled resonance using transmitting (Tx) and receiving (Rx) self-resonant coils of a high-quality factor (Q-factor) was reported
To calculate the optimal impedance matching condition in the proposed WPT system, it is assumed that |Z6 | >> RT2 and Im(Z5 ) ≈ 0 (See Appendix A for validation of assumption)
Summary
Wireless power transfer (WPT) using a magnetic field in a near-field region is a topic that has drawn much attention recently. Few methods for the impedance matching of a WPT system using self-resonant coils of high Q-factor have been reported. A WPT system that consists of two self-resonant coils of high Q-factors and adaptive impedance-matching networks using ferrite core transformers at Tx and. Maximum power transfer efficiency can be achieved by applying the adaptive impedance-matching network according to the changing distance between the self-resonant coils or by varying the load impedance. In [18], the ferrite core transformer was adopted for impedance matching, but it reduced the power transfer efficiency of the system. In the system proposed in this paper, an impedance matching method using ferrite core transformers causes minimal degradation of power transfer efficiency.
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