Abstract
It is obvious that the conventional two-coil wireless power transfer (WPT) system has a short power transmission distance, which is determined by system parameters such as the loaded Q of the resonators and the coupling coefficient. There have been many attempts to improve the power transfer efficiency (PTE) or transmission distance. A typical approach is to use multiple coils like a three- or four-coil WPT system. In this paper, we propose a new method to obtain the PTE of a multi-coil WPT system based on the scattering parameter and impedance parameter. Also, we compare the two- and three-coil WPT systems in terms of their transmission efficiency rather than their system energy efficiency. For high transmission efficiency, we determined that the three-coil WPT system should have a symmetric structure and the coupling coefficient between the transmitting and receiving coils has to be as zero as possible. Additionally, we found that 1/√3 of the conventional critical coupling is the boundary coupling coefficient at which the two- and three-coil WPT systems have the same transmission efficiency. We successfully verified these theoretical analyses by implementing two- and three-coil WPT systems at the operating frequency of 6.78 MHz and measuring their transmission efficiency and spectra.
Highlights
The conventional wireless power transfer (WPT) system is commonly modeled as two magnetically coupled resonators, in which each resonator consists of an inductor, a capacitor, and a parasitic resistor, and is connected to a source or load in series
The derived expression was verified by measuring the power transfer efficiency (PTE) of a symmetric three-coil WPT system, and its expression has a potential to be expanded to a WPT system with an arbitrary number of coils
Through analysis of the PTE of the three-coil WPT system, we showed that the system should have a symmetric structure for a high PTE
Summary
The conventional wireless power transfer (WPT) system is commonly modeled as two magnetically coupled resonators, in which each resonator consists of an inductor, a capacitor, and a parasitic resistor, and is connected to a source or load in series. Compared with the previous literature, the main contribution of this paper is the expression of the transmission efficiency of a multi-coil WPT system, in which all couplings among coils are considered, as well as a useful guideline on which of two- and three-coil WPT systems would be more suitable to use in a specific transmission environment. We consider a multi-coil WPT system as series RLC resonators and express the relationship among the resonators as an impedance matrix Without any assumption such as zero coupling between faraway coils, we derive the transmission efficiency of a multi-coil WPT system in closed-form from the definition of the generalized scattering parameters.
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