Thin Film Coil Design Considerations for Wireless Power Transfer in Flat Panel Display
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Thin Film Coil Design Considerations for Wireless Power Transfer in Flat Panel Display Jun Yu, Kai Ying, David Hasko, Sungsik Lee*, Arman Ahnood*, W. I. Milne and Arokia Nathan Centre for Advanced Photonics and Electronics, Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge, CB3 0FA, UK *London Centre for Nanotechnology, University College London, London, WC1H 0AH, UK
Abstract Wireless power transfer is experimentally demonstrated by transmission between an AC power transmitter and receiver, both realised using thin film technology. The transmitter and receiver thin film coils are chosen to be identical in order to promote resonant coupling. Planar spiral coils are used because of the ease of fabrication and to reduce the metal layer thickness. The energy transfer efficiency as a function of transfer distance is analysed along with a comparison between the theoretical and the experimental results.
I.
Introduction
With the fast development of low-cost display technology for display screen and mobile devices, energy management has become vital to prolong battery life and requires more frequent and easier charging methods. A wireless power transfer system, which could be integrated with display panels using thin film technology, mobile devices and electrical instruments, would enable this requirement. Wireless power transfer has been an interesting and challenging topic since the early 20th century when Tesla made the first studies [1]. Wireless power transfer based on inductively coupled resonance theory was first was introduced by Soljacic et al. [2], using relatively large-sized inductor coils, and as such are not integratable to display panels or small mobile devices. In this paper, a thin film based resonantly coupled wireless power transfer circuit is discussed. A round-cross-section copper wire of typical size 3 mm used for resonance coil at frequency of 9.9 MHz [2] has a skin depth of approximate by 20 μm which is smaller than the radius. The quality factor is reduced from an estimated 2500 to an actual 900 by not fully utilising the coil due to skin effect, so much of the current is constrained by the skin depth. In this work, a thin-film coil pair is introduced, where the coil dimension is minimized which allows easy fabrication. In particular, at high frequency, the skin depth exceeds the metal thickness, so the copper film is fully conductive. We finally investigated the efficiency of the system and analyse it analytically.
System Overview
II.
Fig.1 Equivalent circuits of wireless power transfer Fig.1 shows a schematic diagram with four inductively coupled resonators. On the transmitting side, there is a single-turn driving source coil ( ) which is directly connected to the source ( ) and a multi-turn planar spiral coil ( ) fabricated on top of a glass wafer (discussed next section). When the driving circuit powers up the single coil loop, the magnetic field produced excites the multi-turn coil which stores energy in the LC circuit formed by the coil and i
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