Design, analysis and realization of compact energy efficient RF wireless power transfer systems

Abstract

Nowadays, wireless power transfer (WPT) has become a top area of research due to ease and portable outcome in the present technological scenario. This creates an increased demand globally in various technological sectors such as consumer electronics, bio-medical implants, automotive industry and so on. In consumer electronics, wireless power transfer is used in wireless charging of electronic devices. For example, mobile devices are required to be plugged in manually for charging which limits mobility and disrupts use when charge is depleted. In addition, due to compactness in the overall size of the portable device, the connectors become a larger fraction of device size. As a result, the use of wireless power transfer technique eliminates the connector from portable electronic devices which improves both size and reliability. Furthermore, in biomedical implants, the use of non-rechargeable batteries needs replacement at the end of their life span by a costly surgery. Also, the bulky nature of non-rechargeable batteries creates an obstacle in the design of compact implantable devices. These concerns raise the requirement of wireless power transfer technique in modern applications. Furthermore, wireless power transfer is classified into two different types i.e. near field and far field WPT. This thesis mainly focuses on resonant inductive type near field WPT systems operating in MHz frequency range. This type of WPT system transfers power at higher efficiency and over longer distances than non-resonating WPT systems. In addition, these systems can also transfer power at mid-range separation distance where the distance of power transfer is greater than the size of the resonator. Most importantly, resonant inductive near field WPT is realized using different coil mechanisms i.e. litz/solid wire and printed circuit board (PCB) based spiral coils and planar microstrip line structures such as defected ground structures (DGS). Firstly, the coil mechanism based on litz/solid wire uses two similar coils separated by a distance. This type of coil mechanism has a specific number of turns to obtain a higher value of mutual inductance. The higher the mutual inductance, the greater will be the coupling coefficient which results in power transfer with higher efficiency. Moreover, the litz/solid wire-based coil mechanism mostly operates at single frequency and transfers power at longer distances with optimum efficiency. Secondly, the coil mechanism utilizing printed spiral on the top plane of the PCB can be designed to operate both in single as well as dual frequency band. In addition, it is useful in enabling on-body communication. Finally, the planar microstrip line structure using defect in the ground plane (DGS) is used. The DGS possesses good slow wave characteristics and high electrical length due to which the operating frequency decreases for the fixed physical length. This makes the overall circuit size more compact. Further miniaturization in the circuit size is achieved by adding an external capacitor in the excitation gap of defect. Moreover, DGS helps in achieving the most compact overall circuit size in comparison to both the litz/solid wire and printed spiral based coil mechanism. Furthermore, WPT is achieved by utilizing two similar DGSs placed in a back-to-back manner and separated by a particular distance. The DGS can be tuned to operate at single and multi-band frequency range. This enables power transfer at different frequency bands simultaneously. Here, it is important to note that among all the coil mechanisms as explained above, the DGS based WPT system looks more attractive due to compact size at mega-hertz frequency range. The miniaturized size of DGS based WPT system enables the use in biomedical implants such as defibrillators which checks the abnormal heart rhythm, cochlear implants which helps patients to hear for better understanding of speech, retinal prosthesis which restores perception for blind patients and so on. The compact and light weight characteristic of the DGS-WPT system alongwith the rechargeable battery used in biomedical implants puts a barrier on non- rechargeable, big and bulky batteries. Finally, the objective of this thesis is to address the concerns of size, cost, efficiency and separation distance of the wireless power transfer system. Other objectives include designing of the WPT system operating on multiple frequency bands to enable simultaneous power and data transfer and enabling power transfer using single input-multi output (SIMO) and multi input- multi output (MIMO) wireless topology. Both SIMO and MIMO wireless topologies allow the charging of batteries of multiple portable electronic devices simultaneously