Design and optimization of switched-mode circuits for inductive links

Wireless power transfer (WPT) via magnetic induction is an emerging technology that is a result of the significant advancements in power electronics. Mobiles phones can now be charged wirelessly by placing them on a charging surface. Electric vehicles can charge their batteries while being parked over a certain charging spot. The possible applications of this technology are vast and the potential it has to revolutionise and change the way that we use today’s application is huge. Wireless power transfer via magnetic induction, also referred to as inductive power transfer (IPT), does not necessarily aim to replace the cable. It is intended to coexist and operate in conjunction with the cable. Although significant progress has been achieved, it is still far from reaching this aim since many obstacles and design challenges still need to be addressed. Low power efficiencies and limited transfer range are the two main issues for IPT. A tradeoff is usually associated with these two issues. Higher efficiencies are only achieved at very short transmission distances, whereas transferring large amounts of power at large distances is possible but at reduced efficiencies. This thesis addressed the limitations and design challenges in IPT systems such as low efficiency and short transmission range, in addition to poor power regulation and coil displacement and misalignment sensitivity. Novel circuit topologies and design solutions have developed for DC/AC inverters and DC/AC rectifiers that will allow for increased performance, higher efficiencies and reduced sensitivity to coil misalignments and displacements. This thesis contributes in four key areas towards IPT. Firstly, a detailed mathematical analysis has been performed on the electric circuit model of inductively coupled coils. This allows for better understanding on how power is distributed amongst the circuit’s elements. Equivalent circuit representations were presented to simplify the design process of IPT systems. Secondly, a review of the different classes and configurations of DC/AC inverters that can be used as primary coil drivers in IPT systems were presented. Class E DC/AC inverters were mathematically analysed in great detail and their performance as primary coil drivers in IPT systems was investigated. Thirdly, novel electronic tuning methods were presented to allow Class E primary coil drivers to operate at optimum switching conditions regardless of the distance between the coils of an IPT system and the value of the load. The saturable reactor was used as the electronic tunable element. Lastly, Class D and Class E AC/DC rectifiers have been used for the first time in IPT systems. Detailed mathematical analysis and extensive experimental results show their superior performance over the conventional half-wave and full-wave AC/DC rectifiers.