Design of IPT EV Battery Charging Systems for Variable Coupling Applications

Abstract

‡a Inductive Power Transfer (IPT) is a method and technology through which power may be transferred between two mutually coupled inductors across a relatively large air gap (from 10mm to 100mm). IPT systems have found applications in industry where galvanic isolation is seen as advantageous. This thesis focuses on developing loosely coupled IPT systems for use in hands-free EV battery charging applications. An IPT system can be divided into three main independent blocks: the primary power supply, the secondary pick-up controller, and the coupled magnetic structure of both the primary and the secondary. This thesis presents a new lumped coil magnetic structure and two new pickup controllers. The new lumped coil system, called a Double D charging pad, provides superior magnetic coupling performance over large air gaps when compared to conventional circular charging pads which have been the preferred design for loosely coupled lumped coil systems. A number of practical considerations are addressed and a strategy presented for designing tuning networks within lumped coil systems operating in variable coupling applications. The thesis also develops two new pick-up control topologies, a multi-path pickup controller, and a circulating current controller, both of which are designed for efficient power regulation. The Double D charging pad consists of a flux pipe with two flat Archimedean spiral-wound coils which sit in a co-planar relationship on top of the flux pipe. This structural design guides the generated magnetic flux to exit and to enter at the extremities of the pad in order to generate a high flux profile between the primary and the secondary. As a result, the developed Double D pad prototype has an improved coupling factor, 50% better than a circular pad with similar physical size, when operating with large vertical and lateral displacement. The Double D pad structure presented generates magnetic flux only in one side of the spiral winding and is naturally insensitive to nearby metallic objects. This is important for EV battery charging applications where metal on the car is largely ignored. A number of practical design considerations for variable coupled IPT systems using primary side current control are addressed. The self-inductance of the charging pad can vary with relative movement and misalignment of the ferrimagnetic material in the primary with respect to the secondary coil. As a result, tuning networks become mistuned, introducing additional Abstract ii reactive load into the system. This impact on the power supply has been investigated and a design approach has been proposed to mitigate the burden on the power supply. Two new pick-up controllers, one switched asynchronously and the second one switched synchronously, have been developed to operate efficiently in variable coupling applications. They are called the multi-path pick-up controller and the circulating current controller respectively. The multi-path pick-up controller consists of a series-tuned LC pick-up with multiple series-parallel LCL networks. Power regulation is performed by slowly switching in or out the LCL networks so as to adjust the equivalent number of active LCL networks required to match the output power. The circulating current controller is designed for a series-parallel LCL tuned pick-up. The controller uses two diodes and two switches, synchronously switched with the track frequency, to perform both power regulation and rectification. Both controllers were proven to be efficient when operating at power levels of 1-2kW with variable coupling. The measured efficiency of both controller prototypes is near 95% at rated load and above 85% for part load efficiencies greater than 0.25 per unit. The thesis demonstrates the feasibility of using fixed frequency IPT systems in a single pickup variable coupling application, where in the past this application has been dominated by variable frequency designs. The techniques developed here can be directly applied to systems where single power supplies are used to transfer power to multiple vehicles. Using the developed tuning design methodology, the reactive load in the power supply is minimised to achieve a nearly identical performance to a variable frequency system. Two new pick-up control topologies have been introduced that tolerate the variations in coupling. The development presented in this thesis demonstrates the possibility of using IPT systems for both stationary and Roadway Powering EVs applications, considered not possible in the past.