Power Control Techniques in Wireless Power Transfer System

Department of Electrical EngineeringWireless power transfer (WPT) technology has attract the attentions of researchers and industrial for new method of power transfer mechanism. WPT technology enables contactless energy transfer between two resonators through a magnetic field. WPT is a promising method of powering electrical devices, especially in environments where wired charging is inconvenient of even dangerous. Recently, the interest of WPT has been arise with the increase of mobile devices such as cell phones, PDAs, laptops, tablets, and other handheld gadgets equipped with rechargeable batteries has been widely spreading. In recent years, WPT technology has already been applied to tooth brushes and mobile phones. In addition, many researchers are interested in applying WPT technology to electrical vehicle, cordless zone, and biomedical application and are conducting research to realize it. The area where wireless power transmission technology is most needed is biomedical. Biomedical devices to be implanted in the body are most severely limited by their small volume and battery capacity limitations. WPT technology is the most suitable technology to solve this problem. However, power loss occurs during wireless power transmission, it is necessary to overcome this problem because the harmful effects on the human body. In this dissertation, two new power control techniques are introduced to increase the efficiency of wireless power transmission in biomedical systems. The first proposed technique is a technique for transmitting power more efficiently at a place where the transmission distance is long, the size of the device is small, the position of the device is not fixed, and the efficiency is very low like a capsule endoscopy. The second proposed technique is a power control technique which can increase the power transfer efficiency for applications with close distances for implanted biomedical devices under the skin like a cardiac pacemaker. Chapter II presents a new power control technique to improve efficiency in magnetic resonance (MR)- WPT system for biomedical capsule endoscopy. Recently, capsule endoscopy technology has been developed and emerged as an alternative to small bowel endoscopy, gastroscopy, and colonoscopy, all of which cause discomfort to patients because of their relatively large-diameter and flexible cables. However, commercialized capsule endoscopy still suffers from limited battery capacity. Chapter II presents a theory for power control technique in MR-WPT system, along with its experimental verification. An MR-WPT system with a 9-mm-diameter receiver is implemented, which is small enough to fit in the current capsule endoscope. The proposed system improves the efficiency despite variations in the distance, angle, and displacement. The proposed system is found to have a low specific absorption rate, which demonstrated that it is safe to use in the human body. Chapter III proposes power control technique for inductive power transfer (IPT) battery charging system using in-band communication that aims to minimize number of power stages and increase power transfer efficiency with low-cost hardware. Constant current and constant voltage mode are needed to effectively charge Li-ion batteries to ensure long life-span and maximum capacity utilization. These two charging modes require different feedback loops and circuitry, which increase system complexity and reduces efficiency. One approach is to use additional converter stages that ensure effective battery charging, but this introduces additional conversion losses, which decreases efficiency. The IPT system using proposed step charging method tracks the proper frequency to maintain the desired constant current or voltage for battery charging without the need for additional regulation circuits, and with minimized feedback control signal. In-band communication is used to send feedback signal from secondary side to primary side of the IPT system, which enables effective feedback control without conventional wireless communication module. This power control technique is a technique to eliminate power loss in an unnecessary regulator. This technology is applicable to IPT using in-band communication and is suitable for implantable devices because it reduces receiver loss.ope

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