High-frequency power conversion and variable capacitor control for IPT systems

Abstract

Wireless Power Transfer (WPT) technology can be dated back to more than 120 years ago with some early work by Tesla. However, practical modern WPT technologies only started in the late 19th century with the invention of high-frequency power electronic devices, which made possible the generation of high frequency voltages and currents efficiently with switch mode power converters. High frequency operation is a prominent feature and essential requirement of WPT systems compared to traditional tightly-coupled transformers and electric machines because of the loosely-coupled nature between the primary and secondary sides of WPT systems. To generate the high frequency voltages and currents for WPT systems, switch mode DC-AC power converters are usually employed, and they are preferably soft-switched for resonant operation in order to maximize the system efficiency and minimize EMI (Electromagnetic Interference). This research aims to explore new methods to control the operating frequency of WPT systems. Based on the investigation of different types of high frequency resonant converters, including current-fed energy injection converters, Class E, and autonomous push pull converters, this thesis proposes a DC-voltage Controlled Variable Capacitors (DCVC) to dynamically control the zero voltage switching (ZVS) frequency of autonomous push-pull resonant converters for adjusting or stabilizing the operating frequency, and regulating the power flow to keep the output voltage constant. The equivalent capacitances of DCVCs are varied by controlling the conduction period of a diode in parallel with part of the tuning capacitors of the proposed circuit. In this research the conduction period of the diode is controlled by a DC voltage through two different measures via a bias circuit including a resistor (R-DCVC), or a transistor (T-DCVC). Unlike conventional switch mode capacitors, the proposed DCVCs are controlled smoothly by a DC voltage, so they are more suitable for high frequency operation. In addition, because there are no active switching and related gate drive issues, the EMI (Electromagnetic Interference) of the system can be greatly reduced, and more accurate control can be achieved compared to full switch mode counterpart. The proposed DCVC method and its detailed operation for different applications are fully analysed in theory, and verified by simulation and experimental results. Both the R-DCVC and T-DCVC methods have been applied to adjust the ZVS frequency at the primary side of IPT systems. A PLL controller is designed to stabilize the operating frequency of an IPT system while maintaining soft switching conditions, which helps to simplify the pickup circuit design, particularly with multiple power pickups. It has demonstrated that using the proposed DCVC method the operating frequency of an IPT system can be varied or stabilized in the range of a few hundred kHz to tens of MHz. The T-DCVC is also applied at the secondary side of an IPT system as a series or parallel tuned variable capacitor to regulate the power flow to stabilize the output voltage against the magnetic coupling and load variations. A prototype circuit at about 10W has been built and the experimental results have shown that the output voltage can be stabilized in the range of 5V to 24V with an accuracy of 2%, which is sufficient for driving most low power consumer electronic devices.