A 6.78MHz Adaptive-ZVS Class-D PA with Dynamic Dead-Time for Wireless Power Transfer system

Department of Electrical EngineeringIn this thesis, a class-D power amplifier (PA) with adaptive zero-voltage switching (A-ZVS) technique for Low power 6.78 MHz resonant wireless power transfer (R-WPT) system is proposed. In R-WPT operation, the loading impedance of a PA can be varied by the process tolerance of the LC resonant components and WPT environments, such as the resonant topology, coupling coefficient and loading condition of the receiver. The proposed A-ZVS feedback loop of PA calibrates the equivalent resonant capacitance using PWM-controlled switched capacitor in real-time to achieve ZVS by adjusting the loading impedance to be slightly inductive. Furthermore, the proposed PA adjust the dead-time according to variation of WPT environments. The proposed PA was fully integrated except for one switched capacitor used as the tuning element and fabricated in a TSMC 0.18um BCD process. The measurement results demonstrated robust ZVS operation with a peak system efficiency of 52.7% and an enhanced maximum transmitting power of 107%.ope

Wireless Power Transfer System for Battery-Less Body Implantable Devices

Department of Electrical EngineeringAs the life expectancy is increased and the welfare is promoted, researches on the body implantable medical devices (BIMD) are actively being carried out, and products providing more various functions are being released. On the other hand, due to these various functions, the power consumption of the BIMD is also increased, so that the primary battery alone cannot provide sufficient power for the devices. The limited capacity and life time of batteries force patients to make an additional payment and suffering for the power supply of the BIMD. Wireless power transfer technology is the technology which has been making remarkable progress mainly in wireless charging for personal portable devices and electric vehicles. Convergence of wireless power transfer technology (WPT) and rechargeable battery can extend the life time of the BIMD and reduce the suffering and the cost for battery replacements. Furthermore, WPT enables the devices which do not need to operate consistently such as body implantable sensor devices to be used without batteries. In this dissertation, techniques to support WPT for BIMD are introduced and proposed. First, basic researches on magnetic coupled WPT are presented. The basics which are important factors to analyze power transmission are introduced. In addition, circuits that make up the WPT system are described. There are three common technical challenges in WPT. Those are efficiency degradation on coil geometry, voltage gain variation with coil geometry, and power losses in WPT. The common challenges are discussed in chapter II. Moreover, additional challenges which are arisen in WPT for BIMD and approaches to resolve the challenges are addressed in chapter II. Then, efficiency improvement techniques and control techniques in WPT are presented in chapter III. The presented techniques to improve efficiency are applied in coil parts and circuit parts. In coil parts, efficiency enhancement technique by geometric variation is proposed. In circuit parts, instantaneous power consuming technique for step-down converter is suggested. Li-ion battery charger is also discussed in chapter III. Additionally, the wireless controlled constant current / constant voltage charging mode and the proposed step charging method are described. After that, WPT system for BIMD is discussed one by one with the proposed techniques for each part in chapter IV. A load transformation is suggested to improve efficiency in weak coupling, and suppress voltage gain variation under coil displacement. Power conversion efficiency improvement techniques for rectifier and converter are also proposed. By using the proposed technique for the converter, we can remove the bootstrap capacitors, and reduce the overall size of power circuits. In conclusion, techniques in coil parts and circuit parts to handle challenges in WPT for BIMD are fully investigated in this thesis in addition to the efficiency improvement and control techniques in common WPT. All the techniques are verified through simulations or experiments. The approaches realized in the thesis can be applied to other applications employing the WPT.clos

A GaN-Based Synchronous Rectifier with Reduced Voltage Distortion for 6.78 MHz Wireless Power Applications

The call for a larger degree of engineering innovation grows as wireless power transfer increases in popularity. In this thesis, 6.78 MHz resonant wireless power transfer is explained. Challenges in WPT such as dynamic load variation and electromagnetic interference due to harmonic distortion are discussed, and a literature review is conducted to convey how the current state of the art is addressing these challenges.A GaN-based synchronous rectifier is proposed as a viable solution, and a model of the circuit is constructed. The precisely derived model is compared to a linearized model to illustrate the importance of exactness within the model derivation. The model is then used to quantify the design space of circuit parameters Lr and Cr with regard to harmonic distortion, input phase control, and efficiency. Practical design decisions concerning the 6.78 MHz system are explained. These include gate driver choice and mitigation of PCB parasitics. The model is verified with open loop experimentation using a linear power amplifier, FPGA, electronic load, and two function generators. Current zero-crossing sensing is then introduced in order to achieve self-regulation of both the switching frequency and input phase. The details of the FPGA code and sensing scheme used to obtain this closed loop functionality are described in detail. Finally, conclusions are drawn, and future work is identified

Wireless Power Transfer

Wireless power transfer techniques have been gaining researchers' and industry attention due to the increasing number of battery-powered devices, such as mobile computers, mobile phones, smart devices, intelligent sensors, mainly as a way to replace the standard cable charging, but also for powering battery-less equipment. The storage capacity of batteries is an extremely important element of how a device can be used. If we talk about battery-powered electronic equipment, the autonomy is one factor that may be essential in choosing a device or another, making the solution of remote powering very attractive. A distinction has to be made between the two forms of wireless power transmission, as seen in terms of how the transmitted energy is used at the receiving point: - Transmission of information or data, when it is essential for an amount of energy to reach the receiver to restore the transmitted information; - Transmission of electric energy in the form of electromagnetic field, when the energy transfer efficiency is essential, the power being used to energize the receiving equipment. The second form of energy transfer is the subject of this book

Synchronous operation of high frequency inductive power transfer systems through injection locking

High frequency inductive power transfer systems can be designed for operation with high tolerance to misalignment and large air-gaps, making it possible to operate in highly dynamic environments. Most examples in the literature use a single active transmitter and a single passive receiver (active-passive approach). Such systems are limited to unidirectional power flow and are susceptible to detuning of the transmitter due to changes of reflected reactance stemming from diode non-linearities. This also limits the range of coupling over which the system can be operated efficiently. Therefore there is significant potential for expanding the range of applications of inductive power transfer systems by moving to an active-active configuration. This will enable bidirectional power flow, power routing through several nodes and on-the-fly retuning to eliminate reflected reactances. One of the greatest challenges in achieving an active secondary in an IPT system is obtaining a stable frequency and phase reference for the synchronous rectifier/transceiver with respect to the transmitter coil current and hence magnetic field. Various methods for synchronisation have been proposed in the literature, but they either require a separate, out of band communication link, or are difficult to scale to MHz operation. This paper describes an alternative to the existing solutions, using an injection locked oscillator to provide optimal phase tracking. A series of candidate feedback configurations are also proposed to provide high system resilience. In this work the basic principles of injection locking are described as applied to synchronous IPT transceivers and experimental results are presented demonstrating its application to a bidirectional back-to-back Class-EF configuration operating at 13.56 MHz, with coupling factors ranging from 1.9 % to 8.4 % and power levels of up to 25 W

Design of Power Management Integrated Circuits and High-Performance ADCs

A battery-powered system has widely expanded its applications to implantable medical devices (IMDs) and portable electronic devices. Since portable devices or IMDs operate in the energy-constrained environment, their low-power operations in combination with efficiently sourcing energy to them are key problems to extend device life. This research proposes novel circuit techniques for two essential functions of a power receiving unit (PRU) in the energy-constrained environment, which are power management and signal processing. The first part of this dissertation discusses power management integrated circuits for a PRU. From a power management perspective, the most critical two circuit blocks are a front-end rectifier and a battery charger. The front-end CMOS active rectifier converts transmitted AC power into DC power. High power conversion efficiency (PCE) is required to reduce power loss during the power transfer, and high voltage conversion ratio (VCR) is required for the rectifier to enable low-voltage operations. The proposed 13.56-MHz CMOS active rectifier presents low-power circuit techniques for comparators and controllers to reduce increasing power loss of an active diode with offset/delay calibration. It is implemented with 5-V devices of a 0.35 µm CMOS process to support high voltage. A peak PCE of 89.0%, a peak VCR of 90.1%, and a maximum output power of 126.7 mW are measured for 200Ω loading. The linear battery charger stores the converted DC power into a battery. Since even small power saving can be enough to run the low-power PRU, a battery charger with low IvQ is desirable. The presented battery charger is based on a single amplifier for regulation and the charging phase transition from the constant-current (CC) phase to the constant-voltage (CV) phase. The proposed unified amplifier is based on stacked differential pairs which share the bias current. Its current-steering property removes multiple amplifiers for regulation and the CC-CV transition, and achieves high unity-gain loop bandwidth for fast regulation. The charger with the maximum charging current of 25 mA is implemented in 0.35 µm CMOS. A peak charger efficiency of 94% and average charger efficiency of 88% are achieved with an 80-mAh Li-ion polymer battery. The second part of this dissertation focuses on analog-to-digital converters (ADCs). From a signal processing perspective, an ADC is one of the most important circuit blocks in the PRU. Hence, an energy-efficient ADC is essential in the energy-constrained environment. A pipelined successive approximation register (SAR) ADC has good energy efficiency in a design space of moderate-to-high speeds and resolutions. Process-Voltage-Temperature variations of a dynamic amplifier in the pipelined-SAR ADC is a key design issue. This research presents two dynamic amplifier architectures for temperature compensation. One is based on a voltage-to-time converter (VTC) and a time-to-voltage converter (TVC), and the other is based on a temperature-dependent common-mode detector. The former amplifier is adopted in a 13-bit 10-50 MS/s subranging pipelined-SAR ADC fabricated in 0.13-µm CMOS. The ADC can operate under the power supply voltage of 0.8-1.2 V. Figure-of-Merits (FoMs) of 4-11.3 fJ/conversion-step are achieved. The latter amplifier is also implemented in 0.13-µm CMOS, consuming 0.11 mW at 50 MS/s. Its measured gain variation is 2.1% across the temperature range of -20°C to 85 °C

Nano-Power Integrated Circuits for Energy Harvesting

The energy harvesting research field has grown considerably in the last decade due to increasing interests in energy autonomous sensing systems, which require smart and efficient interfaces for extracting power from energy source and power management (PM) circuits. This thesis investigates the design trade-offs for minimizing the intrinsic power of PM circuits, in order to allow operation with very weak energy sources. For validation purposes, three different integrated power converter and PM circuits for energy harvesting applications are presented. They have been designed for nano-power operations and single-source converters can operate with input power lower than 1 μW. The first IC is a buck-boost converter for piezoelectric transducers (PZ) implementing Synchronous Electrical Charge Extraction (SECE), a non-linear energy extraction technique. Moreover, Residual Charge Inversion technique is exploited for extracting energy from PZ with weak and irregular excitations (i.e. lower voltage), and the implemented PM policy, named Two-Way Energy Storage, considerably reduces the start-up time of the converter, improving the overall conversion efficiency. The second proposed IC is a general-purpose buck-boost converter for low-voltage DC energy sources, up to 2.5 V. An ultra-low-power MPPT circuit has been designed in order to track variations of source power. Furthermore, a capacitive boost circuit has been included, allowing the converter start-up from a source voltage VDC0 = 223 mV. A nano-power programmable linear regulator is also included in order to provide a stable voltage to the load. The third IC implements an heterogeneous multisource buck-boost converter. It provides up to 9 independent input channels, of which 5 are specific for PZ (with SECE) and 4 for DC energy sources with MPPT. The inductor is shared among channels and an arbiter, designed with asynchronous logic to reduce the energy consumption, avoids simultaneous access to the buck-boost core, with a dynamic schedule based on source priority