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  1. In this paper, a wireless charging architecture employing a multilevel switched-capacitor (MSC) AC-DC rectifier is investigated. The proposed MSC rectifier features a multilevel design which is scalable to accommodate different power ratings and load ranges. The topology showcases advantages for wireless power transfer (WPT) systems in terms of compactness, efficiency, impedance tunability, and harmonic attenuation. The single-stage active topology is capable of varying its low-distortion staircase input voltage to tune the wireless power transfer system for high system-wide efficiency. A 7-level, 20 W prototype is used to verify the WPT loading and loss analysis. The prototype operates at 150 kHz with up to 3:1 step-down conversion ratio to an output voltage of 5.0 V. The experimental peak DC-to-DC efficiency is 93.8% and the rectifier peak efficiency is 98.3%. The rectifier demonstrates low waveform distortion and high efficiency across many WPT loading conditions, solidifying its place as a strong candidate for wireless power applications. 
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  2. Active rectifiers enhance WPT systems via tunability, high efficiency, and low waveform distortion. However, utilizing these benefits requires that two circuit characteristics are managed simultaneously: the switching frequency must be synchronized to the transmitter and the output must be regulated. Furthermore, the fundamental benefit of impedance tunability inherent to the active rectifier necessitates that this dual-objective control problem remains stable over a wide range of operating points. Either control loop can be designed in isolation, and under this premise, this work contributes a closed form derivation for the cross-coupling behaviors in the control architecture for a 7-level switched capacitor WPT system. Finally, regions of attenuated cross-coupling effects are identified and used to experimentally demonstrate wide-range control with stable output regulation and frequency synchronization. 
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  3. null (Ed.)
    In wireless power transfer (WPT) applications, the multi-level switched capacitor topology achieves significant advantages in terms of efficiency, system loading, THD, and output regulation. The topology requires dual-loop control in order to harness these benefits. First, a small signal discrete time model for the 7-level rectifier WPT system is developed. Then, a control loop is designed that enables the rectifier to regulate DC load voltage by varying its modulation scheme. Next, the WPT carrier frequency is sensed and a phase-locked loop is used in combination with the small signal power stage model to design a closed-loop controller that synchronizes frequency and regulates control phase through adjustments of the switching period. Finally, cross-coupling interactions between the two control loops are modeled, and stable dual-loop operation is shown. 
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  4. null (Ed.)
    In wireless power transfer systems, active rectifiers demonstrate improved efficiency and regulation capability. To enable impedance or output regulation, ensure stable operation, and maximize the efficiency, switching actions of the rectifier have to be synchronized with the magnetic field generated from the transmitter coil. This work presents an implementation of a phase- locked-loop synchronization controller using commercial components, including a low-cost microcontroller. A discrete-time small-signal model is used to derive the transfer function of the inherent feedback and design a compensator stabilizing the synchronization loop. Large-signal state-space modeling is used to design a high-efficiency, soft-switching, 6.78MHz power stage. A low-profile, 40W, GaN-based rectifier prototype is designed and built to experimentally verify the ability to synchronize and achieve high efficiency due to soft-switching. 
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  5. Active rectifiers in wireless power transfer systems exhibit many benefits compared to diode rectifiers, including increased efficiency, controllable impedance, and regulation capability. To achieve these benefits, the receivers must synchronize their switching frequency to the transmitter to avoid sub-fundamental beat frequency oscillations. Without additional communication, the receiver must synchronize to locally-sensed signals, such as voltages and currents induced in the power stage by the transmitter. However, the waveforms in the receiver are dependent on both the transmitter and receiver operation, resulting in an internal feedback between sensing and synchronization which prohibits the use of traditional phase-locked-loop design techniques. In this digest, a discrete time state space model is developed and used to derive a small signal model of these interactions for the purpose of designing stable closed-loop synchronization control. A prototype 150 kHz wireless power transfer converter is used to experimentally validate the modeling, showcasing stable synchronization. 
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