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Class $E^2$ DC–DC converter composed of Class $E$ power amplifier (PA) and rectifier is a promising candidate for MHz wireless power transfer. It is soft-switching based and able to achieve high efficiency at MHz frequency. However, the converter implemented through traditional static design is sensitive to the variations of operation condition. Its performance gets deteriorated when DC load and coil coupling deviate from their respective optimum values. This paper demonstrates that the degradation of system efficiency is mostly due to the mismatch of PA load, and the efficiency drop can be efficiently improved by adding an L-type impedance matching network (IMN) after PA. A fixed IMN is sufficient to maintain a high efficiency, while a tunable IMN is required to ensure stable output power when operation condition dramatically changes. Key techniques, particularly system-level optimization, are discussed in this paper that ensure high efficiency over a wide range of variation in operation condition and also with reduced capacitor/inductor tuning ranges in the IMN. The 6.78-MHz Class $E^2$ DC–DC converters with and without the fixed/tunable IMN are fabricated and measured for validation purposes. The experimental results show that both high efficiency ( $>$66%) and stable output power (around 9 W) are maintained for the tunable converter when there are variations in the DC load and coil coupling. [ABSTRACT FROM AUTHOR]

High-resistivity(HR) silicon-on-insulator (SOI) substrates provide low substrate loss, so planar spiral inductors integrated on them presenting higher quality factor ( $Q$ ) than those on traditional Si substrates. However, the parasitic surface conduction (PSC) effect in the SOI substrate constitutes a conductive layer underneath the buried oxide layer, which deteriorates the inductors performance. This effect can be effectively eliminated by introducing a trap-rich layer. In this paper, physical models that can accurately characterize the behavior of inductors integrated on the HR and radio frequency enhanced signal integrity (RFeSI) SOI substrates are presented, and the analysis and evaluation of PSC effect on the performance of inductors, i.e., the inductance, the quality factor, the self-resonant frequency, and the frequency, where $Q$ peaks, are shown. Planar spiral inductors integrated on HR and RFeSI SOI substrates are fabricated and measured, validating the feasibility of the models we use. The experiment results show that the value of $Q$ and the frequency where it peaks can be improved significantly by eliminating the PSC effect. The temperature effects are also explored, showing that the PSC effect gets worse with raising temperature and accelerates the degradation rate of $Q$ . [ABSTRACT FROM PUBLISHER]

The high-efficiency current-mode (CM) and voltage-mode (VM) Class-E power amplifiers (PAs) for MHz wireless power transfer (WPT) systems are first proposed in this paper and the design methodology for them is presented. The CM/VM Class-E PA is able to deliver the increasing/decreasing power with the increasing load and the efficiency maintains high even when the load varies in a wide range. The high efficiency and certain operation mode are realized by introducing an impedance transformation network with fixed components. The efficiency, output power, circuit tolerance, and robustness are all taken into consideration in the design procedure, which makes the CM and the VM Class-E PAs especially practical and efficient to real WPT systems. 6.78-MHz WPT systems with the CM and the VM Class-E PAs are fabricated and compared to that with the classical Class-E PA. The measurement results show that the output power is proportional to the load for the CM Class-E PA and is inversely proportional to the load for the VM Class-E PA. The efficiency for them maintains high, over 83%, when the load of PA varies from 10 to 100  $\Omega$, while the efficiency of the classical Class-E is about 60% in the worst case. The experiment results validate the feasibility of the proposed design methodology and show that the CM and the VM Class-E PAs present superior performance in WPT systems compared to the traditional Class-E PA. [ABSTRACT FROM AUTHOR]

Simultaneous wireless charging of multiple devices is a unique advantage of wireless power transfer (WPT). Meanwhile, the multiple-receiver configuration makes it more challenging to analyze and optimize the operation of the system. This paper aims at providing a general analysis on the multiple-receiver WPT systems and compensation for the influence of the cross coupling. A two-receiver WPT system is first investigated as an example. It shows that theoretically by having derived optimal load reactances, the important system characteristics can be preserved, such as the original system efficiency, input impedance, and power distribution when there is no cross coupling between receivers. The discussion is then extended to general multiple-receiver WPT systems with more than two receivers. Similar results are obtained that show the possibility of compensating the cross coupling by having the derived optimal load reactances. Finally, the theoretical analysis is validated by model-based calculation and final experiments using real two- and three-receiver systems. [ABSTRACT FROM PUBLISHER]

RF losses and nonlinearities of the commercially available Soitec radio frequency enhanced signal integrity (RFeSI) high-resistivity silicon-on-insulator substrates are investigated through investigation of 50- $\Omega $ coplanar waveguide lines manufactured on them. It is shown that the losses of the RFeSI substrates are very small. They have a temperature minimum value at around 70 °C. The generated second- and third-harmonic powers, due to the nonlinearities of the substrates, also have a similar temperature minimum value. The results indicate that the RFeSI substrates present a good RF performance over a temperature range from 0 °C to 115 °C. The RF performance of the two generations of RFeSI substrates is also compared. The losses of the two RFeSI substrates remain the same over the explored temperature range, whereas the harmonic performance of generation 2 (RFeSI 90) is better than that of generation 1 (RFeSI 80). This stems from the increased effective resistivity. [ABSTRACT FROM PUBLISHER]

Wireless power transfer has shown revolutionary potential in challenging the conventional charging method for consumer electronic devices. Through magnetic resonance coupling, it is possible to supply power from one transmitter to multiple receivers simultaneously. In a multiple-receiver system, the overall system efficiency highly depends on the loads’ and receivers' positions. This paper extends the conventional circuit-model-based analysis for the one-receiver system to investigate a general one-transmitter multiple-receiver system. It discusses the influence of the loads and mutual inductance on the system efficiency. The optimal loads, the input impedance, and power distribution are analyzed and used to discuss the proper system design and control methods. Finally, systems with a different number of receivers are designed, fabricated, and measured to validate the proposed method. Under the optimal loading conditions, an efficiency above 80% can be achieved for a system with three different receivers working at 13.56 MHz. [ABSTRACT FROM PUBLISHER]

Wireless power transfer (WPT) has attracted an ever increasing interest from both industry and academics over the past few years. Its applications vary from small power devices such as mobile phones and tablets to high power electric vehicles and from small transfer distance of centimeters to large distance of tens of centimeters. In order to achieve a high-efficiency WPT system, each circuit should function at a high efficiency along with the proper impedance matching techniques to minimize the power reflection due to the impedance mismatch. This paper proposes an analysis on the system efficiency to determine the optimal impedance requirement for coils, rectifier, and dc–dc converter. A novel cascaded boost–buck dc–dc converter is designed to provide the optimal impedance matching in WPT system for various loads including resistive load, ultracapacitors, and batteries. The proposed 13.56-MHz WPT system can achieve a total system efficiency over 70% in experiment. [ABSTRACT FROM PUBLISHER]

In this paper, an analytical model for the Class-F rectifier RF-dc conversion efficiency is presented. This model analyzes each kind of diode power losses in the rectifier due to the diode series resistor, junction capacitor, built-in potential, and breakdown voltage based on the time-domain diode voltage and current waveforms. The model provides a simple calculation routine to determine the threshold input power at which the peak reverse voltage across the diode starts to exceed the diode breakdown voltage and cause the diode efficiency to decrease. Two sets of closed-form equations are derived to calculate the Class-F rectifier efficiency in the input power region where the input power is either smaller or larger than the determined threshold input power, respectively. The implementation of a MATLAB code for the model calculation is also presented. Using this tool, the diode efficiency in Class-F rectifiers of different diode parameters can be easily determined, which provides a useful guideline for the optimal diode selection in various applications. The calculated diode efficiency using this model is compared with the Class-F rectifier ADS simulation at different load conditions, and all the results agree well with the model. To verify the model at different frequencies, two Class-F rectifiers working at 900 MHz and 5.8 GHz, respectively, are designed with the highest measured efficiency of 80.4% for the 900-MHz rectifier and 79.5% for the 5.8-GHz rectifier. Both of the measurement results agree well with the model prediction, which further verified the accuracy of the proposed model. [ABSTRACT FROM AUTHOR]

A device architecture for improving the linearity of barium-strontium-titanate (BST) capacitors is proposed, analyzed, and experimentally demonstrated. The basic concept is to reduce the RF swing across the ferroelectric varactors by connecting multiple capacitors in series. With proper biasing networks, the overall device preserves its tunability. In this paper, the proposed architecture is analyzed to derive the equations that relate its quality factor and tuning speed to the design parameters. Parallel-plate BST capacitors are fabricated based on the architecture. Various measurements are performed to demonstrate the efficacy of the technique. The third-order intercept point at input (IIP3) is found to be improved by 16 dB. The hot-tuning test shows that the tunability is maintained for up to 20-V peak-to-peak swing. BST capacitors utilizing the proposed technique are used in a phase-shifter design. The 2.4-GHz phase shifter exhibits high IIP3 greater than 40 dBm. [ABSTRACT FROM AUTHOR]