Saad Pervaiz

Investigation of power transfer density enhancement in large air-gap capacitive wireless power transfer systems

This paper introduces a new capacitive wireless power transfer approach with the potential to significantly enhance power transfer density in large air-gap applications. This enhancement is achieved through the use of multiple phase-shifted capacitive plates that reduce fringing fields in areas where field levels must be limited for safety reasons. The effectiveness of the proposed approach is evaluated using an analytical framework and validated using finite-element and circuit simulations. It is shown that a 6.78-MHz eight plate-pair system based on the proposed approach reduces fringing fields by a factor of five relative to a two plate-pair system while retaining the same power transfer density.

Energy density enhancement of unipolar SSC energy buffers through capacitance ratio optimization

The recently proposed Stacked Switched Capacitor (SSC) energy buffer architecture with film or ceramic capacitors can increase the life of single-phase ac-dc converters by eliminating electrolytic capacitors needed for twice-line-frequency energy storage, while maintaining comparable energy density. This paper presents a methodology for further increasing the energy density of enhanced unipolar SSC energy buffers by optimizing the capacitance ratio of the capacitors used in the energy buffer. It is shown that the relative enhancement in energy density depends on the required ripple ratio and the number of supporting capacitors in the energy buffer. The presented methodology is validated using a 1-2 enhanced unipolar SSC energy buffer designed for an 8-W offline LED driver.

Design of efficient matching networks for capacitive wireless power transfer systems

High-power large air-gap capacitive wireless power transfer (WPT) systems require matching networks that provide large voltage or current gain and reactive compensation. This paper introduces an analytical optimization approach for the design of L-section multistage matching networks for capacitive WPT systems. The proposed approach maximizes the matching network efficiency and identifies the optimal distribution of gains and compensations among the L-section stages. The results of the proposed approach are validated using an exhaustive-search based numerical optimization for a 12-cm air-gap, 6.78-MHz, 125-W capacitive WPT system. A 6.78-MHz, 15-W prototype comprising a two-stage matching network is also designed using the proposed analytical approach and the theoretical predictions are validated experimentally.

High power density impedance control network DC-DC converter utilizing an integrated magnetic structure

This paper introduces a high-power-density high-efficiency isolated dc-dc converter based on the impedance control network (ICN) resonant converter architecture. The ICN converter maintains very high efficiency by achieving zero voltage switching (ZVS) and near zero current switching (ZCS) across a wide range of input voltages, output voltages and output power. High power density is achieved by combining the three inductors of the ICN converter into a single integrated magnetic structure with two coupled windings. Power losses in this integrated magnetic structure are minimized using a finite element analysis (FEA) based design optimization approach. A prototype 550-W, 1-MHz ICN converter incorporating the integrated magnetic structure, designed to operate over an input voltage range of 36 V to 60 V and an output voltage range of 34 V to 55 V is built and tested. The prototyped ICN converter achieves a power density of 462 W/in3, a peak efficiency of 96.7% and maintains efficiencies above 94.8% across its entire operating range.

A high power density single-phase inverter using stacked switched capacitor energy buffer

This paper presents a high power density 2 kW single-phase inverter, with greater than 50 W/in3 power density and 90% line-cycle average efficiency. This performance is achieved through innovations in twice-line-frequency (120 Hz) energy buffering and high frequency dc-ac power conversion. The energy buffering function is performed using an advanced implementation of the recently proposed stacked switched capacitor (SSC) energy buffer architecture, and the dc-ac power conversion is performed using a soft-switching SiC-FET based converter, with a digital implementation of variable frequency constant peak current control.

30 W capacitive wireless power transfer system with 5.8 pF coupling capacitance

This paper presents a new capacitive wireless power transfer (WPT) system that achieves effective power transfer even with very low values of coupling capacitance between two metal plates separated by an air-gap. A prototype of the proposed capacitive WPT system has been built and shown to transfer 30 W across two 5-cm by 5-cm metal plates separated by an air-gap of 0.5 cm. The power transfer per unit capacitance of 5.5 W/pF achieved by this system is 20 times greater than the highest value previously reported in literature.

Improved capacitance ratio optimization methodology for stacked switched capacitor energy buffers

The recently proposed stacked switched capacitor (SSC) energy buffers with film or ceramic capacitors can increase the lifetime of single-phase ac/dc converters by eliminating the relatively short lifetime electrolytic capacitors used for twice-line-frequency energy buffering. This paper presents an improved methodology to minimize the passive volume of SSC energy buffers by optimally selecting their capacitance ratios while taking into account the variation in capacitor energy density with voltage rating. The proposed methodology is compared with the state-of-the-art approach for both unipolar and bipolar SSC energy buffers. It is shown that for designs with 10% voltage ripple, it reduces the passive volume by up to 43% depending on the capacitor technology and type of SSC energy buffer. The effectiveness of the proposed optimization methodology is validated using a prototype SSC energy buffer designed for an 8-W offline LED driver.

High-performance large air-gap capacitive wireless power transfer system for electric vehicle charging

This paper introduces a high-performance large air-gap capacitive wireless power transfer (WPT) module as part of a multi-modular capacitive WPT system for electric vehicle charging. This WPT module utilizes two pairs of metal plates separated by an air-gap as the capacitive coupler, incorporates L-section matching networks to provide gain and reactive compensation, and is driven by a GaN-based inverter operating at 6.78 MHz. The system achieves high efficiency and simplicity by eliminating the need for high-voltage capacitors, and instead utilizes the parasitic capacitances formed between the coupling plates and the vehicle chassis and roadway as part of the matching networks. This paper also presents a comprehensive design methodology for the capacitive WPT system that guarantees high performance by ensuring zero-voltage switching of the inverter transistors, and by selecting matching network component values to maximize efficiency under practical constraints on inductor quality factor and self-resonant frequency. Two prototype 6.78-MHz 12-cm air-gap capacitive WPT systems have been designed, built and tested. The first prototype with 625 cm2 coupling plate area transfers up to 193 W of power and achieves an efficiency greater than 90%, with a power transfer density of 3 kW/m2. The second prototype with 300 cm2 coupling plate area transfers up to 557 W of power and achieves an efficiency of 82%, with a power transfer density of 18.5 kW/m2, which exceeds the state-of-the-art for capacitive WPT systems by more than a factor of four.

A compact electrolytic-free two-stage universal input offline LED driver

This paper presents a single phase, two-stage electrolytic-free offline LED driver that utilizes a hybrid film-ceramic stacked switched capacitor (SSC) energy buffer (EB) in place of limited-life electrolytic capacitors for twice-line-frequency energy buffering. The proposed LED driver comprises a front-end power factor correction (PFC) stage, followed by an isolated dc-dc conversion stage, with an SSC energy buffer connected across the intermediate dc bus. Compared to a single capacitor, the use of SSC energy buffer reduces the passive volume of the energy buffer by a factor of two. A simple and robust control scheme is proposed to interface the SCC energy buffer with the PFC converter. A prototype 300 W LED driver for universal input voltage range (90-265 Vrms) and 12 V output voltage is designed, built and tested based on the proposed architecture. The SSC energy buffer achieves a peak efficiency of 98.5% and maintains an efficiency of above 96% across a wide output power range, thus minimally impacting the overall system efficiency. In addition, the impact of the intermediate dc bus voltage ripple on overall system efficiency is investigated. It is found that there is negligible change in overall system efficiency when the intermediate bus voltage ripple ratio is varied between 3% and 7.5%.

High-power-transfer-density capacitive wireless power transfer system for electric vehicle charging

This paper introduces a large air-gap capacitive wireless power transfer (WPT) system for electric vehicle charging that achieves a power transfer density exceeding the state-of-the-art by more than a factor of four. This high power transfer density is achieved by operating at a high switching frequency (6.78 MHz), combined with an innovative approach to designing matching networks that enable effective power transfer at this high frequency. In this approach, the matching networks are designed such that the parasitic capacitances present in a vehicle charging environment are absorbed and utilized as part of the wireless power transfer mechanism. A new modeling approach is developed to simplify the complex network of parasitic capacitances into equivalent capacitances that are directly utilized as the matching network capacitors. A systematic procedure to accurately measure these equivalent capacitances is also presented. A prototype capacitive WPT system with 150 cm2 coupling plates, operating at 6.78 MHz and incorporating matching networks designed using the proposed approach, is built and tested. The prototype system transfers 589 W of power across a 12-cm air gap, achieving a power transfer density of 19.6 kW/m2.