Khurram K. Afridi

High Efficiency Resonant DC/DC Converter Utilizing a Resistance Compression Network

This paper presents a new topology for a high-efficiency dc/dc resonant power converter that utilizes a resistance compression network (RCN) to provide simultaneous zero-voltage switching and near-zero-current switching across a wide range of input voltage, output voltage, and power levels. The RCN maintains desired current waveforms over a wide range of voltage operating conditions. The use of ON/OFF control in conjunction with narrowband frequency control enables high efficiency to be maintained across a wide range of power levels. The converter implementation provides galvanic isolation and enables large (greater than 1:10) voltage conversion ratios, making the system suitable for large step-up conversion in applications such as distributed photovoltaic converters. Experimental results from a 200-W prototype operating at 500 kHz show that over 95% efficiency is maintained across an input voltage range of 25-40 V with an output voltage of 400 V. It is also shown that the converter operates very efficiently over a wide output voltage range of 250-400 V, and a wide output power range of 20-200 W. These experimental results demonstrate the effectiveness of the proposed design.

Optimal Design of Grid-Connected PEV Charging Systems With Integrated Distributed Resources

The penetration of plug-in electric vehicles and renewable distributed generation is expected to increase over the next few decades. Large scale unregulated deployment of either technology can have a detrimental impact on the electric grid. However, appropriate pairing of these technologies along with some storage could mitigate their individual negative impacts. This paper presents a framework and an optimization methodology for designing grid-connected systems that integrate plug-in electric vehicle chargers, distributed generation and storage. To demonstrate its usefulness, this methodology is applied to the design of optimal architectures for a residential charging case. It is shown that, given current costs, maximizing grid power usage minimizes system lifecycle cost. However, depending upon the locations solar irradiance patterns, architectures with solar photovoltaic generation can be more cost effective than architectures without. Additionally, Li-ion storage technology and micro wind turbines are not yet cost effective when compared to alternative solutions.

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.

Design of Class E Resonant Rectifiers and Diode Evaluation for VHF Power Conversion

Resonant rectifiers have important applications in very-high-frequency (VHF) power conversion systems, including dc-dc converters, wireless power transfer systems, and energy recovery circuits for radio-frequency systems. In many of these applications, it is desirable for the rectifier to appear as a resistor at its ac input port. However, for a given dc output voltage, the input impedance of a resonant rectifier varies in magnitude and phase as output power changes. This paper presents a design methodology for Class E rectifiers that maintain near-resistive input impedance along with the experimental demonstration of this approach. Resonant rectifiers operating at 30 MHz over 10:1 and 2:1 power ranges are used to validate the design methodology and identify its limits. Furthermore, a number of Si Schottky diodes are experimentally evaluated for VHF rectification and categorized based on performance.

Enhanced Bipolar Stacked Switched Capacitor Energy Buffers

The Stacked Switched Capacitor (SSC) energy buffer is a recently proposed architecture for buffering energy between single-phase ac and dc. When used with film capacitors, it can increase the life of grid-interfaced power converters by eliminating limited-life electrolytic capacitors while maintaining comparable energy density. This paper introduces an enhanced version of the bipolar SSC energy buffer that achieves a higher effective energy density and round-trip efficiency, while maintaining the same bus voltage ripple ratio as the original design. Furthermore, the enhanced buffer uses fewer capacitors and switches than the original design. The enhancement in performance is achieved by modifying the control and switching patterns of the buffer switches. A prototype enhanced SSC energy buffer, designed for a 320 V bus and a 135 W load, has been built and tested. The design methodology and experimental results for the enhanced SSC energy buffer are presented and compared with the original design. The paper also presents a comparison of unipolar and bipolar SSC energy buffers. It is shown that while bipolar designs are superior in terms of effective energy density at low ripple ratios, unipolar designs can outperform bipolar designs at high ripple ratios.

A Multilevel Energy Buffer and Voltage Modulator for Grid-Interfaced Microinverters

Microinverters operating into the single-phase grid from solar photovoltaic (PV) panels or other low-voltage sources must buffer the twice-line-frequency variations between the energy sourced by the PV panel and that required for the grid. Moreover, in addition to operating over wide average power ranges, they inherently operate over a wide range of voltage conversion ratios as the line voltage traverses a cycle. These factors make the design of microinverters challenging. This paper presents a multilevel energy buffer and voltage modulator (MEB) that significantly reduces the range of voltage conversion ratios that the dc-ac converter portion of the microinverter must operate over by stepping its effective input voltage in pace with the line voltage. The MEB partially replaces the original bulk input capacitor, and functions as an active energy buffer to reduce the total size of the twice-line-frequency energy buffering capacitance. The small additional loss of the MEB can be compensated by the improved efficiency of the dc-ac converter stage, leading to a higher overall system efficiency. The MEB architecture can be implemented in a variety of manners, allowing different design tradeoffs to be made. A prototype microinverter incorporating an MEB, designed for 27 to 38 V dc input voltage, 230-V rms ac output voltage, and rated for a line cycle average power of 70 W, has been built and tested in a grid-connected mode. It is shown that the MEB can successfully enhance the performance of a single-phase grid-interfaced microinverter by increasing its efficiency and reducing the total size of the twice-line-frequency energy buffering capacitance.

Stacked switched capacitor energy buffer architecture

Electrolytic capacitors are often used for energy buffering applications, including buffering between single-phase ac and dc. While these capacitors have high energy density compared to film and ceramic capacitors, their life is limited and their reliability is a major concern. This paper presents a stacked switched capacitor (SSC) energy buffer architecture and some of its topological embodiments which overcome this limitation while achieving comparable effective energy density without electrolytic capacitors. The architectural approach is introduced along with design and control techniques. A prototype SSC energy buffer using film capacitors, designed for a 320 V dc bus and able to support a 135 W load has been built and tested with a power factor correction circuit. It demonstrates the effectiveness of the approach.

Impedance Control Network Resonant DC–DC Converter for Wide-Range High-Efficiency Operation

This paper introduces a new resonant converter architecture that utilizes multiple inverters and a lossless impedance control network (ICN) to maintain zero voltage switching (ZVS) and near zero current switching (ZCS) across wide operating ranges. Hence, the ICN converter is able to operate at fixed frequency and maintain high efficiency across wide ranges in input and output voltages and output power. The ICN converter architecture enables increase in switching frequency (hence reducing size and mass) while achieving very high efficiency. A prototype 200 W, 500 kHz ICN resonant converter designed to operate over an input voltage range of 25 V to 40 V and output voltage range of 250 V to 400 V is built and tested. The prototype ICN converter achieves a peak efficiency of 97.2%, maintains greater than 96.2% full power efficiency at 250 V output voltage across the nearly 2:1 input voltage range, and maintains full power efficiency above 94.6% across its full input and output voltage range. It also maintains efficiency above 93.4% over a 10:1 output power range across its full input and output voltage range owing to the use of burst-mode control.

Impedance control network resonant step-down DC-DC converter architecture

In this paper, we introduce a step-down resonant dc-dc converter architecture based on the newly-proposed concept of an Impedance Control Network (ICN). The ICN architecture is designed to provide zero-voltage and near-zero-current switching of the power devices, and the proposed approach further uses inverter stacking techniques to reduce the voltages of individual devices. The proposed architecture is suitable for large-step-down, wide-input-range applications such as dc-dc converters for dc distribution in data centers. We demonstrate a first-generation prototype ICN resonant dc-dc converter that can deliver 330 W from a wide input voltage range of 260 V-410 V to an output voltage of 12 V.

Alternative electrical distribution system architectures for automobiles

At present most automobiles use a 12 V electrical system with point-to-point wiring. The capability of this architecture in meeting the needs of future electrical loads is questionable. Furthermore, with the development of electric vehicles (EVs) there is a greater need for a better architecture. In this paper we outline the limitations of the conventional architecture and identify alternatives. We also present a multi-attribute trade-off methodology which compares these alternatives, and identifies a set of Pareto optimal architectures. The system attributes traded off are cost, weight, losses and probability of failure. These are calculated by a computer program that has built-in component attribute models. System attributes of a few dozen architectures are also reported and the results analyzed.