Stanimir Valtchev

Resonant Contactless Energy Transfer With Improved Efficiency

This paper describes the theoretical and experimental results achieved in optimizing the application of the series loaded series resonant converter for contactless energy transfer. The main goal of this work is to define the power stage operation mode that guarantees the highest possible efficiency. The results suggest a method to select the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that efficiency improvement. The research clarifies also the effects of the physical separation between both halves of the ferromagnetic core on the characteristics of the transformer. It is shown that for practical values of the separation distance, the leakage inductance, being part of the resonant inductor, remains almost unchanged. Nevertheless, the current distribution between the primary and the secondary windings changes significantly due to the large variation of the magnetizing inductance. An approximation in the circuit analysis permits to obtain more rapidly the changing values of the converter parameters. The analysis results in a set of equations which solutions are presented graphically. The graphics show a shift of the best efficiency operation zone, compared to the converter with an ideally coupled transformer. Experimental results are presented confirming that expected tendency.

Efficient resonant power conversion

The DC analysis of a series-resonant converter operating above resonant frequency is presented. The results are used to analyze the current form factor and its effect on the efficiency. The selection of the switching frequency to maximize the efficiency is considered. The derived expressions are generalized and can be applied to calculations in any of the switching modes for a series-resonant circuit. For switching frequencies higher than the resonant frequency, an area of more efficient operation is indicated which will aid in the design of this class of converters and power supplies. It is pointed out that (especially for power MOSFETs where ohmic losses dominate) it is more attractive to select switching frequencies that are higher than the resonant frequency because of the possibility of nondissipative snubbers. Slowing down the rise of the gate voltage and, hence, the slow decrease of ON resistance during turn-on is also not a drawback to high-frequency switching. Because of this safer operation, the standard intrinsic diode of the power MOSFET could be used at high frequencies instead of the more expensive FREDFET. >

A multiple-switch high-voltage DC-DC converter

Series connection of power devices has evolved into a mature technique and is widely applied in HV DC power systems. Static and dynamic voltage balance is ensured by shunting individual devices with dissipative snubbers. The snubber losses become pronounced for increased operating frequencies and adversely affect power density. Capacitive snubbers do not exhibit these disadvantages, but they require a zero-voltage switching mode. Super-resonant power converters facilitate the principle of zero-voltage switching. A high-voltage DC-DC power converter with multiple series-connected devices is proposed. It allows the application of nondissipating snubbers to assist the voltage sharing between the multiple series-connected devices and lowers turnoff losses. Simulation results obtained with a circuit simulator are validated in an experimental power converter operating with two series-connected devices. The behavior of the series connection is examined for MOSFETs and IGBTs by both experimental work with a 2 kW prototype and computer simulation. Applications can be found in traction and heavy industry, where the soft-switching power converter is directly powered from a high-voltage source.

A test rig for thrust force measurements of an all HTS linear synchronous motor

This paper presents the design of a test rig for an all HTS linear synchronous motor. Although this motor showed to have several unattractive characteristics, its design raised a number of problems which must be considered in future HTS machines design. HTS electromagnetic properties led to the development of new paradigms in electrical machines and power systems, as e. g. in some cases iron removal and consequent assembly of lighter devices. This is due to superconductors ability to carry high currents with minimum losses and consequent generation in the surrounding air of flux densities much higher than the allowed by ferromagnetic saturation. However, severe restrictions in HTS power devices design that goes further beyond cryogenic considerations must be accounted in. This is usually the case when BSCCO tapes are used as conductors. Its bending limitations and the presence of flux components perpendicular to tape surface, due to the absence of iron, have to be considered for it may turn some possible applications not so attractive or even practically unfeasible. An all HTS linear synchronous motor built by BSCCO tapes as armature conductors and two trapped-flux YBCO bulks in the mover was constructed and thrust force measurements are starting to be performed. Although the device presents severe restrictions due to the exposed and other reasons, it allowed systematising its design. A pulsed-field magnetiser to generate opposite fluxes for both YBCO bulks is also detailed. Thrust force numerical predictions were already derived and presented.

1 kW/250 kHz full bridge zero voltage switched phase shift DC-DC converter with improved efficiency

This paper presents the analysis, design, simulation and experimental results of a full bridge zero voltage switched phase shift DC-DC converter, working with an internal frequency of 250 kHz, with an output power of 1 kW. The converter uses a two windings inductor with the secondary connected to the output by means of two clamping rectifier diodes, to allow resonant commutation of the passive to active leg of the converter. With this process it is possible to improve the efficiency of the converter.

Improved full bridge zero voltage switched phase shift DC/DC converter using a secondary clamped inductor

This paper presents the analysis, simulation and experimental results of a full-bridge zero voltage switched phase shift DC-DC power converter, working with an internal frequency of 250 kHz, with an output power of 1 kW. The power converter uses a two windings inductor with the secondary connected to the output by means of two clamping rectifier diodes, to allow resonant commutation of the passive to active leg of the power converter. With this process it is possible to improve the efficiency of the power converter.

Pespectives of TFET devices in ultra-low power charge pumps for thermo-electric energy sources

The superior electrical characteristics of the heterojunction III-V Tunnel FET (TFET) devices can outperform current technologies in the process of energy harvesting conversion at ultra-low power supply voltage operation (sub-0.25 V). In this work, it is shown by simulations that a cross-coupled switched-capacitor topology with GaSb-InAs TFET devices present better conversion performance compared to the use of Si FinFET technology at low temperature variations (ΔT <; 3 °C) when considering a thermo-electric energy harvesting source (with α = 80 mV/K). At higher ΔT, the conversion process is degraded with the increase of the transistor losses. Considering a ΔT of 1 °C (2 °C), one cross-coupled stage with TFET devices can achieve 74 % (69 %) of power conversion efficiency when considering an output load of 0.4 μA (6 μA). At the same conditions, the FinFET charge pump is shown inefficient.

Tuning techniques for kinetic MEMS energy harvesters

The paper considers piezoelectric vibrating micro electromechanical system (MEMS) harvesters with variable effective spring stiffness. The structure of the harvesters is consists of proof mass suspended by an elastic beam. A viscous damper reflects the energy dissipation of the system. Piezoelectric layers, disposed on the maximum stress zone of the beam surface, are used as generators of electric energy. The stiffness of the beam is varied by capacitors, which are positioned at the maximum deflection zone of the beam. Different types of feedback circuits are used in order to investigate their influence on the resonant frequency of the system. Furthermore, two types of capacitors with transverse and lateral action are applied. According to the type of these capacitors and to the feedback circuits twelve different systems are considered. The dynamic models of those systems have been investigated in order to compare their tuning capability. The results, presented in the paper, could be useful for the designers of tunable kinetic MEMS harvesters.

Insights Into Tunnel FET-Based Charge Pumps and Rectifiers for Energy Harvesting Applications

In this paper, the electrical characteristics of tunnel field-effect transistor (TFET) devices are explored for energy harvesting front-end circuits with ultralow power consumption. Compared with conventional thermionic technologies, the improved electrical characteristics of TFET devices are expected to increase the power conversion efficiency of front-end charge pumps and rectifiers powered at sub-

Three-phase magnetic field system for wireless energy transfer

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