A Thermal Management Solution for Compact Power Converters

Regulated Transformer Rectifier Units contain several power electronic boards to facilitate AC to DC power conversion. As these units become smaller, the number of devices on each board increases while their distance from each other decreases, making active cooling essential to maintaining reliable operation. Although it is widely accepted that liquid is a far superior heat transfer medium to air, the latter is still capable of yielding low device operating temperatures with proper heat sink and airflow design. The purpose of this study is to describe the models and methods used to design and build the thermal management system for one of the power electronic boards in a compact, high power regulated transformer rectifier unit. Maximum device temperature, available pressure drop and manufacturability were assessed when selecting the final design for testing. Once constructed, the thermal management system’s performance was experimentally verified at three different power levels.

UV and visible photoconductivity of undoped diamond films: morphology and related electrical transport phenomena

UV and visible photoconductivity and electrical features of undoped diamond thin films grown by microwave plasma-assisted chemical vapour deposition (MP-CVD) on silicon and copper substrates are studied. The results are correlated with morphology properties analysed by atomic force microscopy (AFM) and micro-Raman. The photoconductivity presents several bands from 1.8 to 3.8 eV that are dependent on the substrate used to grow the samples in spite of some common bands observed. The J-V curve tin DC) in samples grown on Si has a rectifier behaviour (Schottky emission) in opposition to the samples grown on Cu that have no rectification (SCLC conduction). With these results we can conclude that diamond based optoelectronic devices behaviour is controlled by two kinds of structural defects localized in microcrystal and in its boundaries. A general structure model for the optoelectronic behaviour is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

Electrical AC behaviour of MPCVD diamond Schottky diodes

The present work reports some experimental results on the electrical AC behaviour of metal-undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 mum thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs, frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole-Cole plot. Taking into account the existence of traps in the depletion region, a best fit to the experimental results was obtained. The difference between the Fermi level and the band edge of 0.2-0.3 eV is in agreement with the activation energy found from the loss tangent analysis. The capacitance with applied voltage (Mott-Schottky plots) gives a defect density of 10(16) cm(-3) with contact potentials near 0.5 V and the profile of defect density obtained shows a major density (approx. 10(17) cm(-3)) in a layer with a thickness less than 50 nm from the junction, decreasing by one order of magnitude with increasing distance. Finally a structural model is proposed to explain the AC behaviour found. (C) 2001 Elsevier Science B.V. All rights reserved.

Positive biocompatibility of nanocrystalline glass-like carbon thin films

The interest in carbon nanomaterials with high transparency and electrical conductivity has grown within the last decade in view of a wide variety of applications, including biocompatible sensors, diagnostic devices and bioelectronic implants. The aim of this work is to test the biocompatibility of particular nanometer-thin nanocrystalline glass-like carbon films (NGLC), a disordered structure of graphene flakes joined by carbon matrix (Romero et al., 2016). We used a cell line (SN4741) from substantia nigra dopaminergic cells derived from transgenic mouse embryo cells (Son et al., 1999). Some cells were cultured on top of NGLC films (5, 20 and 80 nm) and other with NGLC nanoflakes (approx. 5-10 mm2) in increasing concentrations: 1, 5, 10, 20 and 50 μg/ml, during 24 h, 3 days and 7 days. Cells growing in normal conditions were defined under culture with DMEM supplemented with 10% FCS, Glucose (0,6%), penicillin-streptomycin (50U/ml) and L-glutamine (2mM) at 5%CO2 humidified atmosphere. Nanoflakes were resuspended in DMEM at the stock concentration (2 g/l). The experiments were conducted in 96 well plates (Corning) using 2500 cells per well. For MTT analysis, the manufacturer recommendations were followed (Roche, MTT kit assay): a positive control with a 10% Triton X-100 treatments (15 minutes) and a negative control without neither Triton X-100 nor NGLC. As apoptosis/necrosis assay we used LIVE/DEAD® Viability/Cytotoxicity Assay Kit (Invitrogen). In a separate experiment, cells were cultured on top of the NGLC films for 7 days. Primary antibodies: anti-synaptophysin (SYP, clone SY38, Chemicon) and goat anti-GIRK2 (G-protein-regulated inward-rectifier potassium channel 2 protein) (Abcom) following protocol for immunofluorescence. WB for proteins detection performed with a polyclonal anti-rabbit proliferating cell nuclear antigen (PCNA). Results demonstrated the biocompatibility with different concentration of NGLC varying the degree of survival from a low concentration (1 mg/ml) in the first 24 h to high concentrations (20-50 g/ml) after 7 days as it is corroborated by the PCNA analysis. Cells cultured on top of the film showed after 7 days axonal-like alignment and edge orientation as well as net-like images. Neuronal functionality was demonstrated to a certain extent through the analysis of coexistence between SYP and GIRK2. In conclusion, this nanomaterial could offer a powerful platform for biomedical applications such as neural tissue engineering

High Frequency Class DE Converter Using a Multilayer Coreless PCB Transformer

In modern power electronics equipment, it is desirable to design a low profile, high power density, and fast dynamic response converter. Increases in switching frequency reduce the size of the passive components such as transformers, inductors, and capacitors which results in compact size and less requirement for the energy storage. In addition, the fast dynamic response can be achieved by operating at high frequency. However, achieving high frequency operation while keeping the efficiency high, requires new advanced devices, higher performance magnetic components, and new circuit topology. These are required to absorb and utilize the parasitic components and also to mitigate the frequency dependent losses including switching loss, gating loss, and magnetic loss. Required performance improvements can be achieved through the use of Radio Frequency (RF) design techniques. To reduce switching losses, resonant converter topologies like resonant RF amplifiers (inverters) combined with a rectifier are the effective solution to maintain high efficiency at high switching frequencies through using the techniques such as device parasitic absorption, Zero Voltage Switching (ZVS), Zero Current Switching (ZCS), and a resonant gating. Gallium Nitride (GaN) device technologies are being broadly used in RF amplifiers due to their lower on- resistance and device capacitances compared with silicon (Si) devices. Therefore, this kind of semiconductor is well suited for high frequency power converters. The major problems involved with high frequency magnetics are skin and proximity effects, increased core and copper losses, unbalanced magnetic flux distribution generating localized hot spots, and reduced coupling coefficient. In order to eliminate the magnetic core losses which play a crucial role at higher operating frequencies, a coreless PCB transformer can be used. Compared to the conventional wire-wound transformer, a planar PCB transformer in which the windings are laid on the Printed Board Circuit (PCB) has a low profile structure, excellent thermal characteristics, and ease of manufacturing. Therefore, the work in this thesis demonstrates the design and analysis of an isolated low profile class DE resonant converter operating at 10 MHz switching frequency with a nominal output of 150 W. The power stage consists of a class DE inverter using GaN devices along with a sinusoidal gate drive circuit on the primary side and a class DE rectifier on the secondary side. For obtaining the stringent height converter, isolation is provided by a 10-layered coreless PCB transformer of 1:20 turn’s ratio. It is designed and optimized using 3D Finite Element Method (FEM) tools and radio frequency (RF) circuit design software. Simulation and experimental results are presented for a 10-layered coreless PCB transformer operating in 10 MHz.

A NEW DC UPS FOR DC POWER DISTRIBUTION SYSTEM IN DATA CENTER

In recent years, the 380V DC and 48V DC distribution systems have been extensively studied for the latest data centers. It is widely believed that the 380V DC system is a very promising candidate because of its lower cable cost compared to the 48V DC system. However, previous studies have not adequately addressed the low reliability issue with the 380V DC systems due to large amount of series connected batteries. In this thesis, a quantitative comparison for the two systems has been presented in terms of efficiency, reliability and cost. A new multi-port DC UPS with both high voltage output and low voltage output is proposed. When utility ac is available, it delivers power to the load through its high voltage output and charges the battery through its low voltage output. When utility ac is off, it boosts the low battery voltage and delivers power to the load form the battery. Thus, the advantages of both systems are combined and the disadvantages of them are avoided. High efficiency is also achieved as only one converter is working in either situation. Details about the design and analysis of the new UPS are presented. For the main AC-DC part of the new UPS, a novel bridgeless three-level single-stage AC-DC converter is proposed. It eliminates the auxiliary circuit for balancing the capacitor voltages and the two bridge rectifier diodes in previous topology. Zero voltage switching, high power factor, and low component stresses are achieved with this topology. Compared to previous topologies, the proposed converter has a lower cost, higher reliability, and higher efficiency. The steady state operation of the converter is analyzed and a decoupled model is proposed for the converter. For the battery side converter as a part of the new UPS, a ZVS bidirectional DC-DC converter based on self-sustained oscillation control is proposed. Frequency control is used to ensure the ZVS operation of all four switches and phase shift control is employed to regulate the converter output power. Detailed analysis of the steady state operation and design of the converter are presented. Theoretical, simulation, and experimental results are presented to verify the effectiveness of the proposed concepts.

Simulation of Malfunctions in a Wind System Powered by a DFIG

The paper is about the simulation of malfunctions in an onshore wind energy conversion system powered by a doubly fed induction generator with a two-level power converter, handling only the slip power. These malfunctions are analysed in order to be able to investigate the impact in the wind power system behaviour by comparison before, during and after the malfunctions. The malfunctions considered in the simulation includes are localized in the DC-link of the converter and in the phase change in rectifier.

Characterization of a 3D-printed low-cost flexible dielectric material for the realization of innovative WPT wearable applications

The aim of this thesis is to demonstrate that 3D-printing technologies can be considered significantly attractive in the production of microwave devices and in the antenna design, with the intention of making them lightweight, cheaper, and easily integrable for the production of wireless, battery-free, and wearable devices for vital signals monitoring. In this work, a new 3D-printable, low-cost resin material, the Flexible80A, is proposed as RF substrate in the implementation of a rectifying antenna (rectenna) operating at 2.45 GHz for wireless power transfer. A careful and accurate electromagnetic characterization of the abovementioned material, revealing it to be a very lossy substrate, has paved the way for the investigation of innovative transmission line and antenna layouts, as well as etching techniques, possible thanks to the design freedom enabled by 3D-printing technologies with the aim of improving the wave propagation performance within lossy materials. This analysis is crucial in the design process of a patch antenna, meant to be successively connected to the rectifier. In fact, many different patch antenna layouts are explored varying the antenna dimensions, the substrate etchings shape and position, the feeding line technology, and the operating frequency. Before dealing with the rectification stage of the rectenna design, the hot and long-discussed topic of the equivalent receiving antenna circuit representation is addressed, providing an overview of the interpretation of different authors about the issue, and the position that has been adopted in this thesis. Furthermore, two rectenna designs are proposed and simulated with the aim of minimizing the dielectric losses. Finally, a prototype of a rectenna with the antenna conjugate matched to the rectifier, operating at 2.45 GHz, has been fabricated with adhesive copper on a substrate sample of Flexible80A and measured, in order to validate the simulated results.