1,720,993 research outputs found
Dynamic RON Characterization Technique for the Evaluation of Thermal and Off-State Voltage Stress of GaN Switches
Full text linkGaN power switches provide remarkable performance in terms of power-density, reduced parasitics, and high-thermal handling capability that enable the realization of very efficient and compact dc/dc converters. Despite exhibiting state-of-the-art channel conductivity, GaN high electron mobility transistor (HEMT) devices are affected by the degradation of the dynamic on-Resistance (RON) at increasing off-state voltages and operative temperatures. In this paper, a novel laboratory setup and characterization procedure for the dynamic RON of GaN HEMT switches in the presence of thermal- and trapping-effects is presented. The proposed setup allows the study of RON transients after the switching event at variable off-state voltages and temperatures. The use of custom-designed differential amplification stages and a voltage-controlled current source enables the accurate characterization of RON even on large periphery devices. At first, the proposed setup is tested with a well-established and mature device technology such as a Si MOSFET. Degradation of the RON up to 120% due to temperature variation is observed with the presented setup. The setup is then used for the characterization of commercial-grade GaN-on-SiC and GaN-on-Si HEMTs. For both technologies dynamic RON degradations up to 75% and 20% are observed for temperature and off-state voltage variations, respectively. These characterization data are fundamental for the accurate estimation of conduction losses during the design of switching-mode power converters
Efficient power amplifiers for amplitude-tapered pulses with improved spectral confinement
No full textThis paper reviews various techniques that allow efficient amplification of amplitude-modulated radar pulses through some type of supply modulation. Examples at S and X-band with GaN PAs will be shown with efficiencies above 50% for 10-W transmitters and greatly reduced spectral sidelobes
X-Band GaN Multi-Level Chireix Outphasing PA with a Discrete Supply Modulator MMIC
No full textThis paper presents the characterization of a multi-level Chireix outphasing PA with a GaN discrete supply modulator MMIC at 9.7GHz. The internal PAs include class-F harmonic terminations, while the Chireix combiner determines the fundamental frequency load modulation. A power-DAC architecture provides 8 supply levels with 3 half-bridges, maintaining more than 85% efficiency. The combination of the two achieves a peak output power of 4.8W, and average total efficiencies for a 6dB PAR QPSK signal of 44.1% and 48.1% with and without considering the supply modulator consumption, respectively, demonstrating the feasibility of this PA architecture with a real discrete supply modulator
A novel sensor to continuously monitor cardiac apex rotation
No full textThe left ventricular (LV) rotation is a key component of cardiac mechanics as recently shown by advanced imaging techniques. In healthy subjects, the cardiac rotation follows a specific pattern within the cardiac cycle in order to be effective: during isovolumic contraction all short-axis levels, from base to apex, rotate counterclockwise, when viewed from the apex, during ejection, the apex continues counterclockwise while the base reverses direction; thus cardiac apex rotation angle (CAR) reaches the maximum at End of Systole (ES). In diastole, the shearing forces built up during systole by cardiac rotation result in rapid untwisting which is thought to contribute to diastolic suction. Alterations in the normal pattern or magnitude of the cardiac rotation have been associated with cardiovascular diseases, such as chronic heart failure, tachycardia-induced dilated cardiomyopathy, dilated cardiomyopathy, hypertrophic cardiomiopathy, myocardial infarction and aortic stenosis. The analysis of CAR signal with respect to systolic and diastolic phases could be useful for the monitoring of cardiac function in patients with various degrees of heart failure as there are often delay and/or dyssynchrony phenomena which considerably reduce the overall mechanical efficiency of the heart. If an altered rotational dynamics determines a delay of maximum CAR into the Isovolumic Relaxation Period (IRP), the effective contribution of left ventricular twisting to systole must be considered the CAR value at the time of ES (CARES).
Continuous monitoring of LV rotation has not been attempted so far but may provide important information for long term management of heart failure patients.
We propose the use of an implantable gyroscopic sensor for the continuous monitoring of CAR. We evaluated in animals the CAR signal with respect to the systolic timing (CARES) and we tested its ability to reflect changes of LV function during acute ischemia
Efficient Programmable Pulse Shaping for X-Band GaN MMIC Radar Power Amplifiers
Full text linkThis paper presents a supply modulated X-band 12-W peak power transmitter that maintains an average efficiency greater than 50% for various shapes of amplitude-modulated pulses. The main power amplifier is a two-stage GaN-on-SiC MMIC with a peak efficiency of 65%, while the pulse envelope modulator is a 95% efficient hybrid 3-b power DAC implemented with GaN-on-Si transistor switches. Envelope shaping of a pulsed waveform results in improved spectral confinement of greater than 15 dB for the first sideband compared with constant-envelope pulses, with over 20 points improvement in total efficiency. The combination of supply modulation and digital predistortion is shown to result in high composite (total) efficiency of over 55%, with simultaneous high dynamic range and with flexible digitally programmable pulse shaping
Envelope Tracking of an RF High Power Amplifier With an 8-Level Digitally Controlled GaN-on-Si Supply Modulator
Full text linkThis paper presents an envelope tracking (ET) transmitter architecture based on the combination of a novel 3-bit (N = 3) supply modulator and digital predistortion (DPD). The proposed power converter is based on a direct digital-to-analog conversion architecture that implements the binary-coded sum of N isolated dc voltages, allowing the synthesis of an output waveform with L = 2N voltage levels, with a binary distribution in the range Δ V = VM-VO (maximum voltage VM, offset voltage Vo). This solution provides a better voltage resolution VS = Δ V/(2N-1) with respect to typical multilevel switched-sources topologies (VS = Δ V/N). The improved voltage resolution enables the correction of the residual discretization error in the ET transmitter by means of DPD of the RF signal without the need of an auxiliary linear envelope amplifier. The proposed ET solution has been tested with an L-band 30-W lateral-diffused MOS RF high power amplifier (RF HPA) with 1.4- and 10-MHz long-term-evolution signals. In these conditions the converter demonstrated 92% and 83% efficiency, respectively, whereas the congregate efficiency of the transmitter are 38.3% and 23.9% at 5.5 and 1.9 W of average RF output power, respectively. These performances correspond to an improvement of 17.2 and 17.9 points for the power-added efficiency of the RF HPA and to 13.4 and 13 points of improvement for the efficiency of the entire transmitter with respect to fixed bias operation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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