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This diploma thesis deals with analysis of function and subsequent construction of a quasi-resonant DC / DC converter 300 V / 50 V for an output of about 1.5 kW. The aim of this work is to test and describe the behavior of an experimental converter at various operating parameters. In the theoretical part, resonant circuits are described, as well as our connection of the resonant converter. Based on the used topology and the simulated behavior of the converter, the individual components of the power circuit and its control and excitation circuit are designed in Chapters 4 and 5. The sixth chapter deals with the construction and testing of the converter, including a description of its behavior. The last chapter contains technical documentation.

In many multilingual communities around the world, speakers need to choose, often at an unconscious level, which language to use in their interactions with other members of the community. One of the choices that bilingual speakers often make is to code switch; that is, speakers switch back and forth between languages (or varieties of the same language), even within the same utterance. This article reviews the major theoretical approaches that have been proposed to answer the question of why bilingual speakers choose to code switch.

Published Article / The issue of language of learning and instruction in communities with diverse linguistic and cultural backgrounds, globally and in South Africa, is always shaped by socio-economical, political, ideological and hegemonic factors. The language of instruction and learning in South Africa is politically charged, not only because of colonial legacy, but more profoundly because of the apartheid legacy. This article reports on the results of the pedagogies of teachers teaching in English. The researcher found that most non-native English teachers use their native language pronunciation when teaching English, which results in mother tongue interference. On-native speakers end up transferring articulation habits of first language to second language. There is thus a need to improve quality in pedagogical practices in this diverse South African background since the language of instruction is different from the learners' first language for the majority of the population. The research in this article aims to investigate current pedagogic practices engaged by teachers from different cultural backgrounds when teaching English as a medium of instruction and how their practices affect learners in acquiring knowledge of the English language. The study uses an interpretive paradigm and in particular, adopts social constructivism to embed discussions. The study mainly implements a qualitative approach although the quantitative approach is used to quantify biographical data. The researcher found that two educational systems seem to exist in South African schools: some educators use English as a medium of instruction only, whereas others allow code switching. Education thus becomes a reproductive mechanism of social class differences.

International Telemetering Conference Proceedings / October 18-21, 2004 / Town & Country Resort, San Diego, California / FLL’s are extensively used for fast carrier synchronization. A common approach to meet the wide acquisition range and sufficiently small tracking error requirements is to adopt the wide or narrow band FLL loop in the acquisition and tracking modes and direct switching the loop. The paper analyze the influence of direct switching on performance, including the narrow band loop convergence, transition time etc. and propose applying the Kalman filtering theory to realize the seamless switching (SS) with time-varying loop gains between the two different loop tracking state. The SS control gains for the high dynamic digital spread spectrum receiver is derived. Simulation results for the SS compared to the direct switching demonstrate the improved performance.

The use of Insulated Gate Bipolar Transistors (IGBT) have enabled better switching performance than the Metal Oxide Semiconductor Field effect Transistor (MOSFET) in medium to high power applications due to their lower on-state power loss and higher current densities. The power ratings of IGBTs are slowly increasing and are envisaged to replace thyristors in medium power applications such as High Voltage Direct Current (HVDC) inverter systems and traction drive controls. Devices such as the MOS Controlled Thyristor (MCT) and Emitter Switched Thyristor (EST) were developed in an effort to further simplify drive requirements of thyristors by incorporating a voltage controlled MOS gate into the thyristor structure. However, the MCT is unable to achieve controlled current saturation which is a desirable characteristic of power switching devices while the EST has only limited control. The IGBT can achieve current saturation, however, due to the transistor based structure it exhibits a larger on-state voltage in high power applications compared with thyristor based devices. MOS Gated Thyristor (MGT) devices are a promising alternative to transistor based devices as they exhibit a lower forward voltage drop and improved current densities. This current research focuses on the Clustered Insulated Gate Bipolar Transistor (CIGBT) whilst being operated under soft-switching regimes. The CIGBT is a MOS gated thyristor device that exhibits a unique self-clamping feature that protects cathode cells from high anode voltages under all operating conditions. The self-clamping feature also enables current saturation at high gate biases and provides low switching losses. Its low on-state voltage and high voltage blocking capabilities make the CIGBT suitable as a contender to the IGBT in medium to high power switching applications. For the first time, the CIGBT has been operated under soft-switching regimes and transient over-voltages at turn-on have been witnessed which have been found to be associated with a number of factors. The internal dynamics of the CIGBT have been analysed using 2D numerical simulations and it has been shown that a major influence on the peak voltage is the P well spacing within the CIGBT structure. For example, Small adjacent P well spacings within the device results in an inability for the CIGBT to switch iv on correctly. Further to this, implant concentrations of the n well region during device fabrication can also affect the turn-on transients. Despite this, the CIGBT has been experimental analysed under soft-switching conditions and found to outperform the IGBT by 12% and 27% for on-state voltage drop and total energy losses respectively. Turn off current bumps have been seen whilst switching the device in zero voltage and zero current switching mode of operation and the internal dynamics have been analysed to show the influence upon the current at turn off. Preliminary results on the Trench CIGBT (TCIGBT) under soft switching conditions has also been analysed for the first time and was found to have a reduced peak over-voltage and better switching performance than the planer CIGBT. Through optimisation of the CIGBT structure and fabrication process, it is seen that the device will become a suitable replacement to IGBT in medium power application.

Although there have been a large number of studies on the use of L1 and L2, there seem to be few on L1 use in Chinese university EFL classrooms, especially investigating the language use of those who teach English to students at different proficiency levels or teach different types of English courses. This thesis aims to analyze four Chinese EFL teachers’ actual use of L1 and L2, to understand their attitudes and beliefs regarding this issue, and their own perceptions of and reasons for their language use, and to explore possible influencing factors. The reading-and-writing lessons and the listening-and-speaking lessons of these four teachers, who were teaching non-English major students at four different levels, were observed and recorded. All the observed lessons were subjected to quantitative analysis with the aim of providing a clear picture of the distribution of their L1 and L2 use. Some episodes selected from these lessons were subjected to further detailed analysis, in order to provide an account of the circumstances, functions, and grammatical patterns of their language use, as well as their language use across different frames of classroom discourse. The teachers were interviewed subsequently about their general beliefs on the use of L1 in L2 teaching and learning. Separately, in a stimulated recall interview, they were invited to provide comments specifically on their language use in the selected episodes that were replayed to them. The quantitative findings show that the amount of the teachers’ L1 use was not necessarily closely related to their students’ English proficiency levels, although the teacher of the students at the lowest level used the highest amount of Chinese in her lessons. However, a noteworthy finding was that all four teachers used more Chinese in the reading-and-writing lessons than in the listening-and-speaking lessons, although with substantial individual variation. The qualitative analysis of classroom data indicates that these teachers switched often at unit boundaries, but rarely at clause boundaries. They also switched frequently within units, especially within noun phrases, and the ‘Chinese determiner + English noun’ pattern is the main one they had in common. Furthermore, the teachers used Chinese as the matrix language in their mixed utterances in most cases, and these mixed utterances nearly always fitted Myers-Scotton’s Morpheme Order principle and System Morpheme principle. The teachers were also found to use Chinese in a variety of circumstances, such as talking about lesson plans or examinations, dealing with exercises, analyzing text, teaching vocabulary, checking the students’ comprehension or retention, giving the students advice on learning, telling anecdotes and assigning homework. The functions for which they used Chinese could be divided into four main categories: facilitating developing lesson content; supporting students and carrying out classroom management; delivering information related to teaching agenda or examinations; and facilitating communication beyond language learning and teaching. The most frequent function common to all four teachers was translation. Furthermore, the study used four different ‘frames’ to analyze classroom discourse, and found that the teachers used the L1 with varying frequency across these frames. Moreover, although all four teachers believed that using the L1 was beneficial to L2 learning, their attitudes towards the medium of instruction were different. While two advocated using the L1, the other two expressed a preference for speaking English-only and perceived their L1 use as a compromise or an expedient. The teachers reported many reasons for their L1 use. The factors that affected their language use consisted of both immediate classroom factors, such as functions of utterances, students’ language use, students’ perceived mood, students’ background knowledge, the difficulty of lesson content, time limitations, teachers’ awareness of their own L1 use, and teachers’ state of mind at a particular moment in a lesson, and relatively static factors, such as the university policy, students’ L2 abilities, teaching objectives, teachers’ beliefs regarding L1 use, and teachers’ L2 abilities. Through its detailed analysis of the teachers’ language use, as well as their relevant beliefs and decision-making, this thesis hopes to make a contribution to L2 teachers’ professional development and L2 teaching, especially in helping to establish a pedagogically principled approach to L1 and L2 use.

With advancing age, many cognitive faculties deteriorate, and navigation abilities may be among those most affected. The majority of previous work investigating navigation impairments in ageing has focused on allocentric processing, attributing deficits to hippocampal dysfunction. However, real-world navigation is dependent upon numerous different strategies, as well as the ability to flexibly switch between them. Outside the context of navigation, it has been demonstrated that strategy switching, thought to be coordinated by regions of prefrontal cortex and the locus coeruleus-noradrenergic system, is also susceptible to the effects of ageing. Deficits in navigational strategy switching, and prefrontal or noradrenergic dysfunction, are therefore also likely to contribute to age-related navigation impairments. The work presented in this thesis aimed to explore age-related impairments in strategy switching within the context of navigation, and the underlying neural mechanisms in terms of a prefrontal-noradrenergic model of switching. The studies presented in Chapter Three assessed the use of allocentric and egocentric navigational strategies by young and older people. Older participants tended to use an egocentric strategy where an allocentric strategy was required, possibly due to a difficulty in switching to the appropriate allocentric strategy. In Chapter Four, I provide an account of two studies directly assessing navigational strategy switching, using two different tasks based in virtual reality. The first study utilised a virtual adaptation of the plus maze task, involving switching between an allocentric place strategy and an egocentric response strategy, and demonstrated that older participants were specifically impaired at switching to the place strategy. The second study used a more realistic task set in a virtual town environment, which involved switching from an egocentric route-following strategy to an allocentric wayfinding strategy, and also demonstrated an age-related deficit in switching to an allocentric strategy. In Chapter Five, I begin to explore the mechanisms underlying impaired navigational strategy switching in ageing. Firstly, I describe a further behavioural study that used variants of the virtual plus maze and a navigational gambling task to demonstrate a contribution of impaired decision making to the deficit in switching to an allocentric strategy. This indicates that the deficit can be attributed, at least in part, to prefrontal dysfunction. A second study presented in the same chapter demonstrated that practising orienteering does not protect against decline in navigational strategy switching ability with ageing. Chapter Six provides an account of my direct assessment of the neural bases of navigational strategy switching using functional magnetic resonance imaging. In young subjects, I found some evidence in support of the roles of prefrontal regions in navigational strategy switching. However, I was unable to complete development of a task suitable for assessing age differences in functional activation of brain regions involved in navigational strategy switching. The final experimental study, included in Chapter Seven, assessed pupil size and heart rate as physiological correlates of noradrenergic activity during performance of the virtual plus maze. Both young and old participants demonstrated a noradrenergic response to all strategy changes, suggesting that impairments are more likely attributable to dysfunction of prefrontal cortex than of the locus coeruleus, although some subtle effects suggested that noradrenergic dysfunction does have some effect on navigational strategy switching deficits. In the same chapter, I report the results of a meta-analysis of data from five of the preceding studies, suggesting that deficits in both strategy switching and allocentric processing combine to produce a greater impairment in switching to an allocentric strategy. The main finding of this series of studies is that navigational strategy switching is impaired in ageing, which may contribute to the more widely reported difficulties that older people have with navigation. My work also provides evidence in support of a prefrontal-noradrenergic model of navigational strategy switching, and suggests that dysfunction of prefrontal cortex and, to a lesser extent, the locus coeruleus-noradrenergic system is responsible for decline in navigational strategy switching ability with ageing. In conclusion, this thesis draws attention to the important role of deficient executive processing and dysfunction of extra-hippocampal brain regions in age-related navigation impairments.

U.S.-based Institutions of Higher Education (IHE) face significant competitive forces including market-based pressures in the form of new and increased competition, an escalating cost base and reduced financial aid sources. The practice of customer loyalty development and the study of the loyalty phenomena are prevalent in the marketing literature. Despite the prevalence of studies focusing on relationship marketing concepts and loyalty as drivers of customer retention, little progress has been made in conceptualizing and testing frameworks that can explain the impact of relationship marketing on attitudinal and behavioral loyalty in an IHE setting. This study proposed a relationship marketing approach towards the challenge of increasing student loyalty and thus ultimately student retention. Both offensive (relationship marketing concepts aimed at increasing trust, commitment and perceived value) and defensive (subjective norms and switching costs) strategies were proposed as antecedents of student loyalty. Several existing marketing and organizational behavior scales were modified for use in an IHE environment. Research into the dynamics of relationship marketing in a real-world setting was conducted to (1) confirm the scales for use in the IHE environment , (2) to broaden the settings in which the selected scales have been deployed, and (3) to offer depth to the relationship marketing and loyalty field of study. A survey was administered in three South Florida IHEs and 549 responses were collected. The use of structural equation modeling permitted the researcher to establish the measurement validity of the relationship marketing survey instrument and the statistical reliability and validity of the of the proposed variables. There were significant relationships noted between: trust and perceived value; perceived value and commitment; commitment and loyalty; front line employee behaviors and trust; and management policies and procedures and trust; confirming past studies. However, counter to past research, results showed no significant relationship between subjective norms and commitment. Some of the limitations from previous research were addressed by proposing a more integrated model that combined both offensive and defensive marketing constructs. Future research is needed to determine the causes for the lack of statistical significance for the proposed defensive marketing strategies.

Independent metering poses a possibility to improve energy efficiency of throttlecontrolled hydraulic single-rod cylinder drives. This paper deals with energetic potentials gained through variable circuitry that come along with independent metering. A method to assess energetic potentials is described, based on load specific, optimal operating modes. As a means of yielding maximum energy efficiency for a wide range of applications, a smooth mode switching algorithm that minimizes losses and allows good motion tracking is proposed. The mode switching algorithm is validated in simulation and on a test stand.