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     

Analysis of Caching and Replication Strategies for Web Applications

Developers often use replication and caching mechanisms to enhance Web application performance. The authors present a qualitative and quantitative analysis of state-of-the art replication and caching techniques used to host Web applications. Their analysis shows that selecting the best mechanism depends heavily on data workload and requires a careful review of the application's characteristics. They also propose a technique for Web practitioners to compare different mechanisms' performance on their own     

Non-uniform hydrogen attack cavitation and the role of interaction with creep

Hydrogen attack (HA) is the development of grain-boundary porosity by cavities filled with high-pressure methane that originates from the reaction of carbides with hydrogen at high temperatures. The cavities grow by grain-boundary diffusion and by creep of the adjacent grain material till they coalesce with neighbouring cavities to form a microcrack. Earlier work on HA has focussed on unit cells containing a single cavity, using average cavitation properties. Here, non-uniform cavitation properties on the grain-size scale are assumed in a polycrystalline aggregate, and unit cell analyses are performed to investigate the influence of the adjacent grains on the development of the grain-boundary HA. The numerical results are explained in terms of two simplified models which highlight the key parameters governing the grain deformation-grain boundary cavitation interaction process.     

Squamous cell carcinoma of the penis. III. Treatment of regional lymph nodes

We analyzed the management of regional lymph nodes in 110 patients with squamous cell carcinoma of the penis treated at the Netherlands Cancer Institute between 1956 and 1989 with curative intent. Of 66 patients who presented with unsuspected nodes 57 were placed on a surveillance program, while lymph node dissection was performed in 5 (with adjuvant external radiation therapy in 1) and 4 were treated with external radiation therapy only. The management of 40 patients with clinically suspected nodes included surveillance in 5, lymph node dissection in 27 (with adjuvant radiotherapy in 11), biopsy in 4 and external radiation therapy in 4. Postoperative radiotherapy had been given if more than 2 nodes were involved or when extracapsular growth was observed. Overall, 25 patients had a regional recurrence, 5 of whom could be cured subsequently. All regional recurrences developed within 2 years after primary treatment. Analysis showed 100% survival in histologically proved node negative patients (stage pN0). The success of lymph node dissection was related to the extent of the metastatic spread and to the number of involved nodes. Patients with 1 positive node and unilateral inguinal involvement showed a statistically significant survival advantage compared to patients with more extensive spread. Considering the indications for node dissection we found a clear relationship among T category, grade and the probability of lymph node invasion. Patients with stage T1 tumors and stage T2, grades 1 and 2 tumors presented significantly less often with lymphatic invasion than those with other categories of disease and were less likely to have a regional recurrence after treatment of the primary tumor only. In these categories we recommend surveillance of the regional lymph nodes in patients who present with unsuspected nodes. However, patients with stage T2 grade 3, stage T3 and operable stage T4 tumors should undergo an immediate inguinal node dissection because of the high probability of clinically occult lymph node invasion (in our material more than 50%). With respect to the extent of the node dissection, we found that the likelihood of spread to the contralateral and/or pelvic regions was related to the number of invaded nodes in the inguinal region. We recommend contralateral node dissection and unilateral pelvic node dissection when 2 or more positive nodes are found in the dissected groin specimen. Primary pelvic node dissection should be performed in patients who present initially with cytologically or biopsy proved positive inguinal nodes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Reliability analysis in wind turbine engineering

Wind turbines are being designed in accordance with deterministic design rules. As wind turbines increase in size and power, these rules may not be adequate to ascertain a safe and well balanced design. Wind turbine industry can take advantage of the safety and reliability practices in e.g. aerospace, nuclear and offshore industry. These practices include System Reliability (such as FMECA, Event Sequence Analysis and Fault Tree Analysis) and Structural Reliability methods. The approach to introduce Probabilistic Safety Assessment (PSA) which incorporates these reliability methods into wind turbine engineering will be described. By means of two recent case studies, the applicability, benefits, and limitations of these methods will be illustrated.     

Comparison of a polymethylmethacrylate and glass-ionomer bone cement using a hemiarthroplasty in the rabbit femur

Polymethylmethacrylate (PMMA) bone cements present various problems with respect to biocompatibility and stability. In order to study the histological changes at the bone-cement interface following total hip replacement, a small animal model was created by implanting hip prostheses using two different bone cements (PMMA and glass-ionomer cement, GIC). Two problems with the use of GIC for fixation of the prosthesis became evident. One year following implantation, histomorphometric analysis of femurs containing the GIC demonstrated significantly higher amounts of osteoid at the bone-cement interface. This disturbance of mineralization is comparable with osteomalacia and probably due to leaching of aluminum ions. In addition the mechanical properties of GIC proved to be inadequate for the loads placed upon it in this hip replacement model.     

Effects of quinine on the mechanical frequency response of the cupula in the fish lateral line

Quinine induces changes in the motion of the cupula in the lateral line canal of the African knife-fish in response to sinusoidal water movements. Two different phases in the action of quinine on the cupular frequency response can be discerned. In the first phase the best frequency, i.e., the frequency at which the cupular vibratory displacement is maximal in response to constant-amplitude sinusoidal canal fluid displacement, shifts toward higher frequencies. During this phase, lasting about 70-100 min, the best frequency increases by a factor between 1.3 and 1.5. In the second phase, during roughly the following 90 min, the best frequency decreases gradually to a value 0.3-0.5 times that observed before the application of quinine.     

Temporary results only in electroconvulsive therapy in therapy-resistant depression; retrospective study

OBJECTIVE: To evaluate the use and efficacy of electroconvulsive therapy (ECT) in refractory major depression according to DSM-III-R criteria, and to look for factors predicting response in the acute phase and the occurrence of relapse or recurrence after recovery. DESIGN: Retrospective. SETTING: University Hospital Rotterdam, The Netherlands. METHODS: Of all patients who received ECT between January 1988 and July 1993 data were collected by study of clinical records and of information by treating physicians after discharge. Every patient was visited once, or received an outpatient department appointment, to obtain informed consent, take a follow-up history and evaluate social functioning by scoring Global Assessment of Functioning and Sickness Impact Profile rating scales. RESULTS: 35 patients received ECT. In clinical practice, the guidelines of the Netherlands Psychiatric Association were not violated; most patients had received adequate pharmacological pretreatment before the decision to start ECT was made. Two patients died in hospital (not from ECT). In the acute phase 25 of the 33 patients still alive upon discharge showed good recovery. Seven of these suffered relapse within six months. The number of patients with a return of depressive symptoms rose to 12 by the end of the first year of follow-up. Sociodemographic variables and treatment characteristics did not appear to influence the result of treatment in the acute phase, nor the occurrence of relapse or recurrence. With less intensive pre- and post-ECT drug treatment the chances of relapse were increased. CONCLUSIONS: ECT is an effective treatment in the acute phase of a depression. Results after a longer period of follow-up are less satisfactory.