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     

Nonparametric estimation from length-biased data under competing risks

A new model for cross-sectional lifetime data is presented. The model is based on the length-bias assumption, and it is adapted to situations in which several types of censoring may occur. The NPMLE of the survival function is derived. An EM-algorithm to approximate the NPMLE is devised. The performance of the introduced estimator is investigated through simulations. A real set of data collected as part of a study on unemployment duration in Spain is used for illustration purposes.     

Private practice: a choice, but what challenges!

Recent changes in the Canadian health care system are having an impact on the way health care services are delivered. As a result, opportunities are opening up for a whole new generation of autonomous and independent nurses who choose to work directly with clients in a private practice setting. In part one of this two-part article, the author describes the main responsibilities of the nurse entrepreneur. Stressed are the areas of competency and the entrepreneurial spirit required. Administrative, professional, legal and fiscal aspects of independent practice are also outlined. Part two of the article focuses on the ethical responsibilities of the nurse entrepreneur and the regulations concerning fees, publicity, insurance, and office and file management. The author recommends keeping current on new developments and establishing a process whereby clients can evaluate the quality of care they receive.     

Analysis of the effect of polypyrrole synthesis conditions on its capacity to reduce hexavalent chromium

This paper reports the capacity of polypyrrole to reduce toxic hexavalent chromium (Cr(VI)). The influence of using different electrolytes during the polymer's synthesis has been studied. To improve the reduction of Cr(VI), the parameters considered were the polypyrrole morphology and the amount of conducting polymer. Polypyrrole obtained by cyclic voltammetry at a low sweep rate using KBr as supporting electrolyte showed better performance for Cr(VI) reduction compared to coatings obtained by constant potential or cyclic voltammetry at high sweep rates.     

Mutagenicity of ambient aerosol collected in an urban and industrial area of The Netherlands

To investigate the influence of the densely populated and heavily industrialized Rijnmond area of The Netherlands on the genotoxicity of the ambient aerosol, aerosol samples were collected at locations within the area, and at a coastal region located upwind. The mutagenicity of extracts of the samples was compared in the Salmonella/microsome test. The dependence of the effects on sampling time and on sampling location was investigated with the aid of a series of simple mathematical models. These models were also used to estimate the increase in mutagenicity above background levels at the sites in the Rijnmond area due to emissions within that area. Application of the models showed that the clear and significant increases are not merely a result of the additions of mutagens emitted, but that possibly interactions between sampling time- and location-dependent factors play a role. Comparison of the results obtained with the different Ames-test variants (different strains, with and without liver homogenate) indicate that the conclusions concerning the time and location dependence of the effect were not dependent on the variant used.     

Structure of d(GT)n.d(GA)n sequences: formation of parallel stranded duplex DNA

Alternating polypurine sequences exhibit remarkable polymorphism. In this study, we report that dGA.dGT sequences form parallel stranded duplex DNA at neutral pH. Using two model hairpins, 3'-d(GT)3-5'5'-T4(AG)3-3' (I) and 3'-d(GT)4-5'5'-T4(AG)4-3' (II), containing 5'5' linkages which direct parallel strand formation, we systematically explored the spectroscopic and thermodynamic properties of parallel stranded d(GA)n.d(GT)n. The parallel stranded hairpins are remarkably stable structures with TM's of 41.5 and 47.5 degreesC (in 0.4 M NaCl) for the shorter and longer hairpins, respectively. The van't Hoff enthalpies of 80.7 and 114 kJ mol-1 are relatively low but are comparable to a parallel stranded d(GA)n duplex. On the basis of the spectroscopic and electrophoretic characteristics, we conclude that parallel strand formation is not restricted to hairpin systems, but also readily occurs in unconstrained dimeric duplexes with the appropriate sequence homologies. Both melting curves and electrophoretic analyses of parallel stranded heteroduplexes in which the sequence enforces specific base pairing demonstrate that G-G and A-T base pairs are formed in d(GA)n.d(GT)n segments.