浅谈开关电源中的平面变压器

与传统变压器相比较,使用平面变压器可以提高开关电源的功率密度,减小开关电源中磁性元件的体积。因此,近年来平面变压器被广泛地研究。文章介绍了平面变压器的结构,将平面变压器与传统变压器进行比较,分析综述了平面变压器在当今开关电源发展应用的优势和前景,并进一步说明了平面变压器中绕组间距选择,绕组交叉技术及PCB绕组设计等要点。     

Optimal shape design for packaging containing heating elements by inverse heat transfer method

The present study is concerned with the determination of the optimal shape for a package containing multiple heating elements. The optimization tool has been developed based on the inverse heat transfer (IHT) approach, incorporating a direct problem solver, a numerical grid generator, a direct-differentiation sensitivity analyzer, and the conjugate gradient method. Shape design that leads to a specified outer surface temperature distribution is predicted by the approach. In this study, the effects of internal heat generation on optimal shapes of the packagings have also been evaluated. Several practical cases with various imbedded heating elements and thermal conditions are studied. Results show that the approach provides an efficient computer-aided design scheme for the shape profile determination.     

用电泳技术制备YBCO高温超导厚膜

介绍了电泳技术制备YBCO高温超导厚膜的实验方法和YBCO高温超导厚膜的电学性质测量 ,讨论了在学生小型科研实验或设计实验中开展此实验的学时安排、注意事项和实验内容的扩展 .     

全光网中基于FBG的OADM设计研究

提出一种全新的OADM(Optical Add—Drop Multiplexers)结构设计，此结构仅采用一个多端口环行器，大大减少了环行器的数量，节约了成本。若要上／下路一个波长．则只需要一个FBG(Fiber Bragg Grating)；若要同时上／下路多个波长，则需要2m个FBG。计算机仿真实验证明此结构比H．Yuan提出的结构具有更低的损耗．对全光网络的实现具有重大的意义。     

Experimental Evaluation of Strain Shielding in Distal Femur in Revision TKA

Theoretical concerns about the use of cemented and press-fit stems in revision total knee arthroplasty (TKA) include stress shielding with adverse effects on prosthesis fixation. Radiological studies have showed distal femoral bone resorption after revision TKA. The revision with use of stems can place abnormal stresses. These stresses can promote the effect of bone stress shielding and may contribute to bone loss. Experimental quantification of strain shielding in the distal synthetic femur following TKA is the main purpose of the present study. Three different constructs of TKA were assessed. The first construct included a stemless femoral component. The other two included a press-fit and a cemented femoral stem. Cortical bone strains were measured experimentally with tri-axial strain gauges in synthetic femurs before and after in-vitro knee surgery. The difference between principal strains of implanted and intact femur was calculated for each strain gauge position. This study indicates that the use of stems in distal femur changes the distribution and magnitude of bone strains. The press-fit stem provoked relevant bone area (stem length) subjected to strain shielding and also originated the highest reduction of strains in the distal region, which can potentially induce bone resorption. The stemless implanted femur produced minor bone strain changes relatively to the intact femur. The use of distal femur stems increases initial stability in the bone, but the observed reduction of strains in this region, relative to the intact femur, provokes strain shielding that can induce bone resorption and may compromise the long term implant stability.     

Effect of Pressure on Fluid Damping in MEMS Torsional Resonators with Flow Ranging from Continuum to Molecular Regime

High quality factor of dynamic structures at micro and nano scale is exploited in various applications of micro electro-mechanical systems (MEMS) and nano electro-mechanical system. The quality factor of such devices can be very high in vacuum. However, when vacuum is not desirable or not possible, the tiny structures must vibrate in air or some other gas at pressure levels that may vary from atmospheric to low vacuum. The interaction of the surrounding fluid with the vibrating structure leads to dissipation, thus bringing down the quality factor. Depending on the ambient fluid pressure or the gap between the vibrating and the fixed structure, the fluid motion can range from continuum flow to molecular flow giving a wide range of dissipation. The relevant fluid flow characteristics are determined by the Knudsen number which is the ratio of the mean free path of the gas molecule to the characteristic flow length of the device. This number is very small for continuum flow and reasonably big for molecular flow. In this paper, we study the effect of fluid pressure on the quality factor by carrying out experiments on a MEMS device that consists of a double gimbaled torsional resonator. Such devices are commonly used in optical cross-connects and switches. We only vary fluid pressure to make the Knudsen number go through the entire range of continuum flow, slip flow, transition flow, and molecular flow. We experimentally determine the quality factor of the torsional resonator at different air pressures ranging from 760 Torr to 0.001 Torr. The variation of this pressure over six orders of magnitude ensures required rarefaction to range over all flow conditions. Finally, we get the variation of quality factor with pressure. The result indicates that the quality factor, Q, follows a power law, Q ∝P ^–r , with different values of the exponent r in different flow regimes. In the second part of the paper, we propose the use of effective viscosity for considering velocity slip conditions in solving Navier–Stokes equation numerically. This concept is validated with analytical results for a simple case and then compared with the experimental results presented in this paper. The study shows that the effective viscosity concept can be used effectively even for the molecular regime if the air-gap to length ratio is sufficiently small (h ₀/L<0.01). As this ratio increases, the range of validity decreases.     

宁波城市设计的若干思考与建议

通过探讨城市设计概念的形成过程和历史积淀，阐明其在城市建设中的意义，并运用城市设计的理论与分析方法，针对目前宁波城市大规模的规划、设计、改造现象，提出若干思考与建议．     

Information and asymptotic efficiency of the case-cohort sampling design in Cox's regression model

Efficiencies of the maximum pseudolikelihood estimator and a number of related estimators for the case-cohort sampling design in the proportional hazards regression model are studied. The asymptotic information and lower bound for estimating the parametric regression parameter are calculated based on the effective score, which is obtained by determining the component of the parametric score orthogonal to the space generated by the infinite-dimensional nuisance parameter. The asymptotic distributions of the maximum pseudolikelihood and related estimators in an i.i.d. setting show that these estimators do not achieve the computed asymptotic lower bound. Simple guidelines are provided to determine in which instances such estimators are close enough to efficient for practical purposes.     

概率罐子模型中一种最优设计

基于错误概率达到最小的原则，该文在罐子模型序贯试验中构造了一种渐近最优设计。在这种设计下，不仅能使病员以较多机会分配到较好的处理，而且能使估计量的统计效率在一定意义下达到最优。