Revisiting the subculture: understanding deviant students in China

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Emphysematous pyelonephritis: an eight-year retrospective review across four hospitals in a single cluster

香港泌尿外科學會Moderated Poster (Free Paper) Session II - Upper Tract and Robotic Surgery: MP.2-1OBJECTIVE: Emphysematous pyelonephritis (EPN) is a rare but life-threatening infection. We aim to review our cluster’s experience of managing this urologic emergency. PATIENTS & METHODS: Case notes of patients with EPN in four acute hospitals in the KWC (PMH, CMC, KWH and YCH) were retrospectively reviewed. The patients’ demographic data, clinical presentation, investigation findings, treatment and outcome were studied. OBJECTIVE: Emphysematous pyelonephritis (EPN) is a rare but life-threatening infection. We aim to review our cluster’s experience of managing this urologic emergency. PATIENTS & METHODS: Case notes of patients with EPN in four acute hospitals in the KWC (PMH, CMC, KWH and YCH) were retrospectively reviewed. The patients’ demographic data, clinical presentation, investigation findings, treatment and outcome were studied.published_or_final_versionThe 17th Annual Scientific Meeting of the Hong Kong Urological Association, Hong Kong, 6 November 2011. In Program Book, 2011, p. 6

Scaling of hysteresis dispersion in a model spin system

2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Low output ripple DC-DC converter based on an overlapping dual asymmetric half-bridge topology

A new converter topology is described for applications requiring very low output current ripple. The proposed converter consists of two asymmetric half-bridge converters whose output voltages overlap in a finite interval of time. This converter provides well regulated and smooth dc output without the need of an output filter. The output voltage is regulated by direct amplitude modulation. Unlike the standard interleaved converters, the proposed converter is robust to input voltage and operating duty cycle variations. Furthermore, equal current sharing is automatically achieved under all conditions, thus ensuring full utilisation of the output rectifiers for wide input and output ranges. The circuit achieves zero-voltage turnon for all primary switches and zero-current turnoff for the output rectifiers. An isolated dc-dc converter prototype with 5-V output voltage and 20-A output current has been built to verify the design. © 2007 IEEE.published_or_final_versio

Essential-coupling-path models for non-contact EMI in switching power converters using lumped circuit elements

This paper proposes a simple lumped circuit modeling approach for describing noncontact EMI coupling mechanisms in switching power converters. The resulting model assumes a minimum number of noise sources and contains essential coupling paths that allow easy physical interpretations. Essentially, all capacitive couplings are represented by an equivalent noise voltage source and six coupling impedances, whereas all inductive couplings are represented by an equivalent noise current source and three coupling impedances. The resulting coupled noise appears as currents flowing into the terminals of the Line-Impedance-Stabilization-Network (LISN). The equivalent voltage source can be conveniently approximated as the switching-node-to-zero voltage, which is typically a rectangular pulse of a few hundred volts. The equivalent current source can be modeled as the current flowing around a loop containing the equivalent voltage source and parasitics such as winding capacitance of the power transformer, the snubber capacitance and connection inductances. Also, the coupling impedances can be estimated by making simplifying assumptions about the geometry of the components and tracks, or by direct measurements. Simulations and experiments verify how inductive and capacitive couplings through each path may produce substantial EMI measured by the LISN. Being based on a lumped circuit approach, the proposed model is easy to apply in practice for understanding, diagnosing and approximating EMI behaviors.published_or_final_versio

Efficacious symmetry-adapted atomic displacement method for lattice dynamical studies

Small displacement methods have been successfully used to calculate the lattice dynamical properties of crystals. It involves displacing atoms by a small amount in order to calculate the induced forces on all atoms in a supercell for the computation of force constants. Even though these methods are widely in use, to our knowledge, there is no systematic discussion of optimal displacement directions from the crystal's symmetry point of view nor a rigorous error analysis of such methods. Based on the group theory and point group symmetry of a crystal, we propose displacement directions, with an equivalent concept of the group of $k$, deduced directly in the Cartesian coordinates rather than the usual fractional coordinates, that maintain the theoretical maximum for the triple product $V$ spanned by the three displacements to avoid possible severe roundoff errors. The proposed displacement directions are generated from a minimal set of irreducible atomic displacements that keep the required independent force calculations to a minimum. We find the error in the calculated force constant explicitly depends on the inverse of $V$ and inaccuracy of the forces. Test systems such as Si, graphene, and orthorhombic Sb2S3 are used to illustrate the method. Our displacement method is shown to be very robust in treating low-symmetry cells with a large `aspect ratio' due to huge differences in lattice parameters, use of a large vacuum height, or a very oblique unit cell due to unconventional choice of primitive lattice vectors. It is expected that our displacement strategy can be used to address higher-order interatomic interactions to achieve good accuracy and efficiency

On the relationship of quality factor and hollow winding structure of coreless printed spiral winding (CPSW) inductor

The principle of using hollow spiral winding is not novel, but the study on this topic is far from complete. In this paper, how hollow the central region of the coreless printed spiral winding (CPSW) inductor should be for a given footprint area in order to achieve the maximal quality factor Q max and to maintain high inductance value is explored. A hollow factor based on the ratio of the inner hollow radius and the outer winding radius τ = R in/R out, is proposed as for optimization and quantifying how hollow a spiral winding is. The relationship between τ and Q max, which depends on the operating frequency and the dimensional parameters of CPSW inductor, is established. For a specific operating frequency, it is discovered that if the conductor width is comparable with the skin depth, or the conductors are placed relatively far away from each others, the hollow design of the CPSW inductor has little improvement on Q but reduces the inductance. If the conductor width is much larger than the skin depth and the conductors are closely placed, the hollow spiral design is recommended. The optimal range of τ with which the Q max can be achieved is found to be around 0.45-0.55. © 2006 IEEE.published_or_final_versio

Synthesis of input-rectifierless AC-DC converters

This paper discusses the basic construction procedure and topological possibilities of creating ac/dc converters out of simple dc/dc conveners. It is shown that two separately controlled dc/dc conveners are sufficient for producing a regulated dc output and shaping the input current, from an ac voltage source, without the need for input rectifiers. Some design constraints are discussed, emanating from the limitation of the conversion ratios that can be achieved by particular dc/dc converters. Selected topologies are verified experimentally.published_or_final_versio

Synthesis of input-rectifierless AC/DC converters

This paper discusses the basic construction procedure and topological possibilities of creating ac/dc converters out of simple dc/dc converters. It is shown that two separately controlled dc/dc converters are sufficient for producing a regulated dc output and shaping the input current, from an ac voltage source, without the need for input rectifiers. Some design constraints are discussed, emanating from the limitation of the conversion ratios that can be achieved by particular dc/dc converters. Selected topologies are verified experimentally. This kind of rectifierless converters find applications in airborne power supplies where zero-crossing distortions are significant because of the inevitable phase-lead effect of the input rectifier bridge.published_or_final_versio

A PFC topology with low input current distortion suitable for aircraft power supplies

This paper presents an AC-DC converter topology for realization of PFC converters for applications where the mains frequency is high and a low input current harmonics is required, e.g., in aircraft power systems. The proposed topology eliminates crossover distortion due to the presence of input rectifiers and the inevitable input current phase lead. The idea is to stack a DC voltage on top of the AC mains voltage to prevent voltage polarity reversal in the input, eliminating the input bridge rectifier. This eliminates the crossover distortion, which can be significant for supply systems having a high mains frequency. In addition, the proposed PFC converter allows bi-directional energy flow, ensuring all inductors work in continuous conduction mode and hence eliminating the distortion due to the abrupt change of dynamic response when the operating mode changes. Analysis and design of the power and control circuits will be given and discussed. An experimental system will be presented for verification purposes.published_or_final_versio