Guipeng Chen

Hypernatremia severity and the risk of death after traumatic brain injury

PURPOSE To investigate the relationship between severity of hypernatremia and the risk of death for patients with traumatic brain injury (TBI) who have been admitted to the neurosurgical intensive care unit (NICU). METHODS A total of 1044 patients with TBI were admitted to our NICU from January 2005 to January 2010. Of these patients, 881 were included in this study. Based on blood serum sodium level in the NICU the 881 patients were divided into four groups: 614 had normal serum sodium (Na<150mmol/L), 34 had mild hypernatremia (Na 150-<155mmol/L), 66 had moderate hypernatremia (Na 155-160mmol/L) and 167 had severe hypernatremia (Na≥160mmol/L). RESULTS The mortality rates for the mild, moderate, and severe hypernatremia groups were 20.6%, 42.4%, and 86.8%, respectively; the mortality rate for the normal group was 2.0%. In multivariable analysis, mild, moderate, and severe hypernatremia were independent risk factors for mortality; compared with the normal group the odds ratios of mild, moderate, and severe hypernatremia were 9.50, 4.34, and 29.35, respectively. CONCLUSIONS Severe hypernatremia is an independent risk factor with extremely high odds ratio for death in patients with TBI who are admitted to the NICU.

Integrated Multiple-Output Synchronous Buck Converter for Electric Vehicle Power Supply

This paper proposes a novel integrated synchronous buck converter for the auxiliary power supply system of electric vehicles, which achieves multiple independently regulated outputs with reduced switching components in comparison with the conventional separate buck converters. In order to obtain a better understanding of the proposed converter, operational principle and performance characteristics of a simplified dual-output buck converter are introduced in detail, as an example. The analysis shows that zero-voltage-switching operation and lower conduction losses could be attained. In addition, its dynamic behavior is similar to the conventional buck converter and thus the controller design is simple. Finally, experimental results based on a prototype circuit in which two inductors are integrated into one magnetic core to achieve further reduced cost are demonstrated to verify the advantages.

Topology Derivation and Generalized Analysis of Zero-Voltage-Switching Synchronous DC–DC Converters With Coupled Inductors

In conventional dc-dc converters with synchronous rectifiers, hard-switching operation of the main switch and reverse-recovery problem of the synchronous switch result in degraded performance. In order to avoid these two undesired shortcomings, employing a coupled inductor is a simple yet effective solution. Apart from the improved soft-switching characteristic, low component count is also achieved since only a coupled winding and a series auxiliary diode are added. In this paper, a general zero-voltage-switching (ZVS) topology with a coupled inductor is proposed and based on which, a family of ZVS synchronous buck, boost, and buck-boost converters is readily obtained. For each fundamental dc-dc converter, several viable ZVS topologies evolve, among which an optimum topology can be chosen according to the different requirement in practical engineering applications. To help the topology selection, generalized analysis and individual comparison are also undertaken. In addition, design consideration is illustrated and experiment results are provided to validate converter effectiveness. Furthermore, thanks to its generality, the topology derivation and generalized analysis can be easily extended to various synchronous dc-dc converters.

Relationship between serum sodium level and brain ventricle size after aneurysmal subarachnoid hemorrhage

OBJECTIVE To study the relationship of serum sodium levels and brain ventricle size after aneurysmal subarachnoid hemorrhage (SAH). METHODS Serum sodium levels and brain computed tomography (CT) scans were obtained simultaneously and within 21 days from onset of SAH in 69 patients. Serum sodium levels were compared with brain ventricle size on CT. The index of third ventricle was calculated from brain CT, and we studied its relationship to GOS (Glasgow Outcome Scale) scores. RESULTS There was obvious correlation between serum sodium levels and index of third ventricle (r = -0.753). GOS scores correlated with serum sodium levels in patients with hypernatremia. CONCLUSION There was a negative correlation between serum sodium levels and cerebral ventricle size in SAH patients. Hypernatremia is one factor leading an unfavorable prognosis in SAH patients.

A Family of Zero-Voltage-Switching Magnetic Coupling Nonisolated Bidirectional DC–DC Converters

This paper proposes a family of magnetic coupling nonisolated bidirectional dc–dc converters, which can achieve soft-switching operation in both power flow directions with a simple auxiliary circuit. Compared with conventional zero-voltage-transition converters, the additional resonant inductor is eliminated and magnetic core number is reduced. Therefore, both high efficiency and low cost are achieved. First, a general zero-voltage-switching (ZVS) magnetic coupling structure is proposed for a bidirectional buck/boost converter, based on which 13 different ZVS topologies with different connections of auxiliary circuit are derived. It is very beneficial in practical industrial applications because engineers can choose an optimal one according to converter performance characteristics, and the topology derivation methodology can be easily extended to other bidirectional dc–dc converters. Second, based on the general structure, operation principle and performance characteristics of all proposed topologies are simultaneously obtained, which can facilitate topology comparison and selection processes. Finally, in order to verify the effectiveness of theoretical analysis, design considerations and experiment results of a 500-W prototype circuit are demonstrated.

Enhanced Power Flow Control for Grid-Connected Droop-Controlled Inverters With Improved Stability

Droop control strategy is widely applied in ac microgrid to realize the seamless mode transition and achieve average power sharing among different inverters. However, there is a lack of analysis about the operation performance of droop control strategy in grid-connected mode. In this paper, grid fluctuation and control accuracy of reactive power are illustrated to be the main influence factors for power flow control according to the small-signal model. The feedforward of grid frequency and voltage magnitude is proposed to mitigate the impacts of grid fluctuation on power flow control. Since the feedforward items are achieved through the synchronous reference frame phase-locked loop with the input of filter capacitor voltage, extra sensors for grid voltage are avoided. Moreover, voltage magnitude control is introduced to enhance the control accuracy of reactive power flow. In addition, modified droop control loops are also proposed to improve the system stability. With the implementation of the proposed control scheme, improved power flow control and enhanced system stability are achieved with the feature of seamless mode transition maintained. Finally, the major contribution of this paper is verified by a laboratory prototype.

Integrated dual-output synchronous DC-DC buck converter

A novel integrated synchronous DC-DC Buck converter with dual-output is proposed. Compared to conventional strategy which employs two separate DC-DC buck converters to generate dual-output voltages, the number of switching elements is reduced in the proposed converter. Furthermore, lower current stresses could be attained for switches, contributing to reduced conduction loss. Moreover, the switching loss is decreased because zero-voltage-switching (ZVS) operation of one switch is achieved. In the paper, the operation principle and steady-state analysis of the proposed converter are illustrated in detail. Then, switches current stresses are analyzed in comparison with the conventional converter. The experimental results are demonstrated to verify the advantage of the proposed converter. Finally, topology extension is presented that multiple isolated and non-isolated outputs can be obtained simultaneously.

An optimized modulation method tor full-bridge/push-pull bi-directional DC-DC converter with wide-range ZVS and reduced spike voltage

An optimized modulation is proposed for the bidirectional DC-DC converter based on the full-bridge/push-pull circuit. With the proposed strategy, the zero-voltage-switching (ZVS) range of the lagging-leg switches in the conventional full-bridge topology can be extended. Moreover, severe voltage overshoots across main switches caused by the leakage inductance of the transformer in the current push-pull circuit are suppressed. Furthermore, the modulation is unified for both the buck mode and the boost mode, which is easy to realize compared with the conventional methods. In addition, no auxiliary circuits are employed, resulting in low cost and small volume. Detailed analysis and design of the proposed modulation are described. The simulation results validate the operation principle and a \kW prototype is built to show the effectiveness of the proposed strategy.

Fault-Tolerant Converter With a Modular Structure for HVDC Power Transmitting Applications

For the high-voltage direct-current (HVDC) power transmission system of offshore wind power, dc/dc converters are the potential solution to collect the power generated by off-shore wind farms to HVDC terminals. The converters operate with high-voltage gain, high efficiency, and fault tolerance over a wide range of operating conditions. In this paper, an isolated ultrahigh step-up dc/dc converter with a scalable modular structure is proposed for HVDC offshore wind power collection. A flyback-forward converter is employed as the power cell to form the expandable electrically isolated modular dc/dc converter. The duty ratio and phase-shift angle control are also developed for the proposed converter. Fault-tolerant characteristics of the converter are illustrated through the redundancy operation and fault-ride-through tests. Redundancy operation is designed to maintain high operation efficiency of the converters and fault-ride-through operation improves the converter reliability under harsh operating conditions. Analytical studies are carried out, and a 750-W prototype with three modular cells is built and experimentally tested to verify the performance of the proposed modular dc/dc converter.

A Fundamental Frequency Sorting Algorithm for Capacitor Voltage Balance of Modular Multilevel Converter With Low-Frequency Carrier Phase Shift Modulation

This paper proposes a fundamental frequency sorting algorithm for balancing the floating capacitors in modular multilevel converters with low-frequency carrier phase shift modulation. The relationship between the driving pulses and submodules (SMs) is rebuilt in every fundamental period by sorting the voltage increments and present voltages. Besides, an improved and simplified strategy is proposed to predict the charging abilities of the driving pulses, so as to avoid sorting the voltage increments. Based on the proposed method, the switching frequency of each power device and the carrier frequency are identical, which reduces the operational losses for applications regarding high power. At the same time, the sorting frequency is as low as the fundamental frequency, which alleviates a large amount of computational cost, and the necessity to measure arm currents is omitted. In this case, the arm current sensors are eliminated, and simplified communication among the central controller and the local ones is set up. Finally, a three-phase 1-MVA simulation platform with 120 SMs and a down-scaled 6-kVA experimental prototype with 48 SMs are constructed to validate the proposed approach.