446 research outputs found
Development of Efficient Soft Switching Synchronous Buck Converter Topologies for Low Voltage High Current Applications
Switched mode power supplies (SMPS) have emerged as the popular candidate in all the power processing applications. The demand is soaring to design high power density converters. For reducing the size, weight, it is imperative to channelize the power at high switching frequency. High switching frequency converters insist upon soft switching techniques to curtail the switching losses. Several soft switching topologies have been evolved in the recent years. Nowadays, the soft switching converters are vastly applied modules and the demand is increasing for high power density and high efficiency modules by minimizing the conduction and switching losses. These modules are generally observed in many applications such as laptops, desktop processors for the enhancement of the battery life time. Apart from these applications, solar and spacecraft applications demand is increasing progressively for stressless and more efficient modules for maximizing the storage capacity which inturn enhances the power density that improves the battery life to supply in the uneven times. Modern trends in the consumer electronic market focus increases in the demand of lower voltage supplies. Conduction losses are significantly reduced by synchronous rectifiers i.e., MOSFET’s are essentially used in many of the low voltage power supplies. Active and passive auxiliary circuits are used in tandem with synchronous rectifier to diminish the crucial loss i.e., switching loss and also it minimizes the voltage and current stresses of the semiconductor devices. The rapid progress in the technology and emerging portable applications poses serious challenges to power supply design engineers for an efficient power converter design at high power density. The primary aim is to design and develop high efficiency, high power density topologies like: buck, synchronous buck and multiphase buck converters with the integration of soft switching techniques to minimize conduction and switching losses sustaining the voltage and current stresses within the tolerable range. In this work, two ZVT-ZCT PWM synchronous buck converters are introduced, one with active auxiliary circuit and the other one with passive auxiliary circuit. The operating principle and comprehensive steady state analysis of the ZVT-ZCT PWM synchronous buck converters are presented. The converters are designed to have high efficiency and low voltage that is suitable for high power density application. The semiconductor devices used in the topologies in addition to the main switch operate with soft switching conditions. The viii Abstract topologies proposed render a large overall efficiency in contrast to the contemporary topologies. In addition the circuit’s size is less, reliable and have high performance-cost ratio. The new generation microprocessor demands the features such as low voltage, high current, high power density and high efficiency etc., in the design of power supplies. The supply voltage for the future generation microprocessors must be low, in order to decrease the power consumption. The voltage levels are dripping to a level even less than 0.7V, and the power consumption increases as there is an increase in the current requirement for the processor. In order to meet the demands of the new generation microprocessor power supply, a soft switching multiphase PWM synchronous buck converter is proposed. The losses in the proposed topology due to increasing components are pared down by the proposed soft switching technique. The proposed converters in this research work are precisely described by the mathematical modelling and their operational modes. The practicality of the proposed converters for different applications is authenticated by their simulation and experimental results
Synthesis and superconductivity of new BiS2 based superconductor PrO0.5F0.5BiS2
We report synthesis and superconductivity at 3.7K in PrO0.5F0.5BiS2. The
newly discovered material belongs to the layered sulfide based REO0.5F0.5BiS2
compounds having ZrCuSiAs type structure. The bulk polycrystalline compound is
synthesized by vacuum encapsulation technique at 7800C in single step. Detailed
structural analysis has shown that the as synthesized PrO0.5F0.5BiS2 is
crystallized in tetragonal P4/nmm space group with lattice parameters a =
4.015(5) {\AA}, c = 13.362(4) {\AA}. Bulk superconductivity is observed in
PrO0.5F0.5BiS2 below 4K from magnetic and transport measurements. Electrical
transport measurements showed superconducting transition temperature (Tc) onset
at 3.7K and Tc ({\rho}=0) at 3.1K. Hump at Tc related to superconducting
transition is not observed in heat capacity measurement and rather a
Schottky-type anomaly is observed at below ~6K. The compound is slightly
semiconducting in normal state. Isothermal magnetization (MH) exhibited typical
type II behavior with lower critical field (Hc1) of around 8Oe.Comment: Short note 10 pages text+figs. First report on PrO.5F.5BiS2 Su
Superconductivity at 5K in NdO0.5F0.5BiS2
We report appearance of superconductivity at 5K in NdO0.5F0.5BiS2 and
supplement the discovery [1] of the same in layered sulfide based ZrCuSiAs type
compounds. The bulk polycrystalline compound is synthesized by conventional
solid state route via vacuum encapsulation technique. Detailed structural
analysis showed that the studied compound is crystallized in tetragonal P4/nmm
space group with lattice parameters a = 3.9911(3) {\AA}, c = 13.3830(2) {\AA}.
Bulk superconductivity is established in NdO0.5F0.5BiS2 at 5K by both transport
and magnetic measurements. Electrical transport measurements showed
superconducting Tc onset at 5.2K and Tc ({\rho}=0) at 4.7K. Under applied
magnetic field both Tc onset and Tc ({\rho} =0) decrease to lower temperatures
and an upper critical field [Hc2(0)] of above 23kOe is estimated. Both AC and
DC magnetic susceptibility measurements showed bulk superconductivity below 5K.
Isothermal magnetization (MH) exhibited typical type II behavior with lower
critical field (Hc1) of around 15Oe. Isothermal magnetization (MH) exhibited
typical type-II behavior with lower critical field (Hc1) of around 15Oe.
Specific heat [Cp(T)] is investigated in the temperature range of 1.9-50K in
zero external magnetic field. A Schottky-type anomaly is observed at low
temperature below 7K. This low temperature Schottky can be attributed to the
change in the entropy of the system.Comment: 10 pages text + Figs (New Version):comments/suggestion welcome
([email protected]
Superconducting Mechanism through direct and redox layer doping in Pnictides
The mechanism of superconductivity in pnictides is discussed through direct
doping in superconducting FeAs and also in charge reservoir REO layers. The
un-doped SmFeAsO is charge neutral SDW (Spin Density Wave) compound with
magnetic ordering below 150 K. The Superconducting FeAs layers are doped with
Co and Ni at Fe site, whereas REO layers are doped with F at O site. The
electron doping in SmFeAsO through Co results in superconductivity with
transition temperature (Tc) maximum up to 15 K, whereas F doping results in Tc
upto 47 K in SmFeAsO. All these REFe/Co/NiAsO/F compounds are iso-structural to
ZrCuSiAs structure. The samples are crystallized in a tetragonal structure with
space group P4/nmm. Variation of Tc with different doping routes shows the
versatility of the structure and mechanism of occurrence of superconductivity.
It seems doping in redox layer is more effective than direct doping in
superconducting FeAs layer.Comment: 4 Pages text + Figs: ([email protected]
Bulk Superconductivity in Bismuth-oxy-sulfide Bi4O4S3
Very recent report [1] on observation of superconductivity in Bi4O4S3 could
potentially reignite the search for superconductivity in a broad range of
layered sulphides. We report here synthesis of Bi4O4S3 at 5000C by vacuum
encapsulation technique and basic characterizations. Detailed structural,
magnetization, and electrical transport results are reported. Bi4O4S3 is
contaminated by small amounts of Bi2S3 and Bi impurities. The majority phase is
tetragonal I4/mmm space group with lattice parameters a = 3.9697(2){\AA}, c =
41.3520(1){\AA}. Both AC and DC magnetization measurements confirmed that
Bi4O4S3 is a bulk superconductor with superconducting transition temperature
(Tc) of 4.4K. Isothermal magnetization (MH) measurements indicated closed loops
with clear signatures of flux pinning and irreversible behavior. The lower
critical field (Hc1) at 2K, of the new superconductor is found to be ~39 Oe.
The magneto-transport R(T, H) measurements showed a resistive broadening and
decrease in Tc (R=0) to lower temperatures with increasing magnetic field. The
extrapolated upper critical field Hc2(0) is ~ 310kOe with a corresponding
Ginzburg-Landau coherence length of ~100{\AA} . In the normal state the {\rho}
~ T2 is not indicated. Our magnetization and electrical transport measurements
substantiate the appearance of bulk superconductivity in as synthesized
Bi4O4S3. On the other hand same temperature heat treated Bi is not
superconducting, thus excluding possibility of impurity driven
superconductivity in the newly discovered Bi4O4S3 superconductor.Comment: 12 pages Text + Fig
Magnetization and Magneto-resistance in Y(Ba1-xSrx)2Cu3O7-{\delta} (x = 0.00 - 0.50) superconductor
Here we present the magnetic properties and upper critical field (BC2) of
polycrystalline Y(Ba1-xSrx)2Cu3O7-{\delta} superconductors, which are being
determined through detailed ac/dc susceptibility and resistivity under magnetic
field (RTH) study. All the samples are synthesized through solid state reaction
route. Reduction in Meissner fraction (the ratio of field cooled to zero field
cooled magnetization) is observed with increasing Sr content, suggesting
occurrence of flux pining in the doped samples. The ac susceptibility and
resistivity measurements reveal improved grain couplings in Sr substituted
samples. Consequently the inter-grain critical current density (Jc), upturn
curvature near the Tc in temperature dependence of upper critical field
[BC2(T)], and BC2 are enhanced. Both Jc and BC2 increase in lower Sr
substitution (up to x = 0.10) samples followed by decrease in higher doping due
to degradation in effective pining and grain coupling.Comment: 17 pages text + Figs, [email protected]
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