Symmetric Dc Research Articles

Comparative evaluation of multilevel DC link inverter using symmetrical and asymmetrical DC sources

Abstract This paper presents a comparative evaluation of multilevel DC link inverter of photovoltaic (PV) renewable energy system using either symmetrical or asymmetrical dc sources. The four units of dc source can have the same dc voltage level, ie 81.32 V, resulting in symmetrical dc sources. The asymmetrical dc sources consist of input dc voltages in binary sequence, 21.68 V, 43.37 V, 86.74 V and 173.48 V. The boost converters with maximum power point tracking (MPPT) capability which is regulated by perturb and observe (P&O) based control are connected between the PV panels and the four dc sources. The varying dc voltages from the PV panels are regulated to track the maximum power available regardless of the irradiance and temperature conditions. The symmetrical dc sources will facilitate the generation of 9 levels of staircase ac waveform rms of 230 V after the H-bridge inverter. Whereas, the asymmetrical dc sources are able to produce 31 levels of staircase ac waveform also with rated rms 230 V. Detailed analysis and comparison on the powers, ac output voltage, output current, total harmonic distortions, and MPPT achievement are described.

Generalized Topology of a Hybrid Switched- Capacitor Multilevel Inverter for High- Frequency AC Power Distribution

The generalized topology of a hybrid switched-capacitor multilevel inverter (SCMLI) for high-frequency ac (HFAC) power distribution system is presented. By employing the series–parallel conversion technique, the proposed SCMLI can achieve self-balancing of capacitor voltages and voltage step-up with arbitrary numbers of SC cells. The output power quality is improved by the intermediate voltage levels provided by the bidirectional switched converter connected to the dc source string. Besides, several active switches can be replaced with diodes if the large phase difference between output current and voltage is not required. Since the voltage ripple magnitude of the SC units is inversely proportional to the fundamental output frequency, the proposed SCMLI is more preferable for HFac applications. This paper elucidates the working principle of the proposed inverter with the detailed circuit and switching states’ description under staircase modulation. The mathematical analyses associated with the capacitor voltage ripples, sizing, and power loss are also provided. A 19-level MLI with a voltage gain of three was realized by cooperating two SC units with a voltage source string consisting of three symmetric dc sources. The operation and performance of the proposed inverter have been validated by the simulation and experiment on the inverter. The results showed that the inverter performed better at HFac condition and the power quality was satisfactory with the 19-level staircase output modulated at the fundamental frequency.

A Bidirectional High-Efficiency Transformerless Converter With Common-Mode Decoupling for the Interconnection of AC and DC Grids

This paper presents an interface ac–dc converter to connect the 380 V bipolar dc microgrid with the 240 V split-phase single-phase ac utility. The design targets are high efficiency, leakage current attenuation, and symmetric bipolar dc bus generation. A transformerless two-stage topology is proposed to decouple the common-mode (CM) voltage between the connected ac and dc systems. The two-stage structure also reduces the required capacitance in single-phase power conversion by allowing a large voltage ripple on the intermediate dc link. Adaptive dc-link voltage control and interleaving of state-of-the-art silicon carbide (SiC) mosfet s are adopted to achieve high efficiency. Interleaving angles are optimized to minimize the total volume of both differential-mode and CM magnetics. An active CM control method is incorporated to regulate the dc and low-frequency CM voltages on the dc buses with respect to the ground. The resultant dc bus voltages are rendered symmetric to the ground, and the leakage current is suppressed. The generated ±190 V dc bus is suitable for bipolar 380 V dc power distribution systems. Pluggable phase-leg modules with embedded driving and protection circuit are designed and tested, based on which a 10 kW all-SiC converter prototype is built achieving an efficiency >97%.

Correlation between plasma glow intensity distribution and sputtering profile in dc magnetron discharge

The correlation between the plasma glow intensity distribution and the target sputtering profile in an axially symmetric unbalanced dc magnetron discharge with copper cathode in argon is studied. At high pressures the argon atom and ion emission intensities are in good agreement with a sputtering profile. In case of low pressures, a satisfactory agreement is only for the ion emission. The best estimates of the sputtering profile can be obtained from the argon ion emission distribution at not too low pressures.

Topological phase transition underpinning particle-hole symmetry in the Halperin-Lee-Read theory

Long wavelength descriptions of a half-filled lowest Landau level ($\nu = 1/2$) must be consistent with the experimental observation of particle-hole (PH) symmetry. The traditional description of the $\nu=1/2$ state pioneered by Halperin, Lee and Read (HLR) naively appears to break PH symmetry. However, recent studies have shown that the HLR theory with weak quenched disorder can exhibit an emergent PH symmetry. We find that such inhomogeneous configurations of the $\nu=1/2$ fluid, when described by HLR mean-field theory, are tuned to a topological phase transition between an integer quantum Hall state and an insulator of composite fermions with a dc Hall conductivity $\sigma_{xy}^{\rm (cf)} = - {1 \over 2} {e^2 \over h}$. Our observations help explain why the HLR theory exhibits PH symmetric dc response.

AC electrokinetics based capture of yeast cells from ultra‐fast through‐flow for sensitive detection

This work presents a three-dimensional multi-level microfluidic device that enables yeast cell trapping via alternating current (AC) electrokinetics (ACEK) from a rapid through-flow of sample fluids. ACEK is a versatile technique to control and manipulate cells and microparticles at the micro/nano-scale. Here the trapping device works by applying an AC signal over a pair of electrode across a flow-through channel to capture bioparticles from an ultra-fast through-flow. The particle capture effect by ACEK was experimentally studied by measuring the cell densities at the inlet and outlet of the micro-channel. Both symmetric AC signal and DC biased AC signal were used to achieve cell trapping. It is found that DC biased ACEK effect is highly capable of capturing cells from a rapid through-flow. A trapping efficiency of 56.8% has been obtained from an ultra-fast external flow with its average velocity at 4.44 mm/s. The merits of ACEK include low-voltage electrical operation, which is suitable for portable lab-on-a-chip systems. These devices offer numerous benefits for biological applications such as medical diagnostics, detection, drug screening, and particle trapping.

A simplified structure for three‐phase 4‐level inverter employing fundamental frequency switching technique

This study describes the design and control of simplified structure of three-phase 4-level inverter. A 4-level dc-link derived from three symmetrical dc voltage supplies and two controlled switches is connected to a three-phase 3-level neutral point clamped (3-level NPC) bridge inverter. By employing the fundamental frequency switching technique, the simplified 4-level inverter can have two distinct modes of operation: 4-level and 2-level. Compared to the well-known three-phase 4-level NPC inverter, the proposed inverter topology comes with series of features including a minimum number of power components, high quality voltage waveform, lower losses and higher efficiency. To ensure the feasibility of the proposed topology with its suggested modulation technique, the prototype of the proposed 4-level inverter is built and the experimental results are given.

A developed single-phase cascaded multilevel inverter with reduced number of circuit devices

ABSTRACTIn this paper, an advanced configuration for multilevel voltage source inverters is proposed. The proposed topology can be used as symmetric and asymmetric inverter. In asymmetric mode, DC sources’ magnitudes are defined in a way that the number of output voltage levels gets more than the case when the symmetrical DC sources are applied. In this regard, to calculate the magnitudes of required DC voltage sources, several different solutions are proposed. In multilevel inverters, the overall cost, circuit size and so installation area, complexity of control scheme and reliability are directly dependent on the number of circuit devices needed. The provided comparison study among suggested inverter in all defined solutions, CHB and recently proposed converters validates that the proposed modular inverter uses reduced number of circuit devices. The provided simulation and experimental results confirm the feasibility of the proposed structure and show that the obtained results are in good agreements.

Study of the Modulation Schemes in the Hybrid Three-Phase Multilevel Inverter

This work proposes and analyzes modulation strategies for a hybrid three- phase multilevel inverter, which employs two different topologies in a cascade connection, a three-phase Neutral Point Clamped (NPC) converter and single- phase half-bridge converters. Two different modulation schemes are proposed: the hybrid modulation and the space vector modulation. The both modulation schemes can synthesize five voltage levels in the output phase voltage at the hybrid three- phase multilevel inverter with symmetrical dc sources. The modulation schemes operate the three-phase NPC in low-switching frequency and the half- bridge converters in high-switching frequency. Thus, semiconductors with low conduction losses can be used for three-phase NPC, decreasing the semiconductor losses. The hybrid three-phase multilevel inverter structure and modulation schemes are explained in detail and verified by simulation and experimental results.

Characterization of the large area plane-symmetric low-pressure DC glow discharge

Electron density and temperature as well as nitrogen dissociation degree in the low-pressure (10–50mTorr) large area plane–symmetric DC glow discharge in Ar-N2 mixtures are studied by probes and spectral methods. Electron density measured by a hairpin probe is in good agreement with that derived from the intensity ratio of the N2 2nd positive system bands IC,1−3/IC,0−2 and from the intensity ratio of argon ions and atom lines IArII/IArI, while Langmuir probe data provides slightly higher values of electron density. Electron density in the low-pressure DC glow discharge varies with the discharge conditions in the limits of ~108–1010cm−3. The concept of electron temperature can be used in low-pressure glow discharges with reservations. The intensity ratio of (0–0) vibrational bands of N2 1st negative and 2nd positive systems I391.4/I337.1 exhibits the electron temperature of 1.5–2.5eV when argon fraction in the mixture is higher than nitrogen fraction and this ratio quickly increases with nitrogen fraction up to 10eV in pure nitrogen. The electron temperature calculated from Langmuir probe I-V characteristics assuming a Maxwellian EEDF, gives Te~0.3–0.4eV. In-depth analysis of the EEDF using the second derivative of Langmuir probe I-V characteristics shows that in a low-pressure glow discharge the EEDF is non-Maxwellian. The EEDF has two populations of electrons: the main background non-Maxwellian population of “cold” electrons with the mean electron energy of ~0.3–0.4eV and the small Maxwellian population of “hot” electrons with the mean electron energy of ~1.0–2.5eV. Estimations show that with electron temperature lower than 1eV the rate of the direct electron impact ionization of N2 is low and the main mechanism of N2 ionization becomes most likely Penning and associative ionization. In this case, assumptions of the intensity ratio IN2+,391/IN2,337 method are violated.In the glow discharge, N2 dissociation degree reaches about 7% with the argon fraction in the Ar-N2 mixture<10% and decreases afterwards approaching to ~1–2% when the argon percentage becomes 90% and higher. The atomic nitrogen species is produced by electron-impact processes such as, collisions between electrons and nitrogen molecules or between electrons and N2+ ions. At small Ar fraction in Ar-N2 mixtures, the atomic nitrogen species is most likely produced by the collisions between electrons and N2+ ions.

High‐power bidirectional resonant DC–DC converter with equivalent switching frequency doubler

A high-power double frequency bidirectional symmetrical DC–DC resonant converter is proposed to reduce the sizes of the passive components and increase the cost-effectiveness. Inspired by the bipolar and unipolar modulations for the single-phase inverter, an asymmetrical duty cycle double frequency modulation is originated for high-power bidirectional resonant converters whose switching frequency and power density are significantly limited by the high-power devices. The equivalent operating frequency of the resonant tank becomes twice of the switching frequency, which effectively reduces the volumes and costs of the passive components, especially for high-power high-voltage applications. Moreover, the proposed converter has the same output characteristics for the two power flow directions, which not only symmetrises the bidirectional frequency control, but also simplifies the design of the resonant components. The converter operation principle is illustrated and the converter performances are analysed including the principle of the double frequency modulation, output characteristics and soft switching condition. Finally, a 5 kW prototype is built to verify the analysis, and the resonant components for the conventional bidirectional resonant converter are also designed and compared to illustrate the advantages of the proposed converter.

Reduction of Circuit Devices in Developed Symmetrical/Asymmetrical Multilevel Converters

In this paper, a developed configuration for multilevel voltage source inverters (MVSIs) is proposed. The proposed topology can be used as symmetric and asymmetric inverters. In asymmetric mode, DC source magnitudes are defined in a way that the number of output levels gets more than the case when the symmetrical DC sources are applied. In this regard, to calculate the required magnitudes of DC voltage sources several different solutions are proposed. In multilevel inverters, the overall costs, circuit size and installation area, complexity of control scheme and reliability are directly dependent on the number of circuit devices needed. The provided comparison study among suggested inverter in all defined solutions, cascaded H-bridge (CHB) and recently proposed converters, validates that the proposed inverter uses reduced number of circuit devices. The given simulation results confirm the feasibility of the proposed modular structure. Also, to approve the practicability of the proposed inverter, a prototype of the proposed topology has been implemented. Finally, simulation and experimental results are compared with each other and the provided comparison shows that the obtained results are in good agreements.

Ripple Minimization Through Harmonic Elimination in Asymmetric Interleaved Multiphase DC–DC Converters

Symmetric multiphase dc–dc converters are widely used in power electronics, as they enable the processing of high power through splitting the overall load-current into multiple phases. Distributing the processed power symmetrically between the phases and performing ripple minimization through interleaving is well understood. However, in recent applications such as maximum power point tracking for solar photovoltaic, converters are forced to operate under asymmetric conditions, due to differences in the sources or loads of each converter. This study presents a control technique, based on harmonic elimination, that allows for ripple minimization under asymmetric conditions. The mathematical derivations are outlined and simulations are used to evaluate the performance of the proposed technique. Measurements taken from an experimental prototype, consisting of three dc–dc buck converters, demonstrate significant improvements in ripple reduction over conventional interleaving techniques. When the multiphase converter is operated at the optimum asymmetric phase-shift found through the techniques presented here, a more than 3x reduction in net current ripple is observed under realistic operating conditions. Additionally, the undesirable first harmonic ripple component is reduced by 14.8 dB with the proposed technique.

Current-pressure characteristics of dc magnetron discharge for high-rate sputtering

The current-pressure (I-P) characteristics of axially symmetric dc discharge of planar magnetron sputter were studied. The characteristics in argon and krypton gases for copper and aluminum cathodes and the spatial distributions of the plasma glow at different pressures are obtained. The sequence of local maximum and minimum is observed on current- pressure characteristics under defined conditions. The necessity of taking into account the gas heating by sputtered atoms for explaining the observed features of I-P characteristics is shown. Numerical simulation has allowed explaining the observed significant difference between the values of the discharge current density in argon and krypton.

Design of Integrated Bipolar Symmetric Output DC–DC Power Converter for Digital Pulse Generators in Ultrasound Medical Imaging Systems

This paper presents a bipolar symmetric output switching dc–dc converter for digital pulse generators in ultrasound medical imaging systems. The proposed power stage architecture can generate symmetric bipolar supply voltages, while operating in a time-interleaving manner. Meanwhile, in order to power unipolar pulse generators with the same peak-to-peak pulse amplitude, the power stage can also be reconfigured to deliver a 2× unipolar output voltage. The power converter employs an adaptive <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\Delta}I$</tex></formula> charge balance control scheme to ensure that the outputs are symmetrically balanced and well regulated. At the circuit level, to utilize a single feedback controller for both power stage topologies, the assignment of a suitable chip ground is discussed. A prototype is fabricated using a 0.25-μm CMOS process and occupies a chip area of 6 mm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{2}$</tex></formula> . For an input supply voltage of 1.5 V, the dc–dc converter can regulate the bipolar outputs at a nominal voltage level of ±2.5 V and at 5 V for the unipolar output configuration and a maximum load current of 45 mA for both. The bipolar output power converter achieves a maximum efficiency of 83.4% at a load power of 187.5 mW, with the voltage balance error maintained within ± 2% over the entire load range.

ASYMMETRIC RECTANGULAR WAVEFORM IN STIMULATION WITH HIGH FREQUENCY ALTERNATING CURRENT REDUCES THE THRESHOLD FOR NEURAL CONDUCTION BLOCK

Introduction Abnormal neural impulses in the nervous system may lead to various diseases and disabilities. High frequency alternating currents (HFAC) has been used to block the propagation of such impulses and improve the symptoms or disabilities. The technique is safe, reversible, and relatively selective, and its reliability, the optimum stimulation parameters, and elimination of the onset response have been the focus of related studies in the last decade. In this work, a series of computational simulations were performed to evaluate the performance of asymmetric biphasic rectangular waveforms for HFAC. Materials and Methods Computer simulations were carried out in NEURON software based on the MRG model, a detailed model of mammalian peripheral nerve fibers. The current threshold for the block and the injected charge per phase were assess for different forms of this waveform and compared with symmetric rectangular, sinusoidal, DC, and monophasic stimulations. The effect of fiber diameter and the stimulation frequency were also evaluated for this waveform. Results The threshold charge per phase to induce nerve conduction block was significantly lower for the proposed asymmetric biphasic stimulation. The minimum thresholds were achieved for the waveforms with short anodic long cathodic phases. The threshold was reduced with increasing the asymmetry of the waveform and reduction of the frequency. Conclusion Simulations performed in this study demonstrated that the proposed stimulation with asymmetric biphasic rectangular waveforms significantly reduces the current threshold and requires much less charge injection per phase to induce nerve conduction block. This is very important for clinical use due to less damage to the tissue.

Multilevel inverter topology based on series connected switched sources

A new topology for multilevel DC–AC conversion is presented in this study. It consists of isolated symmetric input DC sources alternately connected in opposite polarities through power switches. The structure allows synthesis of multilevel waveform using reduced number of power switches as compared to the classical topologies. The working principle of the proposed topology is explained with the help of a single-phase five-level inverter. Simulation studies are carried out in MATLAB/Simulink environment and experimental validations are obtained on a laboratory prototype. An exhaustive comparison of the proposed topology against the classical cascaded H-bridge topology indicates reduction in number of power switches, losses, installation area and converter cost.

Theoretical analysis and experimental validation of a novel multilevel inverter topology for renewable energy interfacing applications

This paper presents a new topological structure for symmetrical cascaded multilevel inverters. The proposed circuit consists of cross-connected separate symmetrical DC sources which can generate number of levels equal to that of a cascaded H-bridge topology with reduced number of power switches. It is especially suitable for utility interfacing of renewable energy sources. It also offers the possibility of a transformer less and filter less operation. The working principle of the proposed topology is explained with the help of a single-phase five level inverter. Simulation results are obtained in matlab/simulink environment and are verified experimentally on a laboratory prototype. An exhaustive comparison of the proposed topology is made with the classical cascaded H-bridge topology, especially with the considerations of conduction losses and cost-effectiveness. The proposed topology results in the reduction of number of power switches, losses, installation area, and converter cost and thus can be implemented as an effective and efficient power electronic interface for renewable sources of energy with the utility grid.

Corrections to “Impedance-Transforming Symmetric and Asymmetric DC Blocks” [Sep 10 2463-2474

In the above titled paper (ibid., vol. 58, no. 9, pp. 2463-2474, Sep. 10), there is a publication error [Fig. 8]. The correct Fig. 8 is presented here.

Impedance-Transforming Symmetric and Asymmetric DC Blocks

Design formulas of symmetric and asymmetric dc blocks are presented for achieving Chebyshev and Butterworth frequency responses. They can also be used for impedance transformers. Design formulas of symmetric dc blocks are first derived and those of asymmetric dc blocks are also computed based on the symmetric ones. For easy derivation of design formulas of the symmetric dc blocks, coupling coefficient, being proportional to bandwidth, is defined. It is then clarified that symmetric Chebyshev dc blocks are not possible for the impedance transforming. For the symmetric Butterworth dc blocks, three design methods are exploited. To verify the symmetric Butterworth dc blocks, a microstrip dc block with 50- and 25-Ω termination impedances designed at a center frequency of 2 GHz was fabricated. The measured <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> are - 0.19 and - 57 dB around 2 GHz, showing quite good agreement with simulation results. Based on synthesized symmetric dc blocks, the coefficients of transfer function | <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> | <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are calculated and design formulas of the asymmetric dc blocks are derived based on the symmetric ones. Asymmetric Chebyshev dc blocks are verified by the measurements provided in a previously published paper and asymmetric Butterworth dc blocks by the symmetric Butterworth dc block measured in this paper.