Variable Pulse Width Research Articles

Variable switching frequency hybrid PWM technique for switching loss reduction in a three-phase two-level voltage source inverter

Power loss reduction is one of the main objectives in power electronic system design for achieving higher efficiencies and enhancing system reliability. In this regard, this paper aims to develop a variable switching frequency hybrid pulse width modulation (VSF-HPWM) method to maximize loss saving while maintaining the same current total harmonic distortion (THD) of the conventional scheme. In doing so, this technique utilizes the modulation signal and the switching frequency simultaneously. Using a predefined mathematical representation of current ripple in every switching cycle, the modulating signal (either discontinuous or conventional space vector modulation) with lower RMS current ripple is applied. Meanwhile, the number of commutations under the constrains of constant RMS ripple is reduced by varying the switching frequency. The performance analysis is validated through MATLAB Simulink which shows that the proposed strategy can save up to 53% of switching losses with a similar THD of the conventional scheme.

Pulse-width variation of power supply for evaluating quasi-steady state of magneto-plasma-dynamic thruster operation

Magnetoplasmadynamic (MPD) thrusters are operated in a quasisteady state with about 1.0 ms pulse created by a pulse forming network (PFN). However, there is still no precedent to verify the operation time quantitatively. The nonsteady region of the pulse can lead to an error of the thrust performance against that of steady state operation. In addition, the propellant gas outside the discharge chamber can be consumed since the exhaust velocity exceeds the estimated velocity. This paper shows the first step in quantitative evaluation of the quasisteadiness of an MPD thruster operation. First, we developed a new power supply that outputs a flat-topped and less nonsteady region pulse with a variable pulse width. Compared with that of a PFN, the nonsteady region “tr + tf” decreased from 0.532 to 0.110 ms. By implementing the circuit shorter and adjusting the gate resistance, the surge voltage in the experiment was suppressed to 309 V, which is less than 2% error of that in the PSIM simulation, 305 V. Second, we operated an MPD thruster using the new power supply for discharge and the external magnetic field. As a result, we obtained operation time characteristics by sweeping the operation time from 0.3 to 5.0 ms. The current waveforms are in a range of 620 ± 70 A. We confirmed the consistency of the thrust, 0.32 ± 0.03 N. From the correlation between the input energy and the impulse, it is possible to discuss quasisteadiness of the MPD thruster operation using the determination coefficient and the offset.

Effects of laser irradiation on growth factors and cell apoptosis of in vitro cultured infant hemangioma endothelial cells

AimsThe present study aimed to investigate the effects of laser irradiation on the growth factors and cell apoptosis of in vitro cultured infant hemangioma endothelial cells. Main methodsEndothelial cells of infant hemangioma were cultured in vitro and irradiated using a variable pulse width 1064 nm Nd:YAG laser and intense pulsed light (IPL), the expression of VEGF, VEGFR-2, bFGF and their mRNAs before and after irradiation were measured by ELISA, western blot, RT-PCR and flow cytometry, and changes in the apoptotic rate of endothelial cells in hemangioma were monitored. Key findingsThe mRNA and protein expressions of VEGF, VEGFR-2, bFGF in hemangioma endothelial cells were inhibited by both Nd:YAG laser and ILP compared to the control cells. The apoptotic rates of hemangioma endothelial cells were also decreased after both laser irradiation treatments in comparison to the blank group. The differences were statistically significant. SignificanceLaser irradiation treats hemangioma not only through a selective photothermal mechanism, but also through cytokine signaling pathways.

The effect of laser pulsewidth on the selenium nanoparticles mass yield

High-throughput synthesis of selenium (Se) nanoparticles in the form of colloidal solutions was carried out by 1030 nm laser ablation of a bulk Se target in deionized water and isopropyl alcohol by laser pulses of variable pulse widths (0.3–8.6 ps). The extinction coefficient of the colloidal solutions exhibits an overall decreasing trend with the increase of the laser pulse width in deionized water, also demonstrating a local minimum at 2–4 ps. This can be explained by the growing ablation thresholds, related to the bigger impact of thermal processes during the transition from femto- to picosecond temporal stages.

Microprocessing of a steel surface by single pulses of variable width

Single-pulse microprocessing of a steel surface with a variable pulse width (0.3–12.3 ps) and a wavelength of 515 nm was performed. The morphology of the craters was visualized by a scanning electron microscope and an optical profilometer. The nonmonotonic behavior of ablation thresholds with a minimum at ≈1.5 ps, due to achieving the electron–phonon relaxation time of the absorbed energy in the steel was revealed. It is shown that, with an increase in the pulse width in the considered width range, the efficiency of the ablation decreases by a factor of 2, which is explained by the partial transition from the phase explosion to the surface evaporation.

376 - Impact of variable pulse width and pulse modulation on dusting effect for holmium laser lithotripsy: In vitro evaluation with calcium oxalate monohydrate stones

376 - Impact of variable pulse width and pulse modulation on dusting effect for holmium laser lithotripsy: In vitro evaluation with calcium oxalate monohydrate stones

Mode-locked thulium-fluoride fibre laser with an adjustable pulse width using a nonlinear optical loop mirror

A mode-locked thulium-fluoride fibre laser with a tunable pulse width using a nonlinear optical loop mirror (NOLM) for operation in the S-band region is proposed and demonstrated. Stable dissipative soliton resonance (DSR) mode-locked pulses are obtained at a threshold pump power of 186.8 mW and maintain stability until a maximum pump power of 249.7 mW. Pulse width modulation of between 70.8 ns and 120.3 ns is obtained over the operating pump power range, with a fundamental frequency at 97.4 kHz. A maximum output power of 0.99 mW and a pulse energy of 10.21 nJ are obtained. The output pulse has a signal-to-noise ratio of 69 dB at the maximum pump power, indicating a highly stable output. The variable pulse width will find significant applications as modulated light sources for various optical and optoelectronic systems.

Variable Pulse Width Unipolar Orthogonal Frequency Division Multiplexing for Visible Light Communication Systems

Visible light communication (VLC) systems working as high-speed and short-range wireless connection methods have attracted more and more attention. This paper proposes a variable pulse width unipolar optical orthogonal frequency-division multiplexing (VPW-OFDM) scheme for indoor VLC systems. Similar to the polar-based optical OFDM, Hermitian symmetric data are not required to generate real signals. Instead, the magnitude and phase components of the complex-valued OFDM signal are transmitted successively. Unlike the polar-based optical OFDM, the proposed VPW-OFDM allows using variant pulse widths to reduce the effects of the noise added on the phase component. By adaptively optimizing the pulse widths, the proposed VPW-OFDM outperforms the polar-based OFDM and other state-of-the-art optical OFDM techniques, such as dc-biased optical OFDM, asymmetrically clipped optical OFDM, and unipolar-OFDM. For indoor VLC systems, illumination requirement is crucial and needs to be considered. In this paper, we analyze the effects of the illumination requirements on system performance. Bit error rate and transmitted bit rate are used as criteria to evaluate the communication performances for the techniques tested. In addition, the system channel with bandwidth limitation is taken into account. To enhance the data throughput, a single-tap equalizer at the receiver and the bit loading algorithm are applied. From the numerical results, the proposed VPW-OFDM outperforms DCO-, ACO-, U-, and polar-based OFDM for a wide channel bandwidth.

Rapid dual-RF, dual-echo, 3D ultrashort echo time craniofacial imaging: A feasibility study.

To develop a dual-radiofrequency (RF), dual-echo, 3D ultrashort echo-time (UTE) pulse sequence and bone-selective image reconstruction for rapid high-resolution craniofacial MRI. The proposed pulse sequence builds on recently introduced dual-RF UTE imaging. While yielding enhanced bone specificity by exploiting high sensitivity of short T2 signals to variable RF pulse widths, the parent technique exacts a 2-fold scan time penalty relative to standard dual-echo UTE. In the proposed method, the parent sequence's dual-RF scheme was incorporated into dual-echo acquisitions while radial view angles are varied every pulse-to-pulse repetition period. The resulting 4 echoes (2 for each RF) were combined by view-sharing to construct 2 sets of k-space data sets, corresponding to short and long TEs, respectively, leading to a 2-fold increase in imaging efficiency. Furthermore, by exploiting the sparsity of bone signals in echo-difference images, acceleration was achieved by solving a bone-sparsity constrained image reconstruction problem. In vivo studies were performed to evaluate the effectiveness of the proposed acceleration approaches in comparison to the parent method. The proposed technique achieves 1.1-mm isotropic skull imaging in 3 minutes without visual loss of image quality, compared to the parent technique (scan time = 12 minutes). Bone-specific images and corresponding 3D renderings of the skull were found to depict the expected craniofacial anatomy over the entire head. The proposed method is able to achieve high-resolution volumetric craniofacial images in a clinically practical imaging time, and thus may prove useful as a potential alternative to computed tomography.

Selective Laser Sintering of Laser Printed Ag Nanoparticle Micropatterns at High Repetition Rates.

The increasing development of flexible and printed electronics has fueled substantial advancements in selective laser sintering, which has been attracting interest over the past decade. Laser sintering of metal nanoparticle dispersions in particular (from low viscous inks to high viscous pastes) offers significant advantages with respect to more conventional thermal sintering or curing techniques. Apart from the obvious lateral selectivity, the use of short-pulsed and high repetition rate lasers minimizes the heat affected zone and offers unparalleled control over a digital process, enabling the processing of stacked and pre-structured layers on very sensitive polymeric substrates. In this work, the authors have conducted a systematic investigation of the laser sintering of micro-patterns comprising Ag nanoparticle high viscous inks: The effect of laser pulse width within the range of 20–200 nanoseconds (ns), a regime which many commercially available, high repetition rate lasers operate in, has been thoroughly investigated experimentally in order to define the optimal processing parameters for the fabrication of highly conductive Ag patterns on polymeric substrates. The in-depth temperature profiles resulting from the effect of laser pulses of varying pulse widths have been calculated using a numerical model relying on the finite element method, which has been fed with physical parameters extracted from optical and structural characterization. Electrical characterization of the resulting sintered micro-patterns has been benchmarked against the calculated temperature profiles, so that the resistivity can be associated with the maximal temperature value. This quantitative correlation offers the possibility to predict the optimal process window in future laser sintering experiments. The reported computational and experimental findings will foster the wider adoption of laser micro-sintering technology for laboratory and industrial use.

Effect of Pulse Width in Pulsed Nd:YAG Dissimilar Laser Welding of Austenitic Stainless Steel (304 L) and Carbon Steel (st37)

In the recent development of the automobile, power generation and petrochemical industries the application of dissimilar welds are gaining importance. In the present work, pulsed Nd:YAG laser welding of austenitic stainless steel (304 L) and carbon steel (st37) of 1.4 mm thickness for butt joint configuration have been investigated for automotive industries. The effect of pulsed width on weld bead width (top bead width (TBW) and bottom bead width (BBW), depth of penetration (DOP) and heat affected zone (HAZ) and fusion zone area have been studied. The microstructure characterization of the fusion zone, HAZ and base metals have been compared at varying pulse width using optical microscope. DOP decreases with increase in pulse width beyond 5 ms and fusion zone area decreases with increase in pulse width. Bottom weld bead width decreases with increase in pulse width and HAZ width of st37 side decreases with increase in pulse width. The mechanical properties such as microhardness and tensile strength of the welded joints at varying pulse width have been investigated. Ultimate tensile strength of all the welded joint at different pulse width was equal to the base metal st37 and fracture location was away from the fusion zone. The average microhardness of the fusion zone decreases with increase in pulse width due to the presence of martensite at low pulse width.

A 128-Tap Highly Tunable CMOS IF Finite Impulse Response Filter for Pulsed Radar Applications

A configurable-bandwidth (BW) filter is presented in this paper for pulsed radar applications. To eliminate dispersion effects in the received waveform, a finite impulse response (FIR) topology is proposed, which has a measured standard deviation of an in-band group delay of 11 ns that is primarily dominated by the inherent, fully predictable delay introduced by the sample-and-hold. The filter operates at an IF of 20 MHz, and is tunable in BW from 1.5 to 15 MHz, which makes it optimal to be used with varying pulse widths in the radar. Employing a total of 128 taps, the FIR filter provides greater than 50-dB sharp attenuation in the stopband in order to minimize all out-of-band noise in the low signal-to-noise received radar signal. Fabricated in a 0.18- $\mu \text{m}$ silicon on insulator CMOS process, the proposed filter consumes approximately 3.5 mW/tap with a 1.8-V supply. A 20-MHz two-tone measurement with 200-kHz tone separation shows IIP3 greater than 8.5 dBm.

Direct measurement of bipolar cell responses to electrical stimulation in wholemount mouse retina

Objective. This in vitro investigation examines the response of retinal bipolar cells to extracellular electrical stimulation. Approach. In vitro investigations characterizing the response of retinal neurons to electrical stimulation have primarily focused on retinal ganglion cells because they are the output neurons of the retina and their superficial position in the retina makes them readily accessible to in vitro recording techniques. Thus, the majority of information regarding the response of inner retinal neurons has been inferred from ganglion cell activity. Here we use patch clamp electrophysiology to directly record electrically-evoked activity in bipolar cells within the inner retina of normal Tg(Gng13-EGFP)GI206Gsat and degenerate rd10 Tg(Gng13-EGFP)GI206Gsat mice using a wholemount preparation. Main results. Bipolar cells respond to electrical stimulation with time-locked depolarizing voltage transients. The latency of the response declines with increases in stimulation amplitude. A desensitizing response is observed during repeated stimulation with 25 ms biphasic current pulses delivered at pulse rates greater than 6 pps. A burst of long-latency (200–1000 ms) inhibitory postsynaptic potentials are evoked by the stimulus and the burst exhibits evidence of a lower and upper stimulation threshold. Significance. These results provide insights into the various types of bipolar cell activity elicited by electrical stimulation and may be useful for future retinal prosthesis stimulation protocols. This investigation uses patch clamp electrophysiology to provide direct analysis of ON-type bipolar cell responses to electrical stimulation in a wholemount retina preparation. It explores the effects of variable stimulus amplitudes, pulse widths, and frequencies in both normal and degenerate retina. The analysis adds to a body of work largely based upon indirect measurements of bipolar cell activity, and the methodology demonstrates an alternative retina preparation technique in which to acquire single-cell activity.

Photokinetic Drug Delivery: Near infrared (NIR) Induced Permeation Enhancement of Bevacizumab, Ranibizumab and Aflibercept through Human Sclera

PurposePermeation studies, with near infrared (NIR) light and anti-aggregation antibody formulation, were used to investigate the in vitro permeation of bevacizumab, ranibizumab and aflibercept through human sclera.MethodsA vertical, spherical Franz cell diffusion apparatus was used for this scleral tissue permeation model. A photokinetic ocular drug delivery (PODD) testing device accommodated the placement of NIR LEDs above the donor chambers. An adjustable LED driver/square wave generator provided electrical energy with a variable pulse rate and pulse width modulation (duty cycle).ResultsExposure to non-thermal NIR light had no effect on mAbs with regard to monomer concentration or antibody binding potential, as determined by SE-HPLC and ELISA. The optimal LED wavelength was found to be 950 nm. Duty cycle power of 5% vs 20% showed no difference in permeation. When compared to controls, the combination of non-aggregating antibody formulation and NIR illumination provided an average transscleral drug flux enhancement factor of 3X.ConclusionNarrow wavelength incoherent (non-laser) light from an NIR LED source is not harmful to mAbs and can be used to enhance drug permeation through scleral tissue. The topical formulation, combined with pulsed NIR light irradiation, significantly improved scleral permeation of three anti-VEGF antibody drugs.

Optimization Model Based Sub-Nyquist Sampling of Pulses With Various Shapes and Its Application to ECG Signals

Recently developed variable pulse width finite rate of innovation (VPW-FRI) theory offers an efficient way for sampling pulse streams with various shapes at the sub-Nyquist rate. Unfortunately, for real signals, noise, and model mismatch will induce inaccuracies to such scheme in the reconstruction process. In this paper, an optimization model-based sub-Nyquist sampling system for pulses with various shapes is proposed, which improves the performance of VPW-FRI scheme under noise and model mismatch situation. Since the real pulse streams with various shapes may be modeled as the sum of an unknown number of Lorentzian pulses and a model mismatch error signal, we build an optimization object function with the purpose of minimizing the energy of this model mismatch error signal. Then, for solving such function, we propose a two-channel sub-Nyquist sampling system to obtain a Fourier coefficients subset and several discrete samples from the input signal. We demonstrate that the best number of Lorentzian pulses and the corresponding pulse parameters can be found by using an improved particle swarm optimization algorithm. Finally, simulations with the electrocardiogram signals in MIT-BIH database have shown that the proposed method has better performance and stability than traditional VPW-FRI scheme.

Confined Band Variable Switching Frequency Pulse Width Modulation (CB-VSF PWM) for a Single-Phase Inverter With an LCL Filter

Variable switching frequency pulse width modulation (VSF PWM) has been proposed in the past to optimize the total harmonic distortion (THD) performance and switching losses of an inverter. Nevertheless, such VSF PWMs give rise to switching harmonics which vary across a wide frequency spectrum, where the lower frequencies could resonate with the filter while the upper frequency could exceed the maximum switching frequency. This complicates VSF PWM design, restricting their practical uses. In this paper, a confined band VSF PWM (CB-VSF PWM) for a single-phase inverter with an LCL filter is presented. By confining the switching frequencies within a practical band, the design of the filter becomes easier while maintaining low current THD with low switching losses. The choices of the upper and lower bands of the proposed VSF are defined and the design criterion is introduced. It is found that the proposed method allows reduction in switching losses as well as reduction in current THD when the effect of the LCL filter is taken into consideration. The performance analysis is validated through simulation and experimental results in a 1 kW inverter.

A pulse-burst laser system for Thomson scattering on NSTX-U

A pulse-burst laser system has been built for Thomson scattering on NSTX-U, and is currently being integrated into the NSTX-U Thomson scattering diagnostic system. The laser will be operated in three distinct modes. The base mode is continuous 30 Hz rep rate, and is the standard operating mode of the laser. The base mode will be interrupted to produce a “slow burst” (specified 1 kHz rep rate for 50 ms) or a “fast burst” (specified 10 kHz rep rate for 5 ms). The combination of base mode→ interruption→ burst mode is new and has not been implemented on any previous pulse-burst laser system. Laser pulsing is halted for a set period (∼ 1 minute) following a burst to allow the YAG rods to cool; this type of operation is called a heat-capacity laser. The laser is Nd:YAG operated at 1064 nm, q-switched to produce ⩾ 1.5 J pulses with ∼ 20 ns FWHM. It is flashlamp pumped, with dual-rod oscillator (9 mm) and dual-rod amplifier (12 mm). Variable pulsewidth drive of the flashlamps is accomplished by IGBT (insulated gate bipolar transistor) switching of electrolytic capacitor banks. Direct control of the laser Pockels cell drive enables optimal pulse energy extraction. The laser system has demonstrated compliance with all specifications, and is capable of exceeding design specifications by significant margins, e.g., higher rep rates for longer burst periods. Burst operation of this laser system will be used to capture fast time evolution of the electron temperature and density profiles during events such as ELMs, the L-H transition, and various MHD modes.

The mechanism of plasma plume termination for pulse-excited plasmas in a quartz tube

Although the formation and propagation of plasma plume for atmospheric pressure plasmas have been intensively studied, how does the plasma plume terminate is still little known. In this letter, helium plasma plumes are generated in a long quartz tube by pulsed voltages and a constant gas flow. The voltages have a variable pulse width (PW) from 0.5 μs to 200 μs. It is found that the plasma plume terminates right after the falling edge of each voltage pulse when PW < 20 μs, whereas it terminates before the falling edge. When PW is larger than 30 μs, the duration of plasma plume starts to decrease, and the termination is found to occur at the current zero moment of the discharge current through the high-voltage electrode, which is much different from that through the ground electrode. This indicates that part of the discharge current is shunted by the plasma plume to its downstream gas region. An equivalent circuit model is developed, from which the surface charge deposited on the quartz tube is found crucial for accelerating the termination of a plasma plume when PW > 30 μs.

Pulsed characterization of a UV LED for pulsed power applications on a silicon carbide photoconductive semiconductor switch.

The electrical and optical characteristics of a high-power UV light emitting diode (LED) (365 nm wavelength) were evaluated under pulsed operating conditions at current amplitudes several orders of magnitude beyond the LED's manufacturer specifications. Geared towards triggering of photoconductive semiconductor switches (PCSSs) for pulsed power applications, measurements were made over varying pulse widths (25 ns-100 μs), current (0 A-250 A), and repetition rates (single shot-5 MHz). The LED forward voltage was observed to increase linearly with increasing current (∼3.5 V-53 V) and decrease with increasing pulse widths. The peak optical power observed was >30 W, and a maximum system efficiency of 23% was achieved. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a 4H-SiC PCSS. The lowest measured on-resistance of SiC was approximately 67 kΩ.

Laser Pulse Width Dependence and Ionization Mechanism of Matrix-Assisted Laser Desorption/Ionization.

Ultraviolet laser pulses at 355 nm with variable pulse widths in the region from 170 ps to 1.5 ns were used to investigate the ionization mechanism of matrix-assisted laser desorption/ionization (MALDI) for matrices 2,5-dihydroxybenzoic acid (DHB), α-cyano-4-hydroxycinnamic acid (CHCA), and sinapinic acid (SA). The mass spectra of desorbed ions and the intensity and velocity distribution of desorbed neutrals were measured simultaneously for each laser shot. These quantities were found to be independent of the laser pulse width. A comparison of the experimental measurements and numerical simulations according to the multiphoton ionization, coupled photophysical and chemical dynamics (CPCD), and thermally induced proton transfer models showed that the predictions of thermally induced proton transfer model were in agreement with the experimental data, but those of the multiphoton ionization model were not. Moreover, the predictions of the CPCD model based on singlet-singlet energy pooling were inconsistent with the experimental data of CHCA and SA, but were consistent with the experimental data of DHB only when some parameters used in the model were adjusted to extreme values. Graphical Abstract ᅟ.