Showing papers on "Rise time published in 2017"

Binary response Se/ZnO p-n heterojunction UV photodetector with high on/off ratio and fast speed

Abstract: A high-performance UV photodetector (PD) based on a p-Se/n-ZnO hybrid structure with large area (more than 1×1 cm) is presented in this study. The device is theoretically equivalent to a parallel-connection circuit for its special structure and shows multifunction at different voltage bias, which means the output signal can be tailored by an applied voltage. The Se/ZnO PD shows binary response (positive and negative current output under on/off periodical light illumination) under small reverse bias (–0.05 V and –0.1 V) which efficiently reduces the negative effect of noise signal in weak-signal detection applications. At zero bias, with the aid of a p-n heterojunction, a high on/off ratio of nearly 104 is achieved by this device at zero set bias under 370 nm (∼0.85 mW cm−2) illumination and this on/off ratio can be achieved in 0.5 s. The device also shows a fast speed with rise time of 0.69 ms and decay time of 13.5 ms measured by a pulse laser, much faster than that of a pure ZnO film. The Se/ZnO PD in this research provides a new pathway to fabricate multifunctional high-speed, high signal-to-noise ratio, high detectivity and high selectivity UV photodetectors.

A process modification for CMOS monolithic active pixel sensors for enhanced depletion, timing performance and radiation tolerance

Abstract: For the upgrade of its Inner Tracking System, the ALICE experiment plans to install a new tracker fully constructed with monolithic active pixel sensors implemented in a standard 180 nm CMOS imaging sensor process, with a deep pwell allowing full CMOS within the pixel. Reverse substrate bias increases the tolerance to non-ionizing energy loss (NIEL) well beyond 1 0 13 1 MeV n e q ∕ cm 2 , but does not allow full depletion of the sensitive layer and hence full charge collection by drift, mandatory for more extreme radiation tolerance. This paper describes a process modification to fully deplete the epitaxial layer even with a small charge collection electrode. It uses a low dose blanket deep high energy n-type implant in the pixel array and does not require significant circuit or layout changes so that the same design can be fabricated both in the standard and modified process. When exposed to a 55 Fe source at a reverse substrate bias of −6 V, pixels implemented in the standard and the modified process in a low and high dose variant for the deep n-type implant respectively yield a signal of about 115 mV, 110 mV and 90 mV at the output of a follower circuit. Signal rise times heavily affected by the speed of this circuit are 27 . 8 + ∕ − 5 ns, 23 . 2 + ∕ − 4 . 2 ns, and 22 . 2 + ∕ − 3 . 7 ns rms, respectively. In a different setup, the single pixel signal from a 90 Sr source only degrades by less than 20% for the modified process after a 1 0 15 1 MeV n e q ∕ cm 2 irradiation, while the signal rise time only degrades by about 16 + ∕ − 2 ns to 19 + ∕ − 2 . 8 ns rms. From sensors implemented in the standard process no useful signal could be extracted after the same exposure. These first results indicate the process modification maintains low sensor capacitance, improves timing performance and increases NIEL tolerance by at least an order of magnitude.

Passive Balancing of Peak Currents Between Paralleled MOSFETs With Unequal Threshold Voltages

Abstract: The peak switching currents of two paralleled MOSFETs turned on/off by one gate driver could differ significantly owing to the mismatch in threshold voltages (Vth). The passive balancing method described herein employs one inductor and one resistor per MOSFET to force the currents to track with negligible penalty in loss. Sensors, feedbacks, and knowledge of gate-related parameters (such as gate charge, polarity of Vth difference, gate impedances, etc.) are not required. The passive components are designed using an inequality involving Vth, rise time, and unbalance percentage. The mismatch in peak currents is reduced from 15% to 1% between SiC MOSFETs tested at 20 A and 300 V with 19% Vth variation.

Demonstration of a scalable frequency-domain readout of metallic magnetic calorimeters by means of a microwave SQUID multiplexer

Abstract: We report on the first demonstration of a scalable GHz frequency-domain readout of metallic magnetic calorimeters (MMCs) using a 64 pixel detector array that is read out by an integrated, on-chip microwave SQUID multiplexer. The detector array is optimized for detecting soft X-ray photons and the multiplexer is designed to provide a signal rise time τrise<400ns and an intrinsic energy sensitivity ϵ<30h. This results in an expected energy resolution ΔEFWHM<10eV. We measured a signal rise time τrise as low as 90ns and an energy resolution ΔEFWHM as low as 50eV for 5.9keV photons. The rise time is about an order of magnitude faster compared to other multiplexed low-temperature microcalorimeters and close to the intrinsic value set by the coupling between electron and spins. The energy resolution is degraded with respect to our design value due to a rather low intrinsic quality factor of the microwave resonators that is caused by the quality of the Josephson junction of the associated rf-SQUID as well as an e...

Enhanced Response Time of Electrowetting Lenses with Shaped Input Voltage Functions

Abstract: Adaptive optical lenses based on the electrowetting principle are being rapidly implemented in many applications, such as microscopy, remote sensing, displays, and optical communication. To characterize the response of these electrowetting lenses, the dependence upon direct current (DC) driving voltage functions was investigated in a low-viscosity liquid system. Cylindrical lenses with inner diameters of 2.45 and 3.95 mm were used to characterize the dynamic behavior of the liquids under DC voltage electrowetting actuation. With the increase of the rise time of the input exponential driving voltage, the originally underdamped system response can be damped, enabling a smooth response from the lens. We experimentally determined the optimal rise times for the fastest response from the lenses. We have also performed numerical simulations of the lens actuation with input exponential driving voltage to understand the variation in the dynamics of the liquid–liquid interface with various input rise times. We furt...

Gain and time resolution of 45 μm thin Low Gain Avalanche Detectors before and after irradiation up to a fluence of 1015 neq/cm2

Abstract: Low Gain Avalanche Detectors (LGADs) are silicon sensors with a built-in charge multiplication layer providing a gain of typically 10 to 50. Due to the combination of high signal-to-noise ratio and short rise time, thin LGADs provide good time resolutions. LGADs with an active thickness of about 45 μm were produced at CNM Barcelona. Their gains and time resolutions were studied in beam tests for two different multiplication layer implantation doses, as well as before and after irradiation with neutrons up to 1015 neq/cm2. The gain showed the expected decrease at a fixed voltage for a lower initial implantation dose, as well as for a higher fluence due to effective acceptor removal in the multiplication layer. Time resolutions below 30 ps were obtained at the highest applied voltages for both implantation doses before irradiation. Also after an intermediate fluence of 3×1014 neq/cm2, similar values were measured since a higher applicable reverse bias voltage could recover most of the pre-irradiation gain. At 1015 neq/cm2, the time resolution at the maximum applicable voltage of 620 V during the beam test was measured to be 57 ps since the voltage stability was not good enough to compensate for the gain layer loss. The time resolutions were found to follow approximately a universal function of gain for all implantation doses and fluences.

Movable Noncontact RF Current Measurement on a PCB Trace

Abstract: This paper develops a movable noncontact probing method of a radio frequency current on a printed trace based on the electromagnetic induction The current measurement method is validated by comparing a series of reconstructed periodic and pulse currents with those as known input signals on a microstrip line According to our measurement, the transfer impedance from the current under measurement to the probe has a linear frequency range from 10 MHz to 22 GHz The reconstruction measurement can be achieved even for a random periodic noise with a duration of 1 ns and a single pulse with a rise time of 2 ns Owing to the flexible relocation of the probe, the measurement accuracy due to the errors of spatial displacement and the probe placement angle is investigated in detail It is shown that the dynamic parameters of reconstructed current, such as fall time and rise time, are insensitive to the spatial errors, and instead the amplitudes of both periodic and pulse currents are highly sensitive, especially to a horizontal displacement

Properties of HPK UFSD after neutron irradiation up to 6e15 n/cm2

Abstract: In this paper we report results from a neutron irradiation campaign of Ultra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15, 3e15, 6e15 n/cm2. The UFSD used in this study are circular 50 micro-meter thick Low-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. They have been produced by Hamamatsu Photonics (HPK), Japan, with pre-radiation internal gain in the range 10-100 depending on the bias voltage. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particle (MIPs) from a 90Sr based \b{eta}-source. The leakage current, internal gain and the timing resolution were measured as a function of bias voltage at -20C and -30C. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both. The dependence of the gain upon the irradiation fluence is consistent with the concept of acceptor removal and the gain decreases from about 80 pre-irradiation to 7 after a fluence of 6e15 n/cm2. Consequently, the timing resolution was found to deteriorate from 20 ps to 50 ps. The results indicate that the most accurate time resolution is obtained at a value of the constant fraction discriminator (CFD) threshold used to determine the time of arrival varying with fluence, from 10% pre-radiation to 60% at the highest fluence. Key changes to the pulse shape induced by irradiation, i.e. (i) a reduce sensitivity of the pulse shape on the initial non-uniform charge deposition, (ii) the shortening of the rise time and (iii) the reduced pulse height, were compared with the WF2 simulation program and found to be in agreement.

Comparison of Logic Performance of CMOS Circuits Implemented With Junctionless and Inversion-Mode FinFETs

Abstract: In this paper, we report the logic performance of CMOS circuits implemented with n- and p-channel junctionless (JL) FinFETs. A one-to-one comparison of the performance is made between such circuits and those implemented with n- and p-channel conventional inversion-mode (IM) FinFETs. The logic performance of a CMOS inverter is evaluated in terms of rise time and fall time for three different future technology nodes with the help of extensive 3-D device and mixed-mode circuit simulators. Frequency of oscillation of a three-stage ring oscillator (RO) is also obtained for such nodes. In spite of reduced ON-state current arising out of the higher channel doping and, hence, reduced carrier mobility in the JL devices, somewhat improved performance of the JL CMOS circuits over their IM counterparts are observed. For example, improvement of ~28% and ~10% in the rise time and fall time, respectively, for the inverter and ~12% in the frequency of oscillation of RO are observed for 10-nm technology node. Our investigations reveal reduced gate capacitance of the JL devices that in turn result in such improved performance of the JL CMOS circuits. Reduced gate capacitance also results in reduced dynamic power dissipation of JL CMOS circuits.

Circuit-Parameter-Independent Nonlinearity Compensation for Boost Converter Operated in Discontinuous Current Mode

Abstract: This paper proposes a current control method for discontinuous current mode (DCM) in order to achieve the same control performance as continuous current mode (CCM) in a boost converter. By utilizing the duty ratio at the previous calculation period to compensate for a DCM nonlinearity, the controller, which is designed for CCM, can also be used in DCM. In the frequency analysis, the cutoff frequency of the proposed DCM current control agrees exactly to the design value, which is 2 kHz, whereas the cutoff frequency of the conventional DCM current control results in high error of 47.5%. In the current step response experiment with a 360-W prototype and the switching frequency of 20 kHz, the experimental DCM current response almost agrees with the conventional CCM current response, which are 380-μs rise time for both CCM and DCM, 9% and 8% overshoot for CCM and DCM, respectively. Furthermore, the computation time of the proposed DCM current control is shorter 35% than the conventional DCM current control.

DC-Current-Limiting Characteristics of YBCO Tapes for DC Currents of 50 A to 10 kA

Abstract: The amplitude and rise time of fault currents can be reduced by installing a resistive-type superconducting fault current limiter (SFCL) in a voltage-source converter high-voltage DC (VSC-HVDC) system. Such an SFCL can provide sufficient time for a mechanical-type dc circuit breaker to interrupt a fault current. The objective of this paper is to elucidate the dc-current-limiting characteristics of Yttrium Barium Copper Oxygen (YBCO)-coated conductor tapes for a wide range of prospective currents (ranging from 50 to 10 kA). The factors investigated include critical currents (I c ) of the tape of 102 and 225 A, circuit time constants of 1 and 3 ms, and rated dc voltages of 250 and 500 V. The experimental results show that the limited current remains nearly constant with prospective current when the prospective current is higher than ten times I c . The circuit time constant affects only the peak value and rise rate of the limited current. A single tape (I c = 102 A) can limit a 10-kA prospective current to a peak value of 555 A. The limited current for a rated voltage of 500 V is always higher than that for a rated voltage of 250 V.

An Empirical Fitting Method for Type Ia Supernova Light Curves. II. Estimating the First-light Time and Rise Time

Abstract: We investigate a new empirical fitting method for the optical light curves of Type Ia supernovae (SNe~Ia) that is able to estimate the first-light time of SNe~Ia, even when they are not discovered extremely early. With an improved ability to estimate the time of first light for SNe Ia, we compute the rise times for a sample of 56 well-observed SNe~Ia. We find rise times ranging from 10.5 to 20.5 days, with a mean of 16.0 days, and confirm that the rise time is generally correlated with the decline rate $\Delta m_{15}(B)$, but with large scatter. The rise time could be an additional parameter to help classify SN~Ia subtypes.

Note: An avalanche transistor-based nanosecond pulse generator with 25 MHz repetition rate.

Abstract: We have developed an avalanche transistor-based pulse generator for driving the photocathode of an image intensifier, which comprises a mainly capacitive load on the order of 100 pF. The circuit produces flat-top pulses with a rise time of 2 ns, a FWHM of 10 ns, and an amplitude of tens of V at a high repetition rate in the range of tens of MHz. The generator is built of identical avalanche transistor sections connected in parallel and triggered in a sequence, synchronized to a reference rf signal. The described circuit and mode of operation overcome the power dissipation limit of avalanche transistor generators and enable a significant increase of pulse repetition rates. Our approach is naturally suited for synchronized imaging applications at low light levels.

Note: A high-frequency signal generator based on direct digital synthesizer and field-programmable gate array

Abstract: A high-frequency signal generator based on direct digital synthesizer (DDS) and field-programmable gate array (FPGA) is presented. The FPGA provides the controlling time sequence for the DDS, which has a highest output frequency of 1.4 GHz and a frequency resolution of 190 pHz. At an output frequency of 1.2 GHz, the measured phase noise, including the contribution of the reference clock, is -65 dBc/Hz@1 Hz, while the intrinsic phase noise is -82 dBc/Hz@1 Hz. Time delay of the DDS is measured to be less than 150 ns. The signal generator is used to drive an acousto-optic modulator, and the rise time due to the whole link is 24 ns. The developed signal generator can be used in many precision measurement experiments in the fields of atomic, molecular, and optical physics.

Rise time of proton cut-off energy in 2D and 3D PIC simulations

Abstract: The Target Normal Sheath Acceleration regime for proton acceleration by laser pulses is experimentally consolidated and fairly well understood. However, uncertainties remain in the analysis of particle-in-cell simulation results. The energy spectrum is exponential with a cut-off, but the maximum energy depends on the simulation time, following different laws in two and three dimensional (2D, 3D) PIC simulations so that the determination of an asymptotic value has some arbitrariness. We propose two empirical laws for the rise time of the cut-off energy in 2D and 3D PIC simulations, suggested by a model in which the proton acceleration is due to a surface charge distribution on the target rear side. The kinetic energy of the protons that we obtain follows two distinct laws, which appear to be nicely satisfied by PIC simulations, for a model target given by a uniform foil plus a contaminant layer that is hydrogen-rich. The laws depend on two parameters: the scaling time, at which the energy starts to rise, a...

A Fully Integrated Fast-Response LDO Voltage Regulator with Adaptive Transient Current Distribution

Abstract: A fully integrated low-dropout (LDO) regulator with fast transient response is proposed in this paper. The capacitor-less LDO (CL-LDO) regulator incorporates both assisted pass-transistors and control circuit to realize adaptive transient current distribution during the load current transition, thereby enhancing the transient response and minimizing the output voltage's spike. In 65-nm CMOS process, the CL-LDO regulator occupies an active area of 0.0088 mm2. It supplies an output voltage of 1.2 V, while the input supply ranges from 1.5 V to 2.5 V. Subjected to a 100 µA ± 10 mA step change of load current with 1-µs rise time and fall time, the regulator can settle the output to a stable voltage within 1.1 µs and the output voltage's spike is reduced to less than ± 20 mV. The line regulation and load regulation of this regulator are 0.52 mV/V and 0.01 mV/mA, respectively.

Robust fast controller design via nonlinear fractional differential equations.

Abstract: A new method for linear system controller design is proposed whereby the closed-loop system achieves both robustness and fast response. The robustness performance considered here means the damping ratio of closed-loop system can keep its desired value under system parameter perturbation, while the fast response, represented by rise time of system output, can be improved by tuning the controller parameter. We exploit techniques from both the nonlinear systems control and the fractional order systems control to derive a novel nonlinear fractional order controller. For theoretical analysis of the closed-loop system performance, two comparison theorems are developed for a class of fractional differential equations. Moreover, the rise time of the closed-loop system can be estimated, which facilitates our controller design to satisfy the fast response performance and maintain the robustness. Finally, numerical examples are given to illustrate the effectiveness of our methods.

The influence of magnitude and rise time of applied voltage and the type of oil on streamer growth in a wet-mate DC connector

Abstract: For the safe design and operation of wet-mate (WM) DC power connector, a time-dependent full thermo-electrodynamic model comprised of Poisson's equation, three charge continuity equations-one each for the positive and negative ions and one for the electrons-, and a thermal diffusion equation is developed to study the streamer initiation and propagation in the oil portion of a WM DC chamber. The electric field dependent molecular ionization mechanism accounts for the source term for free charge carriers, and positive ion/electron recombination, positive/negative ion recombination and electron attachment represent sink terms in the oil section. The solid portion of the WM DC connector is modeled as a perfect insulator. Considering a needle-sphere electrode geometry with electrodes covered by a dielectric solid and oil enclosed by the dielectric solid, it is approached the complicated solid-liquid insulation system envisaged in a WM DC connector after mating. By using the model, the influence of three parameters including magnitude and rise time of applied voltage as well as the type of oil on streamer initiation and propagation is investigated. It is found that the shorter rise time the more prominent streamer growth in the oil portion. For oil comprising only aromatics, an electric field magnitude larger than about 2×108 V/m is needed to propagate streamers, while this value for the oil comprising naphthenics/paraffinics will be exceeding 4×108 V/m.

Optimal Pulse Processing, Pile-Up Decomposition, and Applications of Silicon Drift Detectors at LCLS

Abstract: Silicon drift detectors (SDDs) revolutionized spectroscopy in fields as diverse as geology and dentistry. For a subset of experiments at ultrafast, X-ray free-electron lasers (FELs), SDDs can make substantial contributions. Often the unknown spectrum is interesting, carrying science data, or the background measurement is useful to identify unexpected signals. Many measurements involve only several discrete photon energies known a priori , allowing single-event decomposition of pile-up and spectroscopic photon counting. We designed a pulse function and demonstrated that the signal amplitude (i.e., proportional to the detected energy and obtained from fitting with the pulse function), rise time, and pulse height are interrelated, and at short peaking times, the pulse height and pulse area are not optimal estimators for detected energy; instead, the signal amplitude and rise time are obtained for each pulse by fitting, thus removing the need for pulse shaping. By avoiding pulse shaping, rise times of tens of nanoseconds resulted in reduced pulse pile-up and allowed decomposition of remaining pulse pile-up at photon separation times down to hundreds of nanoseconds while yielding time-of-arrival information with the precision of 10 ns. Waveform fitting yields simultaneously high energy resolution and high counting rates (two orders of magnitude higher than current digital pulse processors). At pulsed sources or high photon rates, photon pile-up still occurs. We showed that pile-up spectrum fitting is relatively simple and preferable to pile-up spectrum deconvolution. We developed a photon pile-up statistical model for constant intensity sources, extended it to variable intensity sources (typical for FELs), and used it to fit a complex pile-up spectrum. We subsequently developed a Bayesian pile-up decomposition method that allows decomposing pile-up of single events with up to six photons from six monochromatic lines with 99% accuracy. The usefulness of SDDs will continue into the X-ray FEL era of science. Their successors, the ePixS hybrid pixel detectors, already offer hundreds of pixels, each with a similar performance to an SDD, in a compact, robust and affordable package.

Ultrafast Reverse Recovery Time Measurement for Wide-Bandgap Diodes

Abstract: A system is presented that is capable of measuring subnanosecond reverse recovery times of diodes in wide-bandgap materials over a wide range of forward biases (0 – 1 A) and reverse voltages (0 – 10 kV). The system utilizes the step recovery technique and comprises a cable pulser based on a silicon (Si) Photoconductive Semiconductor Switch (PCSS) triggered with an Ultrashort Pulse Laser, a pulse charging circuit, a diode biasing circuit, and resistive and capacitive voltage monitors. The PCSS-based cable pulser transmits a 130 ps rise time pulse down a transmission line to a capacitively coupled diode, which acts as the terminating element of the transmission line. The temporal nature of the pulse reflected by the diode provides the reverse recovery characteristics of the diode, measured with a high bandwidth capacitive probe integrated into the cable pulser. This system was used to measure the reverse recovery times (including the creation and charging of the depletion region) for two Avogy gallium nitride diodes; the initial reverse recovery time was found to be 4 ns and varied minimally over reverse biases of 50–100 V and forward current of 1–100 mA.

Axial magnetic field injection in magnetized liner inertial fusion

Abstract: MagLIF is a fusion concept using a Z-pinch implosion to reach thermonuclear fusion. In current experiments, the implosion is driven by the Z-machine using 19 MA of electrical current with a rise time of 100 ns. MagLIF requires an initial axial magnetic field of 30 T to reduce heat losses to the liner wall during compression and to confine alpha particles during fusion burn. This field is generated well before the current ramp starts and needs to penetrate the transmission lines of the pulsed-power generator, as well as the liner itself. Consequently, the axial field rise time must exceed hundreds of microseconds. Any coil capable of being submitted to such a field for that length of time is inevitably bulky. The space required to fit the coil near the liner, increases the inductance of the load. In turn, the total current delivered to the load decreases since the voltage is limited by driver design. Yet, the large amount of current provided by the Z-machine can be used to produce the required 30 T field b...

An avalanche transistor-based nanosecond pulse generator with 25 MHz repetition rate

Abstract: We have developed an avalanche transistor-based pulse generator for driving the photocathode of an image intensifier, which comprises a mainly capacitive load on the order of 100 pF. The circuit produces flat-top pulses with rise time of 2 ns, FWHM of 10 ns and amplitude of tens of V at a high repetition rate in the range of tens of MHz. The generator is built of identical avalanche transistor sections connected in parallel and triggered in a sequence, synchronized to a reference rf signal. The described circuit and mode of operation overcome the power dissipation limit of avalanche transistor generators and enable a significant increase of pulse repetition rates. Our approach is naturally suited for synchronized imaging applications at low light levels.

Abatement of NOX using compact high voltage pulse power supply: Towards retrofitting to automobile vehicle

Abstract: The research work in this paper is based on generation of high voltage pulses using compact power supply sources for abatement of NOX from the diesel engine exhaust. The high voltage source is made up of zero voltage switch cascaded with flyback converter to produce high voltage AC, a Cockroft-Walton voltage multiplier to generate high voltage DC and a rotary spark gap for the production of high voltage pulses of rise time in the range of nano seconds. The rectifier based power supply is validated by a 12 V battery supply. To corroborate the feasibility of proposed methodology in diesel operated vehicles, the experiments are conducted with a 12 V battery. Performance of various shapes of electrodes (cylindrical 5mm diameter, square 5 mm diagonal and hexagonal with vertex to vertex diameter of 5 mm) at different flow rates of 2 l/min, 4 l/min and 6 l/min are also examined.

A Nonlinear Transmission Line Based Harmonic RF Tag

Abstract: This paper describes the design and development of a nonlinear transmission line based passive RFID tag. When the tag receives a signal from the interrogator, it generates harmonics which are transmitted back to interrogator. The reflected signal is distorted in time domain due to the harmonic contents. Hence, a change in the rise time would be observed in the received signal. Experimentally, a decrease in rise time by 14% is demonstrated using the wireless tag at a distance of 100mm from the interrogator. The tag was designed to operate in the range of 100 MHz to 500 MHz with Bragg's cutoff frequency of 1.2 GHz. This circuit will find applications in tagging, tracking and sensing.

A Sizing Methodology for Rise-Time Minimization of Dickson Charge Pumps With Capacitive Loads

Abstract: A novel sizing methodology for Dickson charge pumps with pure capacitive loads is presented. The methodology is based on dynamic analysis to minimize the rise time of the charge pump up to 25% under a given circuit area. The methodology is validated through the implementation of a six-stage charge pump-based driver in 180-nm standard low-voltage CMOS technology. The driver is used for the excitation of ultrasonic transducers with 34 V at a resonance frequency of 220 KHz. A rise time of only 512 nS is achieved. The driver consumes 10.6 mA drawn from a 5-V supply at a pumping frequency of 50 MHz and occupies an area of 0.2 mm2.

Partial discharge characteristics of electrical treeing in XLPE insulation exposed to voltages of different rise times

Abstract: The use of pulse width modulated (PWM) waveforms allows a more flexible energy applications and management. The downside is that under such operating conditions the voltage stress imposed on the insulation systems increases, particularly as the frequency content is considerably higher than at the conventional 50 Hz sinusoidal waveform. Among various degradation mechanisms, electrical treeing is one important processes and it can be linked with a presence of partial discharge (PD) activity. In this paper, the changes in PD characteristics at different stages of degradation by electrical treeing were investigated for XLPE based insulation exposed to rapidly changing voltages of varying magnitude. The tested material samples were subjected to two square voltage shapes, each characterized by their rise-time. Here the shorter was 1 ps and the longer 0.5 ms, all tests run at 414 Hz. To compare the voltage endurance, the voltage level was gradually increased until detectable PD activity and tree initiation could be observed, both electrically and optically. Typically continuous PD appearance was observed between 23 and 32 kVpp. The total number of PDs and their characteristics were monitored as the trees were gradually growing. Here the experimental results shows that the PDs appears at lower voltage magnitudes for the shorter rise times and that the trees had a significantly less branched appearance as compared to the longer voltage rise time. These results resemble the observations that recently been presented with tests on DC voltage pre-stressed material samples exposed to impulses of reversed polarity. Additionally the measured PD characteristics were non-symmetric, having a clear polarity dependence. This finding suggests a possibility to use the presented approach for a more detailed investigating of degradation processes in solid insulation systems suitable for both HVAC and HVDC applications.