Bipolar AC Research Articles

An Isolated Identical Bipolar Buck-Boost AC–AC Converter With Step-Changed Frequency Operation and Reduced Active and Passive Components

An Isolated Identical Bipolar Buck-Boost AC–AC Converter With Step-Changed Frequency Operation and Reduced Active and Passive Components

Nondifferential AC Choppers Based Identical Bipolar Buck–Boost AC–AC Converter Without Commutation Issue

This article proposes a new H-bridge structured bipolar pulsewidth modulation (PWM) buck–boost ac–ac converter based on the nondifferential ac choppers. The proposed converter can generate simple identical noninverting and inverting buck–boost voltage outputs by modulating one phase leg with single control duty ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${d}_1$</tex-math></inline-formula> or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${d}_2$</tex-math></inline-formula> ). It can also provide flexible identical noninverting and inverting buck–boost operations by modulating both phase legs with two control duty ratios ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${d}_1$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${d}_2$</tex-math></inline-formula> ). In the proposed topology, the use of nondifferential ac choppers eliminates the complementary switching of ac switches and related inductor open-circuit issue. Moreover, half the switching devices are configured as switching-cell units to avoid shoot-through occurrence due to accidental turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> of complementary switches. Therefore, the proposed topology is free from commutation issue and can abolish PWM deadtimes to produce high-quality output with improved utilization of switch duty ratios. It also provides continuous input current and can perform normally with reactive loads. It has the potential to mitigate both voltage sags and swells (due to bipolar operation) when used as series voltage compensator; and unlike the existing nondifferential bipolar ac–ac converters, it can provide compensation for more intense voltage sags (of above 50%) as well. In-depth analysis of the proposed topology and hardware verifications are provided in this article.

Effects of DC and AC stress on the VT shift of AlGaN/GaN MIS-HEMTs

AlGaN/GaN MIS-HEMTs with adjusted VT were fabricated using a recess gate to investigate the effect on actual operation when the polarity of the gate voltage is opposite in the on- and off-state. The direction and time exponents of VT shift depend on the polarity of the gate bias stress. Electrons detrapping from the Al2O3/AlGaN interface trap site to AlGaN under negative gate bias stress has to overcome the energy barrier, resulting in a higher temperature dependence. In addition, the unaffected gm and SS show that the degradation occurred primarily at the Al2O3/AlGaN interface rather than channel or mobility degradation. For unipolar and bipolar AC stresses, the time exponent of the VT shift during stress time has two values, and a relatively low value during relaxation after bipolar AC stress. These results may be due to the further degradation by Vmin at the broader energy levels of the Al2O3/AlGaN interface.

Optical Detection of Partial Discharges Under Fast Rising Square Voltages in Dielectric Liquids

Partial discharge (PD) measurement of an insulation system is widely used as a diagnostic tool for the quality control of high voltage apparatuses. In ac, the conventional method of PD detection is well established. However, under square voltages such as those generated across power electronic converters, the conventional method cannot discriminate between PDs and switching transients. In this context, this paper demonstrates PD detection under fast-rising square voltages by deploying a photomultiplier tube. For this, an experimental set-up generating unipolar square voltage (USV) waveforms is developed, generating a dV/dt in the range of 10-100 kV/&#x03BC;s. The device under test (DUT) comprises of needle-barrier plane configuration immersed in dielectric liquids. Barriers differing in permittivity and thicknesses are chosen to test with two types of dielectric liquids, to assess varying conditions. The results thus obtained show a lower partial discharge inception voltage (PDIV) in USV than in bipolar ac for the same DUT specimen. The phase-resolved (PR) PD patterns exhibit a higher PD occurrence during rising and falling front of the USV waveform. Experimental results demonstrate the relevance of the PMT as a PD detector for square voltage waveforms exhibiting high dV/dt in dielectric liquids.

Intersystem fault between MMC‐HVDC and AC systems and its impact on DC/AC protection

Flexible DC and AC transmission lines can be arranged across vertically or sharing transmission corridor. Under this condition, the possibility of intersystem fault increases significantly, which endangers the safety of both DC and AC systems. Here, the intersystem fault between bipolar half-bridge MMC-HVDC system and AC system is analysed. The reliability of DC wavefront protection after intersystem fault is analysed based on initial traveling wave, while the reliability of DC intersystem fault protection and AC protection is analysed based on 50 Hz component characteristics. Since wavefront protection cannot be guaranteed to operate reliably, intersystem fault protection based on 50 Hz components is applied to DC system to reliably detect intersystem faults and isolate the fault by commanding DC circuit breaker to trip. Although current differential relaying and zero-sequence overcurrent relaying in AC system can reliably detect intersystem faults, a large DC offset prevents effective fault isolation of AC circuit breaker. This highlights the necessity of MMC-HVDC system to reliably isolate intersystem faults.

A High-Frequency Isolated Multilevel Cascaded-Type Bipolar Direct PWM AC–AC Converter for Utility Voltage Compensation

A high-frequency isolated multilevel cascaded-type bipolar direct PWM ac-ac converter is proposed. By cascading N-unit cells, the converter can produce symmetric bipolar voltage gain of ±ND. It does not require a separate ac source for each unit cell, and can provide higher ac output voltages with multiple levels using low-voltage rating devices, decreasing output voltage harmonics and filtering requirements. Moreover, phase-shift PWM switching is employed, increasing the effective frequency of the output filter, further reducing its requirements. The converter can be utilized as a dynamic voltage restorer (DVR) for grid voltage regulation, with the following features: No need for an external dc source, separate ac sources, and a line-frequency voltage injection transformer. Injection of higher voltage magnitudes than the line voltage, mitigating deep voltage sags. Injection of a negative series voltage to provide compensation for voltage swells. The detailed circuit operation is explained together with component design guidelines and application as DVR. Finally, experimental results are provided using an 800-W scale-down laboratory prototype.

Bipolar AC (Bipac) Switch With Buried Layer for Specific AC Mains Applications

A new vertical bipolar bidirectional switch (Bipac) with a buried layer is proposed for specific ac mains applications (230 V-50 Hz). It is mainly dedicated for the low load current ones (0.5 Arms) and must support a voltage of 750 V in the blocking state. Moreover, the Si-chip area must not exceed 10 mm2. The study of the new proposed Bipac structure is carried out using 2-D Sentaurus physical simulation. The operating principles are first validated, and then the physical and geometrical parameters of the buried layer are determined to meet the specifications. As compared to the classical Bipac, the Bipac with a buried layer exhibits a much higher current gain that makes it more attractive in replacing the triac in the targeted applications.

Bipolar AC Switch for Specific Mains Applications: Design, Realization, and Characterization

This paper deals with the design of an ac switch structure for specific ac mains applications 230 V–50 Hz. The targeted power level is about a hundred watts, and the currently used converter circuits make use of bidirectional switches that are realized using anti-series connected MOS transistors. Despite the improvements in performance provided by some of these structures, their fabrication cost is still high and limits their widespread diffusion in a market shared with the triac. To replace the triac, an original current and voltage bidirectional bipolar device called a Bipolar ac (Bipac) is proposed, designed, realized, and characterized. It can be controlled both to turn-on and turn-off with respect to a single reference electrode. It exhibits a very low ON-state voltage drop that makes it interesting for specific mains applications with low load current (0.5 $\text{A}_{{\text {rms}}}$ ). The study of the Bipac structure is carried out using 2-D Sentaurus physical simulations. The Bipac structure is realized on n-type and on p-type substrates for two different wafer thicknesses. The operating modes of the monolithic bidirectional Bipac switch were validated through electrical characterizations.

Interface state degradation during AC positive bias temperature instability stress

The reliability of a bulk fin field-effect transistor (FinFET) with a high-k dielectric/metal-gate stack has been investigated by comparing the effects of DC and AC stresses. It is well known that the relaxation during the off-cycle of the AC stress decreases the Vth shift and enhances the device lifetime due to electron detrapping from the high-k dielectric. We found that the relaxation in the interface traps is significantly weaker than that of bulk traps during the unipolar and bipolar AC stresses. The weak recovery is attributed to the concurrent interface state generation during a positive-bias temperature instability (PBTI) stress. Eventually, the interface traps became a major source of the device drift (over 60%) at the high temperature of 400 K. This finding suggests that a new strategy is required to address the PBTI reliability focusing on the residual interface states as well as the bulk trapping, particularly at a high temperature.

Understanding BTI in SiC MOSFETs and Its Impact on Circuit Operation

The threshold voltage hysteresis in SiC power MOSFETs is rarely studied. This paper investigates the captureand emission-time constants of positive and negative charge trapped in the gate oxide and at the interface as a function of gate bias. We present a measurement technique which enables time-resolved measurement of the real ${V} _{\text {th}}$ during application-relevant bipolar ac high temperature gate stress. In addition, we use capture and emission time maps to explain the temperature dependence of $\Delta {\text V}_{\text {th}}$ after stress and are able to simulate $\Delta {\text V}_{\text {th}}$ after positive ac stress considering the full stress-history. Furthermore, we will show that the threshold voltage hysteresis has no harmful impact on switching operation in real applications.

Bipolar Ripple Cancellation Method to Achieve Single-Stage Electrolytic-Capacitor-Less High-Power LED Driver

Conventional topologies for high-power LED drivers with high power factors (PFs) require large capacitances to limit the low frequency (100 or 120 Hz) LED current ripples. Electrolytic capacitors are commonly used because they are the only capacitors with sufficient energy density to accommodate high-power applications. However, the short life span of electrolytic capacitors significantly reduces the life span of the entire LED lighting fixture, which is undesirable. This paper proposes a bipolar (ac) ripple cancellation method with two different full-bridge power structures to cancel the low-frequency ac ripple in the LED current and minimize the output capacitance requirement, enabling the use of long-life film capacitors. Compared with the existing technologies, the proposed circuit achieves zero double-line-frequency current ripple through LED lamps and achieves a high PF and high efficiency. A 100-W (150 V/0.7 A) LED driver prototype was built which demonstrates that the proposed method can achieve the same double-line-frequency LED current ripple with only 44- $\mu \text{F}$ film capacitors, compared with the 4700- $\mu \text{F}$ electrolytic capacitors required in the conventional single-stage LED drivers. Meanwhile, the proposed prototype has achieved a peak power efficiency of 92.5%, benefiting from active clamp technology.

Observation and Control of Hot Atom Damage in Ferroelectric Devices

Ferroelectric materials are the most common example of a Landau structure, defined as a system having an atom/mass moving in a double-well energy landscape. These materials have applications in memories, actuators, low power logic transistors, and so on. For a bipolar ac signal typical in most of the applications, one suspects that the repeated roller coaster shuttling of the moving atoms located microscopically at the domain walls could lead to bond dissociation, suggesting a new channel for defect generation with no classical counterpart. Here, we demonstrate that once the bipolar pulses initiate transfer of atoms between the energy pockets, the transient overshoot away from their equilibrium positions (hot atoms) leads to significant increase in defect generation. We interpret the degradation theoretically and demonstrate a set of soft-switching schemes to control the hot atom damage and to improve the device lifetime dramatically. The damage mechanism should be generic in other Landau structures, such as microelectromechanical systems, nonvolatile memories, and analogous control strategies should improve the lifetime of all such bistable devices.

AC Bias-Temperature Stability of a-InGaZnO Thin-Film Transistors With Metal Source/Drain Recessed Electrodes

In this paper, we fabricated metal source/drain recessed nearly self-aligned amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) that are highly stable under ac bias-temperature stress (BTS). For TFTs of the size W/L=60 μm/10 μm, the stress-induced threshold voltage shifts are all within -0.35 V. A comprehensive investigation of ac BTS stress polarity, frame time, and duty cycle dependence is presented in the context of high-resolution high-refresh rate active-matrix flat-panel displays. We find that higher frequency bipolar ac pulses increase the device instability. The threshold voltage instability may be reduced significantly by decreasing the duty cycle of the stress waveform.

Time-dependent dielectric breakdown (TDDB) distribution in n-MOSFET with HfSiON gate dielectrics under DC and AC stressing

This paper discusses time-dependent dielectric breakdown (TDDB) in n-FETs with HfSiON gate stacks under various stress conditions. It was found that the slope of Weibull distribution of Tbd, Weibull β, changes with stress conditions, namely, DC stress, unipolar AC stress and bipolar AC stresses. On the other hand, the time evolution component of stress-induced leakage current (SILC) was not changed by these stresses. These experimental results indicate that the modulation of electron trapping/de-trapping and hole trapping/de-trapping by stress condition changes the defect size in high-k gate dielectrics. Therefore, the control of injected carrier and the characteristics of trapping can provide the steep Weibull distribution of Tbd, leading to long-term reliability in scaled CMOS devices with high-k gate stacks.

16.3: AC and DC Bias‐Temperature Stability of Coplanar Homojunction a‐InGaZnO Thin‐Film Transistors

AbstractWe fabricated coplanar homojunction a‐IGZO TFTs that are highly stable under AC and DC bias‐temperature‐stress. For TFTs of the size W/L = 60μm/10μm, the stress‐induced threshold voltage shifts are all within −0.35 V. A comprehensive investigation of AC BTS stress polarity, pulse width, and duty cycle dependence is presented We find that higher frequency of bipolar AC pulses increases the device instability, while lower duty cycle values have the opposite effect.

Shielding Effect of HVAC Transmission Lines on the Ion-Flow Field of HVDC Transmission Lines

The HVDC and HVAC transmission lines running on the same tower is a promising technology to improve the transmission capacity of an existing corridor. The ions generated by corona discharge of HVDC transmission lines will strengthen the ground-level electric field, which may endanger human exposure. To investigate how the presence of HVAC transmission lines changes the ion-flow field, experiments are carried out on an indoor test model with bipolar dc and three-phase ac lines. The experiment results are then compared with the simulation results. The experiment and simulation results show that the HVAC transmission lines have a shielding effect on the ion-flow field from the HVDC transmission lines running above. Finally, the ground-level electric field and ion current density of the ±800-kV HVDC and 500-kV HVAC transmission lines running on the same tower are calculated.

Nonlinear magnetoelectric response of a Metglas/piezofiber laminate to a high-frequency bipolar AC magnetic field

A nonlinear magnetoelectric (ME) response has been investigated in a Metglas/piezofiber laminate by applying a bipolar AC magnetic field (Hac) without a DC magnetic bias. The ME voltage (VME) was measured for various amplitudes of Hac of up to 9 Oe over the frequency (f) range 0.1 &amp;lt; f &amp;lt; 40 kHz. Compared to the linear ME behavior, an anomalous response to f and Hac was observed, which is believed to be associated with the Eddy-current loss in the Metglas layers. A frequency multiplication effect was also observed for VME, which was understood by Fourier analysis of the nonlinearity in the magnetostriction.

Polarization Reversal in Relaxor PZN-PT Single Crystals

The polarization reversal under application of bipolar ac field and dielectric properties were investigated in wide temperature range in PZN-xPT (x = 4.5 and 8%) single crystals. The observed temperature dependences of the dielectric permittivity during heating after field-cooling and zero-field-cooling were discussed on the basis of several phase transformations and existing of the heterophase structures in given temperature range. The “doubling” of the hysteresis loops reveled during heating in limited temperature range was explained using quasi-static Preisach approach taking into account the key role of inhomogeneous depolarization field Edep.

Electrical stress-induced instability of amorphous indium-gallium-zinc oxide thin-film transistors under bipolar ac stress

Bipolar ac stress-induced instability of amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors is comparatively investigated with that under a positive dc gate bias stress. While the positive dc gate bias stress-induced threshold voltage shift (ΔVT) is caused by the charge trapping into the interface/gate dielectric as reported in previous works, the dominant mechanism of the ac stress-induced ΔVT is observed to be due to the increase in the acceptorlike deep states of the density of states (DOS) in the a-IGZO active layer. Furthermore, it is found that the variation of deep states in the DOS makes a parallel shift in the IDS-VGS curve with an insignificant change in the subthreshold slope, as well as the deformation of the CG-VG curves.

Reorientational Motion of Polymer-Stabilized Ferroelectric Liquid Crystals

A polymer-stabilized ferroelectric liquid crystals (PSFLCs), in which photocurable mesogenic monomers are doped into an FLC and then a UV photocure is carried out in the SmC* phase under the application of a bipolar AC electric field, can exhibits monostable and V-shaped electrooptical characteristics. In this research, the time-resolved ellipsometry is applied to the PSFLCs, and we investigate the dependence of the molecular reorientational motion of the PSFLC on the photocuring condition such as the tilt angle which FLC molecules take in the photocure stage. The measurement results show that the dynamic behavior of PSFLC-molecular reorientation strongly depends on the photocuring condition of the tilt angle in addition to the concentration of polymer. Furthermore, the molecular reorientation of the PSFLC is 2-steps motion originated in the difference of the threshold voltage between near and far from polymer.