Reverse Capability Research Articles

A current source PWM inverter with actively commutated SCRs

Conventional SCR based current source inverters suffer from poor waveform quality due to six step switching. Pulse width modulated current source inverters typically require gate turn off devices with reverse voltage blocking capability which have limited their application. In this paper, a new pulse width modulated current source inverter topology using one gate turn off switch and six SCRs is presented. The converter uses active commutation to realize pulse width modulation in a conventional SCR based current source inverter. Modulation techniques for the proposed inverter, simulation and experimental results are described in the paper. This topology is suitable for high performance, high power applications.

Selective doping of 4H–SiC by codiffusion of aluminum and boron

A graphite mask was used to realize selective doping of aluminum/boron in 4H–SiC by thermal diffusion at a temperature range of 1800–2100 °C. The doping profiles investigated by secondary ion mass spectrometry show that a high aluminum concentration of 5×1019 cm−3 near the surface and linearly graded boron profile up to several micrometers in depth can be obtained. Hall effect measurement was also employed to obtain the electrical characteristics of the diffused region, from which the carrier concentration (1×1019 cm−3) and hole mobility (7 cm2/V s) at room temperature were extracted. Room temperature photoluminescence indicates that the dominant luminescence is attributed to the donor acceptor pair recombination, in which boron D complex is the prevailing center rather than Al and boron shallow acceptors. Cathodoluminescence micrographs clearly illustrate a pattern with the locally diffused regions. To confirm the viability of the diffusion process, planar p-n diodes with a fairly low forward voltage drop (3.3 V at 100 A/cm2) and high reverse blocking capability (more than 1100 V) were fabricated. Built-in voltage of 2.9 V, which is typical for 4H–SiC p-n diodes, was obtained by capacitance–voltage measurement.

Application of MTO thyristors in current stiff converters with resonant snubbers

This paper addresses the impact of power devices switching characteristics on design and performance of pulsewidth-modulated current stiff converter (CSC) topologies with resonant snubbers. The MOS turn-off (MTO) thyristors were selected for this study due to their high voltage and current ratings, simplified gatedrive circuitry, unity gain turn-off, greatly reduced storage time allowing higher switching frequencies, and reverse voltage blocking capability. The analysis presented in this paper is supported by experimental data obtained from a CSC commutation cell capable of reproducing all the commutation processes in the current stiff converter topologies with active resonant snubbers. Issues involved in the implementation of the commutation cell itself and MTO characteristics relevant for their operation in CSC topologies with resonant snubbers are addressed in detail in this paper.

Propulsion airframe integration design, analysis and challenges going into the 21st century

AbstractPropulsion airframe integration design and analysis has many challenges as we move into the 21st century. Along with the conventional challenges of integrating a propulsion system on an aircraft, new technologies and new business drivers dictate innovative approaches to the design process be developed and applied.On the technical side, new innovations in jet noise suppression and new aircraft concepts such as the blended wing body (BWB) will give rise to many challenges in propulsion system performance, operability, and meeting system requirements such as thrust reverse capability. Aircraft engine and aircraft manufacturers must have the appropriate design and analysis tools in place which provide the ability to react quickly to inevitable design changes, driven by constantly changing requirements, during the product development cycle.On the business side, the rapid globalisation of the business dictates that the latest electronic technology be utilised to enable speed in communication with global customers as well as revenue sharing partners. More than ever, cost and schedules dictate the use of analytical methods to minimise the amount of qualification testing. Design and analysis software must be flexible and capable of integrating CAD/CAM and CAE tools while maintaining configuration control of the product.The following paper describes some of the new technical challenges facing the industry. Innovative methods of addressing those challenges are described.

A comparative study of C plus Al coimplantation and Al implantation in 4Hand 6H-SiC

Coimplantation of equal concentrations of C and Al in 6H- and 4H-SiC has been investigated comparatively with implantation of Al alone in the Al concentration range of N/sub Al/=8/spl times/10/sup 19/ to 2/spl times/10/sup 21/ cm/sup -3/. C-Al coimplantation at either room temperature (RT) or 600/spl deg/C, when implanted with N/sub Al/>1/spl times/10/sup 20/ cm/sup -3/, gives improvements over Al alone implantation in the specific contact resistance /spl rho//sub c/, of Al ohmic contacts on the implanted surfaces. The lowest median values of /spl rho//sub c/ obtained are 3/spl times/ and 6/spl times/10/sup -15/ /spl Omega/ cm/sup 2/ for 6H- and 4H-SiC, respectively. In the mid to high 10/sup 20/ cm/sup -3/ N/sub Al/ range, sheet resistivity of the p-type implanted layers are also reduced by the coimplantation at RT. Temperature-dependent Hall-effect measurements reveal the correlated trends in increased impurity-band conduction and high-temperature (>400 K) hole concentration. Implantation at 600/spl deg/C, by either coimplantation or Al alone implantation, degrades the electrical characteristics in the mid 10/sup 20/ cm/sup -3/ N/sub Al/ range, which implies that amorphization of as-implanted layers is necessary for effective activation of Al acceptors. It will be shown that C-Al coimplantation at RT can produce p/sup +/-n diodes with low reverse leakage currents and high forward current capability as evidenced by the 110 A/cm/sup 2/ forward current density at 2.8 V and 19 nA/cm/sup 2/ leakage current density at -100 V for 4H-SiC p/sup +/-n diodes created by C-Al coimplantation at RT.

High-power rectifier using the BSIT operation

A novel high-power rectifier using the BSIT (Bipolar mode Static Induction Transistor) operation has been demonstrated. In order to improve the reverse blocking capability of this diode, an ion implanted P-type channel structure is introduced. Device simulations have revealed that, as the tradeoff relationships between reverse blocking voltage, forward voltage drop, and reverse recovery time are considered, there exists an optimum set of the channel dosage and the channel width.

Manufacturability of heterojunction tunnel devices: further progress

A second attempt to establish the manufacturability of a simple heterojunction tunnel diode has made some significant progress towards the goal, but has failed to demonstrate a reverse engineering capability, leaving major challenges in design, materials growth and materials qualification.

Double epitaxial layer power Schottky diodes with end in the ramp oxide technique

The paper reports a new power Schottky diode (PSD) structure, with a double epitaxial layer and oxide ramp edge. This structure was designed to enhance the forward current capability and the resilience to reverse current transient pulses of the PSD with only minor decreases in the maximum working reverse voltage capability ( V RWM). Two different metallization types for the Schottky contact were investigated, namely: Ti:Al for devices intended for operation with T jmax = 150°C and Cr:Ni:Ag for diodes with T jmax = 125°C, respectively.

Reverse Rotation Capability of Spiral-Groove Gas Face Seals

The dry-running spiral groove face seals, used in centrifugal compressors, are known for trouble-free operation and reduced operating and maintenance costs. This type of seal creates a gas film between the faces which prevents contact while running. Generally, the film stiffness of a unidirectional spiral groove seal is much higher than that of a corresponding bidirectional seal. In reverse rotation, however, a spiral-groove seal loses its film stiffness and may even become negative depending on operating pressure and speed as well as the groove design parameters. Some compressors during their operating cycle may run in the reverse direction for short periods of time. This paper describes the analysis, design and testing of a spiral groove seal wit reverse rotation capability. A seal designed for a 220 mm shaft was analyzed and the spiral groove geometries optimized for good forward rotational stiffness at design pressure and speed, and acceptable reverse rotation capability at lower speed levels. The parameters for optimization include number of grooves, groove angle, groove depth and land-to-groove width ratio. The optimized spiral groove design was machined into smaller prototype hardware and tested in a test cell. Test conditions were modified to simulate an actual groove operating environment. A computerized real-time data acqusition system was utilized to record speed, pressure, temperature, leakage, and vibration readings. The seals were operated in both directions, forward and reverse, and cycled to insure reliability. These seals showed good operating characteristics in forward rotation even with high vibration levels. At significant pressure levels, the optimized spiral groove configuration is a robust design in both directions of rotation. At reduced pressure levels the reverse rotation testing evidenced slight face contact but the temperature levels and amount of wear were acceptable. The seal performance is directly related to the pressure, speed, and time of reverse rotation.

Residential photovoltaic power generating system connected to utility line

Abstract We successfully developed a utility grid interconnected photovoltaic (PV) power generating system with reverse power flow capability and installed it in an occupied private residence for the first time in Japan. The system, which consists of a 1.8-kW PV array, a 3-kVA inverter, and an interconnection protection device, has been operating satisfactorily since July 31, 1992. On a clear day, about 77% of the PV generated power was reversed to the utility between 10:00 AM and 2:00 PM. For a one-month period in May, 1993, the system generated 180 kWh of power, and fed 61% of this generated power back to the utility line. A 3-kW PV array could be installed on the roof of an ordinary Japanese house, which would supply almost all of the power required for household consumption and alleviate peak power demand.

Low forward drop JBS rectifiers fabricated using submicron technology

This paper demonstrates the impact of using submicron technology (0.5 /spl mu/m design rules) on JBS Rectifiers to achieve very low forward voltage drops while maintaining good high temperature reverse blocking characteristics. Two dimensional numerical simulations show that decreasing P/sup +/-junction width and depth improves the on-state voltage drop by improved utilization of the active area for the Schottky region and improved spreading of majority carrier current from the Schottky contact. Experimental results that demonstrate the capability to reduce the forward drop from 0.5 V to 0.25 V, while operating at up to 125/spl deg/C-175/spl deg/C with good reverse blocking capability, are presented. The tradeoff curves between forward drop and reverse leakage current show 45/spl times/ reduction in leakage current for the same forward drop as compared to previous reports on JBS rectifiers. Power dissipation analysis indicates higher operating temperatures, (100/spl deg/C for Ti-JBS and 175/spl deg/C for Cr-JBS rectifiers) with reduced heat sink sizes for the JBS Rectifiers when compared to the conventional Schottky Barrier Diode (SBD).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-resolution investigations of seismoacoustic propagation in shallow-water areas

Data from a newly developed 30-element accelerometer/hydrophone array are used to study seismoacoustic propagation in shallow-water locations off New Jersey and Martha's Vineyard. The elements of the array have 1-m spacing; each consisting of three orthogonal accelerometers (flat from 2 to 500 Hz), a hydrophone, and a vertical direction sensor. The 120 accelerometer/hydrophone signals and 30 vertical direction signals are digitized and transmitted via 1 km of fiber optic cable to a PC-type recording system. The A/D converters digitize all hydrophones at 2048 samples/s and all accelerometers at 512 samples/s. Good coupling to the bottom is obtained since sensor symmetry has been maximized; coupling to the water has been minimized; and density is matched to the sediments. There are 10-m extensions between the 30-m sensor section and the A/D converter housing at either end of the array. Units containing port-starboard paired, shotgun-shell sources were mounted within the array in these extensions and fired through the array circuitry, providing precise time and distance control and reverse profile capability. Differenced shot pairs emphasize transverse horizontal shear (SH) and Love wave signals; summed pairs emphasize P, SV, and Rayleigh/Stoneley/Scholte waves. Shear boundary wave energy is observed with frequencies greater than 60 Hz and velocities and wavelengths as low as 20 m/s and 1 m, respectively. Considerable energy appears to result from lateral heterogeneity and/or anisotropy. [Research supported by ONR.]

Translating speech by computer

People who cannot speak a word of each other's language will soon be able to talk over the phone using a sytem developed by British Telecom's Research Laboratories. The world's first instantaneous translation of speech by computer was recently carried out when simple sentences in English were translated into French and vice‐versa. The prototype equipment can translate English into German, Spanish, Swedish and Italian and the reverse capability is also being developed. This will then make possible translation between any pair of these languages, such as French‐German or Swedish‐Italian.

Capacitive Load Induced Field Stresses on Generators Connected into DC Systems

Sometimes, a synchronouis machine experiences a sudden increase. in capacitive loading, due for example to a load rejection by the blocking of an hvdc rectifier, or to the tripping of the synchronous machines when connected to a predominantly capacitive load. This sudden increase in capacitive loading can initiate oscillations in the stator circuit, which are reflected as sudden oscilldtions in the machine's field current. In machines with solid state exciters without reverse current capability, severe oscillations may momentarily try to reduce the field current to zero, and result in excessive field overvoltages. It is these overvoltages, that this paper attempts to study, through detailed simulations and also through demonstration on a laboratory setup. The situation discussed here relates to such phenomena occuring on machines connected to hvdc rectifier stations, although similar situations could also exist in other systems.

Evaluation of split-path extended range continuously-variable transmissions for automotive applications

One potentially important application of continuously-variable transmissions (CVT's) is the improvement in fuel economy of internal combustion engine vehicles. However, many otherwise attractive CVT designs have a ratio coverage less than ideal for such applications. Split-path configurations can be used to extend the ratio coverage, providing even a geared neutral and reverse capability with some combinations of parameters. The increased coverage is in general achieved, however, only at the expense of lower overall CVT efficiency and a larger required variable unit capacity. All possible single-regime designs which use a single differential to extend the ratio coverage of the basic variable unit are considered, as well as two 2- regime designs. Data for the CVT efficiency characteristics are developed for both otherwise conventional vehicles and those utilizing an energy-storage flywheel, with steady speeds and three driving cycles used for the evaluation. It is found that the efficiency penalty for extending the ratio coverage can be quite severe for some combinations of system parameters and operating conditions, so that a complete analysis of this aspect is important when considering the concept for any specific application.

Breakdown in concentration profile diodes

Abstract Reverse breakdown capability or a concentration profiled p+ n n+ diode depends upon the surface concentration at the contra and the edge region. To study the effect of various concentration profiles on the breakdown capability, the electric field distribution in the semiconductor and the dielectric is theoretically computed. These theoretical computations are compared with the experimentally determined values. Extremely high breakdown voltages have been achieved in plane p+ n n+ junctions using this technique. The desired concentration profiles are achieved using spin-on sources for boron diffusion. These sources require a curing time, alter preparation, to yield reproducible uniform impurity eurfece concentration. To study the stj-ucturoJ changes taking place in the liquid phase during the curing time, IR spectra of the liquid spin-on source are presented. It is concluded that the interaction between B-O and Si-OH in the liquid phase is responsible for achieving uniform surface concentration, Fu...

Temperature dependence of field-controlled thyristor characteristics

The dependence of the characteristics of field-controlled thyristors upon the ambient temperature has been examined in the range of -30 to 200°C. Unlike conventional thyristors, these devices have been found to continue to exhibit forward blocking capability up to the highest measurement temperature (200°C). In fact, it is shown here that the forward blocking capability as well as the blocking gain improve with increasing temperature with the usual scaling of the leakage current for power devices. The reverse blocking capability is also retained. The forward voltage drop of the device in the conducting state decreases with increasing temperature. This behavior is shown to be similar to that of conventional rectifiers and thyristors operated at high injection levels. Further, the force gate turn-off time of the devices has been found to increase with increasing temperature. This has been correlated with a measured increase in the minority-carrier lifetime. The results of this study demonstrate that field-controlled thyristors are capable of being operated at higher temperatures than conventional thyristors.

Failure Mechanisms in Wet Tantalum Capacitors

The wet tantalum capacitor is well established in both power supply smoothing and timing applications and has a well proven high reliability. This paper reviews the mechanisms of the possible failures arising from mis-application or over-stressing. The effect of over voltage on the anodic oxide is considered and the influence of cathode structure on reverse voltage and ripple current capability is discussed. The silver cathode allows migration of silver ions through the electrolyte under appropriate conditions and hence produces a failure mechanism not present in the tantalum cathode device. The life of a wet tantalum unit is eventually limited by loss of water vapour leading to open-circuit. Results are presented for button style capacitors which suggest that this is in fact not a practical limitation.

Load rejection overvoltages as affected by excitation system control

The effects of static excitation systems with and without reverse current capability are examined in the context of load rejection conditions on synchronous generators connected to high voltage transmission lines.

A high-voltage, high-temperature reverse conducting thyristor

This paper describes the structure and electrical characteristics of a silicon p-n-p-n inverter switch which has been fabricated to demonstrate the feasibility of obtaining both high-voltage and high-temperature capability in the same device while still maintaining reasonable dynamic properties. It was realized that in most inverter systems, feedback rectifiers are required, and thus the reverse blocking capability of a thyristor is not utilized. This was used to advantage in the design of a thyristor by integrating an anti-parallel rectifier into a conventional thyristor structure. Although this approach results in a thyristor having a negligible reverse blocking capability in comparison to commercially available power thyrisitors, the potential advantages of lower conduction losses, high maximum junction temperatures, and integrated antiparallel rectifier may be attractive for use in inverters. Reverse conducting thyristors designed in this fashion have been operated with reasonable turnoff times at a forward blocking voltage of 1000 volts while at case temperatures of up to 230°C.