A Two-Quadrant Transistor Chopper for an Electric Vehicle Drive

A two-quadrant dc-dc transistor converter capable of delivering 400 A motoring current and of generating 200 A braking current is described. The chopper operates from a 108-V dc source (54 lead-acid cells) and supplies the armature current of a separately excited dc machine in an electric vehicle application (3000-lb commuter-type vehicle). The chopper employs high-current transistors specifically developed for the application and power diodes packaged together in power module form. Snubber networks which reduce both turn-on and turn-off device stresses are employed. The interaction of the snubber networks for the motoring and braking transistors is described and design considerations presented. It was found that for these snubbers a minimum on-time and a minimum off-time for the transistors must be maintained to ensure that the transistors' dynamic load lines never enter into the region of forward bias or reversed bias second breakdown. A technique is described which instantaneously detects a transistor failure and initiates the appropriate action in order to prevent machine overcurrent and overtorque. Factors are discussed which are crucial to ensure proper transitions from motoring to braking and to inhibit device power dissipation due to parasitic currents. The selection of a variable-frequency/variable-pulsewidth switching strategy and protection and control techniques unique to high-current transistor choppers are discussed.     

A single-molecule potentiometer

Controlling electron transport through a single-molecule device is key to the realization of nanoscale electronic components. A design requirement for single molecule electrical devices is that the molecule must be both structurally and electrically connected to the metallic electrodes. Typically, the mechanical and electrical contacts are achieved by the same chemical moiety. In this study, we demonstrate that the structural role may be played by one group (for example, a sulfide) while the electrical role may be played by another (a conjugated chain of C═C π-bonds). We can specify the electrical conductance through the molecule by modulating to which particular site on the oligoene chain the electrode binds. The result is a device that functions as a potentiometer at the single-molecule level.     

Single-molecule circuits with well-defined molecular conductance

We measure the conductance of amine-terminated molecules by breaking Au point contacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule-Au junctions is much less than the variability for diisonitrile- and dithiol-Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2-8 carbon atoms in the hydrocarbon chain, our results show a systematic trend in the conductance from which we extract a tunneling decay constant of 0.91 +/- 0.03 per methylene group. We hypothesize that the diamine link binds preferentially to undercoordinated Au atoms in the junction. This is supported by density functional theory-based calculations that show the amine binding to a gold adatom with sufficient angular flexibility for easy junction formation but well-defined electronic coupling of the N lone pair to the Au. Therefore, the amine linkage leads to well-defined conductance measurements of a single molecule junction in a statistical study.     

A study of parasitic reactions between NH3 and TMGa or TMAI

The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH₃ is not substantial in the reactor used in this study. On the other hand, A1N growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, A1N deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAI and NH₃. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAI+NH3 than for TMGa+NH₃.     

Power electronic converter technology

The present state of the art in DC-DC, AC-DC, DC-AC, and AC-AC converter technology is presented along with their typical areas of applications. Advances made in the last decade that are finding use in industrial, commercial, and military applications are emphasized. Soft switching, in which power device switching stresses are reduced due to circuit resonances, has dominated DC-DC power conversion and some high-performance DC-AC power conversion. In the higher voltage and higher power areas, the invention and improvements of gate turnoff thyristors (GTOs), insulated gate bipolar transistors (IGBTs), and integrated gate-commutated thyristors (IGCTs) have led to other unique topologies to take advantage of these devices