Selenium rectifier

Abstract: The low power losses of silicon carbide (SiC) devices provide new opportunities to implement an ultra high-efficiency front-end rectifier for data center power supplies based on a 400-Vdc power distribution architecture, which requires high conversion efficiency in each power conversion stage. This paper presents a 7.5-kW high-efficiency three-phase buck rectifier with 480-Vac,rms input line-to-line voltage and 400-Vdc output voltage using SiC MOSFETs and Schottky diodes. To estimate power devices' losses, which are the dominant portion of total loss, the method of device evaluation and loss calculation is proposed based on a current source topology. This method simulates the current commutation process and estimates devices' losses during switching transients considering devices with and without switching actions in buck rectifier operation. Moreover, the power losses of buck rectifiers based on different combinations of 1200-V power devices are compared. The investigation and comparison demonstrate the benefits of each combination, and the lowest total loss in the all-SiC rectifier is clearly shown. A 7.5-kW prototype of the all-SiC three-phase buck rectifier using liquid cooling is fabricated and tested, with filter design and switching frequency chosen based on loss minimization. A full-load efficiency value greater than 98.5% is achieved.

Abstract: In a rectification system with unity power factor, the input power consists of a dc and a double-line frequency power component. Traditionally, an electrolytic capacitor (E-Cap) is used to buffer the double-line frequency power such that the dc output presents a small voltage ripple. The use of E-Cap significantly limits the lifetime of the rectifier system. In this paper, a differential ac/dc rectifier based on the use of an inductor-current waveform control methodology is proposed such that a single-stage direct ac/dc rectification without the need of an E-Cap for buffering the double-line frequency power, and a front-stage diode rectifier circuit can be achieved. The feasibility of the proposal has been practically confirmed in an experimental prototype.

Abstract: A method of reducing reverse currents and increasing breakdown voltages without inducing negative effects on switching behavior in silicon carbide Schottky diodes is proved successfully. Implantation of p-regions in the surface of the n-drift region below the Schottky metal form face to face p-n junctions which screen the Schottky contact from high electrical fields. This results in a reduction of the reverse current and an increase of the breakdown voltage to the limit of a `pure` SiC p-n diode. It is shown, that in contrast to silicon based devices, SiC merged p-n/Schottky (MPS) rectifier preserve their excellent unipolar switching behavior. (orig.) 5 refs.

Abstract: This paper describes experiments to elucidate the exact physical and chemical structure of the selenium rectifier photocell, especially that of the thin surface film. A technique is described for sputtering films of cadmium oxide which, though transparent in the thickness required for a cell, have an electrical conductivity exceeding that of graphite. The thickness of the films can be closely controlled. With such films, on pure crystalline selenium, cells were produced with white-light sensitivities of over 700 $\mu $A per lumen, open-circuit voltages up to 0$\cdot $5 V under high illumination, and maximum quantum efficiencies up to 70%. The optical properties of the films are described, and the way in which the technique may be used to produce other non-metallic films is indicated. The cadmium oxide is found to have a negative Hall coefficient, and is therefore an N type semi-conductor. Further experiments with single films of gold, and double films of zinc oxide and gold, illustrate the behaviour of these, in intimate contact with selenium. The metal-selenium contact yields a poor photocell, the metal-zinc oxide-selenium contact one whose properties are critically dependent on the thickness of the intermediate oxide layer, and the N type cadmium oxide-selenium contact one for which the efficiency is high, and the thickness of cadmium oxide not critical. It is suggested therefore that in the practical photocell, the essential mechanism is a contact between two suitable semi-conductors of dissimilar types, any extra metal film when present serving simply to raise the lateral conductivity of the intermediate semi-conducting film when this is not high enough to eliminate undesirable effects of a high internal resistance in the finished cell.