This invention relates to improvements in high-power rectifier assemblies including semiconductor devices of the type in which a semiconductor body, located in a sealed housing, is sandwiched under pressure between opposing electroconductive thrust members, or posts.
High-current solid-state rectifiers employing semiconductive material (e.g., silicon) are well known in the art of electric power conversion. A typical device of this kind comprises a semiconductor body in the shape of a broad-area multilayer wafer disposed between flat-surfaced thrust members, or posts, that are joined in sealed relationship to opposite ends of a hollow insulator to form a sealed housing or package for the wafer. If a two-layer (PN) silicon wafer is used, the device is a simple rectifier or diode, whereas if a four-layer (PNPN) wafer with gating means is used, the device is a controlled rectifier known in the art as a thyristor or SCR. In either case, it is common practice to support the device in a pressure assembly that provides a force urging the thrust members together and clamping the semiconductor body between the thrust members and electrically in series therewith. When properly constructed and installed, such a rectifier assembly can safely conduct continuous forward current of 250 amperes or more and brief surges of many thousands of amperes.
Even when properly designed and applied, a high-current semiconductor device may sometimes fail. There are a number of known causes for device failures, such as cyclic fatigue or excessive surge currents. The failure mechanism typically involves overheating localized areas of the silicon wafer which then lose blocking ability and permit the unimpeded flow of reverse current. In practice this will usually occur near the center of the wafer where short circuit current is well contained and will not cause permanent damage outside of the afflicted device itself. The failed device can then be replaced with a sound one, and the associated conversion apparatus can continue operating without expensive repairs or serious interruption of service. Occasionally, however, an electric arc may occur near the edge of a wafer where the housing of the device is especially vulnerable, and in this event external flashing or flame is possible with consequent propagation of the failure and widespread damage to other parts of the apparatus.
When its silicon body fails, the peak magnitude of current (having a given rate of rise and a given duration) to which a semiconductor rectifier assembly can be subjected without external flashing is herein referred to as the "explosion rating" of the assembly. Explosion ratings of 90,000 amperes and even higher are not uncommon.
One arrangement for providing such high explosion ratings is disclosed and claimed in U.S. Pat. No. 3,581,160-- Piccone et al., assigned to the assignee of the present invention. The rectifier assembly of that arrangement comprises a sealed housing in which the silicon wafer is located. Such housing comprises a cylindrical insulator and sheet metal terminal, or sealing, members bonded to the insulator. The above-described occasional arc occurring at the edge of the wafer is a high current arc which can burn a hole through or otherwise perforate one of these sealing members, thereby allowing flame, sparks, or other arcing products to escape from the sealed housing. Piccone et al. prevents escape of these arcing products to the surrounding ambient by providing what amounts to a supplementary enclosure of rugged construction around portions of the housing, which enclosure is able to contain the arcing products escaping from the housing and effectively withstand the high pressures developed by such escaping arcing products. This enclosure includes relatively thick O-rings, metal back-up rings for the O-rings, and also means for compressing the O-rings.