The present-day laser and accelerator technology, new types of radar, controlled fusion research and other areas of science and applications require very high power switching (MW to TW) in the micro-, nano-, subnanosecond ranges. The new principles of switching make use of semiconductor devices for such power switching. The principle of switching involving a controlling plasma layer has been employed to develop three new classes of devices, namely, reversely switched dynistors (RSD), reversely controlled transistors (RCT), and drift step-recovery diodes (DSRD). The highest-power microsecond-range RSDs switch 300 kA single pulse current, and 50 kA current at 100 Hz; high power DSRDs generate current pulses of 103 A in a few nanoseconds at frequencies > 104 Hz. The switching principle based on the impact ionization wave was used to develop diode pulse sharpeners (PS) switching power of a hundreds kW in tens of picoseconds at a frequency > 104 Hz. By this principal parameter, pulsed switching power, all these devices surpass the devices employing the conventional principles.

New principles of high power switching with semiconductor devices

We tested a variety of high-current closing switches for lifetime and reliability on a dedicated 2-MJ, 500-kA capacitor bank facility at Sandia National Laboratories. Interest was in a switch capable of one shot every few minutes, switching a critically damped, de-charged 6.2-mF bank at 24 kV, with a peak current of over 500 kA. The desired lifetime is 24000 shots. Typical of high-energy systems, particularly multimodule systems, the primary parameters of interest related to the switch are 1) reliability, meaning absence of both pre-fires and no-fires; 2) total switch lifetime or number of shots between maintenance; 3) cost. Cost was given lower priority at the evaluation stage because there are uncertainties in estimating higher-quantity prices of these devices, most of which have been supplied before in only small quantities. Cost has more importance in choosing between viable candidates after testing The categories of switches tested are vacuum discharge, high-pressure discharge, and solid-state. Each group varies in terms of triggering ease, ease-of-maintenance, and tolerance to faults such as excess current and current reversal. We tested at least two variations of each technology group. The total number of shots on the switch test facility is about 50000. This paper will present the results from the switch testing. The observed lifetime of different switches varied greatly: the shortest life was one shot; one device was still operating after 6000 shots. On several switches, we measured the voltage drop during conduction and calculated energy dissipated in the switch; we will show these data also.

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Final results from the high-current, high-action closing switch test program at Sandia National Laboratories

A unique type of thyristor, the reverse switching dynistor (RSD), has been studied in specially designed pulsers to evaluate their performance for several high-power applications. The dynistor is an asymmetric thyristor with an alternating p/sup +/ and n/sup +/ structure in its anode. The device operates like a transistor during turn-on and generates a uniform plasma distribution enabling fast turn-on. This reduces commutation dissipation and permits high di/dt operation to be achieved. An 80-mm diameter device was evaluated in a pulser using a 0.5 ms full-width at half maximum (FWHM), 10-m/spl Omega/ pulse-forming network (PFN) with a matched load. Operation was demonstrated at 177 kA and a di/dt of 2.8 kA//spl mu/s. These tests were performed using a stack of two RSDs in series with a diode to protect against voltage reversal. The devices were triggered by a common driver. The driver unit must be capable of holding off the PFN voltage and generating a high-voltage reverse pulse for turn-on. A. Saturable reactor is required to isolate the main discharge circuit for a period of 2 /spl mu/s. This technique can be extended to many devices in series, thus, enabling a high-voltage switch to be built using a single driver. Results with two dynistor pulsers are also discussed.

Reverse switching dynistor pulsers

The switch-on process in 6 kV 4H-SiC junction diodes has been investigated experimentally and theoretically. The results of a detailed computer simulation are compared with the data furnished by the analytical theory. It is demonstrated that, at high current densities exceeding the critical value jcr = eNDvs (e is the elementary charge, ND is the base doping level, and vs is the carrier saturation velocity) and rather short current rise time (1 ns), an extremely fast base modulation can be achieved. In this mode, the base is spanned by an electron front that moves from the n+–n to the p+–n emitter with a velocity vs and by a relatively slow quasi-neutral hole front moving in the opposite direction, from the p+–n to the n+–n junction.

https://iopscience.iop.org/article/10.1088/0268-1242/20/1/010/pdf

Analysis of the turn-on process in 6 kV 4H-SiC junction diodes

High-energy switches and trigger generators are required for MJ-level capacitor banks. We have developed a compact gas switch and a matched series injection trigger generator. A series inductance is required for isolation of the trigger pulse from the surrounded circuit. A saturable inductor is employed here because low inductance is needed after the switch breakdown. The switch is of coaxial two-electrode design with electrodynamic acceleration of a spark channel. The switch operates at atmospheric pressure. The spark gap can be triggered reliably down to zero voltage (at 50 kV self-breakdown voltage) with less than 35 ns timing jitter. Energy losses in this spark gap have been accurately investigated. The main results are as follows: energy losses in the switch do not exceed 4% at voltages higher than 15 kV, i.e., when operation voltage exceeds ∼36% of the self-breakdown voltage. The spark gap is designed for 24 kV charging voltage, at a current up to 250 kA, and ∼70 C charge transfer. In this paper, we present a design for the spark gap, inductor and trigger generator. Test bed schematics and results of the tests are also described.

Two-electrode gas switch with electrodynamical acceleration of a discharge channel.

This paper deals with the uniform plasma injection regime in reversely switched dinistors (RSD) which is similar to the drift double-injection regime in p + nn + -diodes. Such a regime is distinguished as highly efficient and most stable. The anode current retroactive mechanism controlled by the plasma in the collector layer is investigated and criteria of the quasi-diode operation are established. The theory explains the main characteristics of the RSD in both the micro- and submicrosecond range of operation. It also allows the calculation of time dependence of current density and voltage drop. The criteria derived are in good agreement with empirical data.

A.V. Nalivkin

Papers

Theory of quasi-diode operation of reversely switched dinistors