Power Semiconductor Switching Of Low Impedance High Energy Networks

Abstract

The military has been a growing interest in using electric guns over conventional powder based systems. Three types of guns are currently being explored: electromagnetic launchers or railguns, electrothermal chemical guns which use electrical energy to burn the propellant and coil guns which use solenoidal magnetic fields to launch the projectiles. All of these designs are in their infancy and as such the components that make up the system have not been developed enough to make these ideas competitive with conventional cannons. Current designs call for using vacuum switches or spark gaps as the main switching elements. While these switches can operate at the required voltages and currents they have ancillary equipment that is or can be large in size and weight. Spark gaps generally need to be pressurized to operate at high voltages and this requires an air compressor or gas bottles. The trigger generators tend to be large for spark gaps and vacuum switches. Vacuum switches have to operate under certain circuit conditions or they won't recover properly under reapplication of voltage. They have a high prefire rate, and require large trigger currents to keep the trigger pin operational. Certain designs of the modulator call for the switches to fire in a sequence to shape the current pulse delivered to the load. Without steering diodes in each line, switches that are on when the other switches begin to f i e will pass current in the reverse direction; recharging lines that should be discharged and lowering the overall efficiency of the system. Both spark gaps and vacuum switches can conduct current in both directions. At the U S . Army Pulse Power we have been investigating using high voltage thyristors as the main gun switch. Their ancillary requirements such as trigger generators are much smaller, typically rated at a few hundred volts and up to 100 A. No gas is needed, they are more reliable, and have a longer lifetime. Their disadvantage is their low breakdown voltage compared to gas switches, but they are easily operated in series and, if packaged correctly, there can be multiple devices in a single package, such as done by ABBI. Large area devices with amplifying gate structures can operate at currents of 136 kA reliably in millisecond pulses and have been operated over 150 kA. The need for steering diodes is negated if the reverse blocking capability of symmetric thyristors is utilized.