This thesis presents data from a simulation study of the thermal and electrical characteristics of a Gate Turn Off (GTO) thyristor. At present, most of the research on GTO thyristors has focused on their use in power electronic systems at high switching frequencies. As a result, the behavior of GTO thyristors at very low switching frequencies is not well understood. Previous research projects have shown experimentally that GTO thyristors are capable of interrupting significantly more than their nominal turn-off current rating when used in pulsed power applications at low switching frequencies. This work demonstrates the use of physics-based computer simulation to study the electrothermal turn-off characteristics of a GTO thyristor at low switching frequencies. The computer model used in this project simulated both the electrical and the thermal characteristics of a GTO thyristor and allowed its internal properties - such as current density, electric fields, and lattice temperature - to be investigated. The model was used to track the generation, transfer, and dissipation of energy within the structure of the device and show that the current interruption capability of a GTO thyristor may depend on its switching frequency due to the thermal energy that is generated and stored in the device during turn-off.
The research project involved an investigation of the current interruption capability of Gate Turn Off (GTO) thyristors. Specifically, the project examined factors that had the potential to increase the amount of current that could safely be turned off by a GTO thyristor. During the course of the project, a series of GTO thyristors were used in an inductive pulse forming circuit to interrupt varying amounts of current. The primary objective of the research was to determine how the control current used to turn off the GTO thyristors affected their current interruption capability. The initial part of the research project involved designing and building the experimental apparatus required to test the inductive turn-off characteristics of the GTO thyristors. Fundamentally, the test circuit consisted of a high-voltage capacitor bank, a wire-wound inductor, the GTO thyristor with its associated control circuitry, and instrumentation to measure the voltages and currents in the circuit. The capacitor bank supplied the energy for each current pulse while the inductor, hand wound using a polyvinylchloride (PVC) core and a length of magnet wire, controlled the shape and duration of the current pulses; the control circuitry determined the timing of the current pulses used to turn the GTO thyristor on and off. The test circuit utilized a conventional H-bridge driver to control the turn on and turn off of the GTO thyristor. Each leg of the H-bridge was connected to a dedicated capacitor bank that supplied the energy for the turn-on and turn-off gate current pulses. The H-bridge allowed the shape and magnitude of the turn-off gate current pulse to be easily adjusted. The amount of gate turn-off current was adjusted throughout the experiment to determine if the current interruption capacity of the GTO thyristors could be increased beyond their turn-off current rating. The project demonstrated that the GTO thyristors were capable of reliably interrupting twice their nominal turn-off current rating. Furthermore, the project showed that the turn-off speed of the GTO thyristors is dependent upon the amount of gate current used to turn off the GTO thyristors. The GTO thyristors that were turned off using larger gate currents were able to interrupt higher levels of current and turn off faster than the GTO thyristors that were turned off using smaller gate currents. This research project directly complements current and future Department of Defense (DOD) work being performed on railgun pulse power systems. Inductive power supplies may provide a solution to the energy density requirements of the next generation of electric weapon systems. The experimental results of this investigation demonstrate that GTO thyristors can be used in inductive pulse power supplies to safely interrupt significantly more current than they are nominally rated to turn off.
The Emitter Turn Off Thyristor (ETO) is an emerging high power device that can be considered as a Mosfet-GTO hybrid. It is very suitable for high power applications due to many of its significant advantages in terms of usability and performance benefits. It combines the advantages of simplified control and improved switching performance. This thesis is focused on firstly investigating the loss characteristics of the ETO with variation in operating junction temperature. The switching loss, leakage loss and conduction loss of the ETO are recorded as a function of junction temperature in order to gain a better understanding of the thermal performance. Switching loss and conduction loss are found to have a linear dependence on temperature, whereas leakage loss is dependent exponentially on junction temperature. An effort is made in order to understand what limits the operation of semiconductor devices beyond a certain temperature limit. All this is then combined to develop a closed loop thermal model which is used to study the thermal stability. Thermal instability at high temperatures is caused primarily by the large leakage current, which rises exponentially with rise in temperature. Thermal stability can be maintained as long as the change rate of losses remains low enough such that the losses can be carried out of the switching junction of the device by means of the thermal conductance. This condition and the loss characteristics were used to predict the high temperature operating limit of the ETO. It is seen that due to the excellent loss characteristics of the device and due to the low thermal resistance from the junction to ambient, that the ETO should be capable of operating at temperatures well above 160 Degrees Celsius. Tests are done on the ETO in order to corroborate this theory and calculations Multilevel converters have become an important technology in high-power applications, especially for Flexible AC transmission system (FACTS) applications. In this th.
