Application of High-Power Snubberless Semiconductor Switches in High-Frequency PWM Converters
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For many years, power electronics in the high-power area was performed with extremely slow semiconductor switches. These switches, including the thyristor and the Gate Turn-Off (GTO) thyristor, had the capacity to handle very high voltages and currents but lacked the ability to perform high frequency switching. Low-power converters, such as computer power supplies and low horsepower motor drives, have employed high-frequency switching for years and have benefited from very nice output waveforms, good control dynamic performance, and many other advantages compared to low frequency switching. Recent improvements in high-power semiconductor technology has brought switching performance similar to that of the low-power MOSFETs and IGBTs to the high-power area through the advancement of the IGBT's ratings to create the High Voltage IGBT (HVIGBT) and the development of new GTO-derived devices including the Integrated Gate Commutated Thyristor (IGCT) and the Emitter Turn-Off (ETO) thyristor. These new devices all feature high switching speed and the capability to turn off without the requirement for a turn-off snubber. With these new device technologies the high-power field of power electronics can realize dramatic improvements in the performance of systems for utility applications and motor drives. However, with these high-speed switches come new issues relating to noise, protection, performance of diodes, and thermal management in high-frequency applications. This thesis addresses the application of these new devices, especially the ETO and the IGCT. Examples of each device technology (IGBT, IGCT, and ETO) have been characterized in both their switching performance and conduction loss. The tests performed show how these new devices may be applied to various applications. The switching loss, especially related to turn-off, is the dominant factor in the power dissipation of the high-power switches, so knowledge of these characteristics are very important in the system design. To demonstrate the operation of the ETO, two power converters were constructed. The first was a 100 kW DC/DC converter, which demonstrated the operation of the ETO in a typical building block configuration, the half-bridge. The second system, a 1 MegaVolt-Amp (MVA) three-phase inverter, demonstrated the ETO in an application where the switching frequency and power level were both high. The test results demonstrate the expected characteristics of the high-frequency converters. The development of the ETO's gate driver is described. During the inverter testing, a new failure mode was found involving a parasitic diode within the ETO. This failure mode was analyzed and solutions were proposed. One of the proposed solutions was implemented and there were no more failures of this type. Another possible failure mode regarding a circulating current in an IGCT-based system is also analyzed. Soft-switching techniques can help reduce the switching loss in power semiconductor switches. Several topologies were considered for application in the high-power area, and one was selected for further investigation. A prototype Zero Current Transition (ZCT) circuit was developed using an IGCT as the main switch. The turn-off loss was reduced dramatically through the tested ZCT circuit, and the diode recovery was also alleviated.
- Masters Theses