One of the main trend in the development of high power electric machines (motors, generators) is to replace the magnetic components with superconducting wires, this inevitably leads to a critical requirement from the industry (Converteam) to operate power devices at cryogenic temperatures. However, the current understanding of the behaviour power devices at cryogenic temperatures is limited, especially below the liquid nitrogen temperature of 77 K. This is a problem since most of the superconducting wires operate at temperatures below 77 K. Furthermore, it is uncertain which device type is better, if at all suited to cryogenic operation. In order to answer this, a thorough analysis of the known cryogenic behaviour of all the generic power devices was performed, including the physical behaviour of silicon at cryogenic temperatures. It is concluded that the power MOSFET is the best likely candidate for cryogenic operation. To understand the cryogenic behaviour of silicon power MOSFETs especially between the temperatures of 20 K and 100 K, a cryogenic measurement system was built to characterise different types of power MOSFETs. All the measured power MOSFETs exhibited large improvement in on-state resistance down to 50 K and non-linear degradation of breakdown voltages with lower temperatures. Various behaviour was observed below 50 K including carrier freeze-out, electric field dependent ionisation of free charge carriers and large variations in on-state resistance between identical devices. Several power Schottky diodes were also characterised and all exhibited merged PiN Schottky diode behaviour at cryogenic temperatures. Non-silicon devices such as silicon carbide power MOSFETs and gallium nitride HEMTs were also measured. Silicon carbide exhibited no improvements at cryogenic temperatures, whereas gallium nitride HEMTs may prove to be the best power device to be utilised in future cryogenic applications. Since unusual behaviour was observed in power MOSFETs below 50 K, an attempt was made to explain these phenomena using theoretical equations of semiconductor physics and analytical models of power MOSFETs. The author suggested that careful control of the dopant concentration at the accumulation region below the oxide gate is required to improve the power MOSFET operations below 50 K. Moreover, the super-junction power MOSFETs could be optimised for better cryogenic operation. It is the intention of this work to demonstrate the benefits of power MOSFET cryogenic operation in a realistic industrial application. A demonstration model was designed and simulated, this circuit uses a back-to-back power MOSFETs configuration to control the freewheeling current flowing through a high temperature superconducting coil. The electrical and thermal design of the model has been described, simulated and presented in this work.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:560123 |
| Date | January 2011 |
| Creators | Leong, Kennith Kin |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/46807/ |
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