Utilising power devices below 100 K to achieve ultra-low power losses
Leong, Kennith Kin (2011) Utilising power devices below 100 K to achieve ultra-low power losses. PhD thesis, University of Warwick.
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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.
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||T Technology > TK Electrical engineering. Electronics Nuclear engineering|
|Library of Congress Subject Headings (LCSH):||Low temperature engineering, Metal oxide semiconductor field-effect transistors|
|Official Date:||August 2011|
|Institution:||University of Warwick|
|Theses Department:||School of Engineering|
|Supervisor(s)/Advisor:||Mawby, Phil ; Bryant, Angus|
|Sponsors:||Converteam UK Ltd.|
|Extent:||xxv, 227 leaves : ill., charts|
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