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Title: High efficiency GaN power converters
Author: Nam, Kee Beom
ISNI:       0000 0004 9358 2424
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2020
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Industries have been exploring Gallium Nitride (GaN) as a candidate of material choice for the next-generation power semiconductor device technologies due to their superior material properties compared to Silicon (Si) that has been the predominant choice in conventional power electronics applications. One of the main focus of the research work presented in this thesis is the performance evaluation of a new lateral GaN-based semiconductor device technology referred to as 'Polarisation Super Junction' (PSJ). The large-area PSJ samples with the breakdown capability of > 3 kV from POWDEC K.K. were used for this work. Their static on-state and off-state current-voltage characteristics at temperatures up to 1500C were extracted and presented. According to the results, they can offer much lower specific on-state resistance compared to commercial GaN devices of much lower breakdown capability. Also, their typical turn-on and turn-off switching performances under 900V dc link with various load current levels and device temperatures of up to 1500C were evaluated using a double pulse test circuit with an inductive load configuration. The criteria for the selection of voltage and current measurement techniques for switching test are discussed thoroughly as well. The same tests were carried out for cascode GaN devices that were realized via bare GaN PSJ and Si-MOSFET dies attached to a direct copper bond substrate and bonded wires for electrical connections between them. Despite its advantage it can offer in terms of enhanced breakdown voltage and significantly reduced area-specific on-state resistance, the PSJ transistors are inherently depletion-mode (D-mode) or normally-on devices due to the spontaneous formation of 2-dimensional electron gas (2DEG) via polarisation present at the GaN/AlGaN heterointerface and thus negative gate drive voltage is required for them to be turned off. Hence, critical safety concerns arise if the gate drive voltage supplies fail. Also, short-circuit issues may arise even during normal operation as the controller may send a false command signal due to electromagnetic interference (EMI). In order to mitigate these issues, a refined version of the protection circuit proposed in a previous work was proposed, simulated and implemented onto a printed circuit board for demonstration. From both simulation and hardware results, it has been confirmed that the prototyped circuit is capable of providing protection necessary during the normal operation and gate drive supply failure event of D-mode power devices in power converters. Finally, the design process for the bi-directional DC-to-DC converter with efficiency target of >98% using D-mode GaN PSJ transistors is discussed in detail. The converter consists of the 2nd prototype of the gate drive failure protection scheme, a forced convection heatsink for cooling the power devices and passive elements including an inductor and output side capacitor (s) that provides dc current and dc voltage. However, due to the conducted EMI that arised from high dV/dt during the device turn-on, the controller was falsely toggled to command the converter operation to be halted during normal operation at dc supply voltages above 300 V. Nevertheless, the fail-safe operation of the converter under the supply failure event and its system requirement were verified using PSpice simulator with the behavioural model of the large-area PSJ transistor that was fitted to its measurement parameters and the same operating conditions as for its hardware prototype. Potential mitigation strategies on the controller side and the circuit side are proposed, which will pave the way for future high-efficiency power converter design and hardware validation using the latest PSJ device technology.
Supervisor: Madathil, Shankar Narayanan Ekkanath Sponsor: Rolls-Royce plc
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available