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Title: Turn-off voltage sharing of field-stop IGBTs in series connection under inductive load conditions
Author: Zhang, Xueqiang
ISNI:       0000 0004 9359 6850
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2016
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Operating Silicon (Si) Insulated Gate Bipolar Transistors (IGBTs) in series connection is attractive to many power electronic applications, including converters, hybrid circuit breakers, etc. To operate IGBTs in series connection, regulating the voltage sharing between the IGBTs during the IGBT turn-off is essential. The objective of this thesis is to explore practical and efficient methods at IGBT device level for regulating the voltage sharing of IGBTs in series connection during the IGBT turn-off under inductive load condition. The discussions in this thesis are regarding modern Field-Stop (FS) Si IGBTs which are widely adopted. The key mechanisms at IGBT device level are discussed regarding the turn-off voltage sharing of series-connected IGBTs under typical inductive load conditions. A Finite Element Method IGBT model of a 1700 V FS Si IGBT is developed, and simulations of two-in-series IGBT turn-off using this IGBT model in an inductive current commutation loop are conducted to study the effects of a few selective IGBT internal parameter variations and control errors on the turn-off voltage sharing. Following these studies, three types of approaches from different perspectives are attempted to mitigate the turn-off voltage divergence of series-connected IGBTs. For a passive approach, a few selective adjustments to IGBT internal parameters are reviewed in simulation regarding their effects on the turn-off voltage divergence. Some of these parameter adjustments are useful for improving the basis of the turn-off voltage sharing, and appropriate application of them in combination is attractive. Active voltage sharing regulation is essential for achieving closely matched turn-off voltage sharing of series-connected IGBTs. Regarding the direct control of IGBT gate for the voltage sharing regulation, a redesigned direct regulation method based on an Active Voltage Control (AVC) scheme is presented. This direct regulation method is based on differential regulation designed to regulate the voltage sharing as it tends to diverge. The design concept and the mechanism of this direct method is discussed, and the experimental demonstration is shown. Compared with previous relevant researches, the direct regulation method here avoids significant additional losses caused by the voltage sharing regulation process. In addition, an external regulation method using auxiliary Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) controlled by the AVC scheme is presented. This external regulation method is also based on differential regulation designed to regulate the voltage sharing as it tends to diverge. The design concept and the mechanism of this external method is discussed, and the experimental demonstration is shown. The response of the voltage sharing regulation is improved with the auxiliary SiC MOSFETs. Both the direct and the external regulation methods here are effective in the experiments. The conclusions are drawn regarding the work presented in this thesis on the turn-off voltage sharing of series-connected IGBTs and its improvement and regulation. The limits in the experimental implementations here of the two voltage sharing regulation methods are discussed. Future work is suggested regarding modelling and advanced optimisation in simulation and advancing the voltage sharing regulation methods.
Supervisor: Palmer, Patrick Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Insulated Gate Bipolar Transistor (IGBT) ; IGBT turn-off ; series connection ; voltage sharing ; Active Voltage Control (AVC) ; Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)