Fault protection system for Current Source Inverter with normally on SiC JFETs RABKOWSKI Jacek, ZDANOWSKI Mariusz, NOWAK Mietek, BARLIK Roman Warsaw University of Technology, Institute of Control and Industrial Electronics, Koszykowa 75, 00-662 Warszawa, Poland jacek.rabkowski@isep.pw.edu.pl Key words: Normally-on JFET, fault protection, Current Source Inverter. Abstract. This paper describes a simple fault protection system for Current Source Inverter built with normally-on SiC JFETs. Because all transistors are in on-state after loss of the gate drive(s) supply, list of possible fault modes is extended in reference to standard inverters. That is also why an additional normally off switch is introduced in the DC link. Operation principles of the protection system which follows the drain-source voltages of JFETs and the current of the DC link are presented. The 2kVA/100kHz model of the inverter equipped with the proposed system is validated via various laboratory tests including short-circuits and the auxiliary supply turn off. Introduction Normally on Silicon Carbide JFETs with simple structure and very interesting features are in interest area of power electronics. Number of papers describe fundamental properties of these elements [1]-[3]: low on-state resistance at high blocking voltage, fast switching and low switching energies, the excellent short-circuit behavior with self-limitation of drain current, avalanche breakdown, ability to operation in high temperatures. On the other hand an essential disadvantage of SiC JFETs is normally on characteristic while most of today’s applied Silicon devices are normally off. Thus the application of the SiC JFET to power electronics converter brings new challenges and requires more effort on reliability issues. Analysis of possible faults caused by normally on characteristics of the JFET lead to conclusion that most immune topology of the power converter is Current Source Inverter (CSI – see Fig. 1). At first, series diodes (D 1 -D 6 ) prevent from AC side short-circuits even when all JFETs are in on state. Loss of the gate drive supply of one or more JFETs (T 1 -T 6 ) result in the DC side short circuit which is one of regular operation modes (Fig. 1). Furthermore an amount of energy stored in the DC-link inductors L D is much lower than in capacitor of the comparable Voltage Source Inverter (VSI) and might be safely dissipated after any emergency inverters turn-off. Thus, CSI topology was chosen for further studies as most suitable for application of normally on JFETs. Novel fault protection system for three-phase CSI is presented and verified under various circuit conditions in this paper. The laboratory model of fully-SiC CSI (100kHz/2kVA/400VDC/3x400VAC) was built with 2,4x2,4mm 2 JFETs from SICED. While similar systems operating at 150 and 200kHz switching frequencies [4],[5] can be found in literature, lack is relevant information on fault immunity. This issue was only discussed for VSI based systems [6,7]. CSI with normally on switches - possible fault modes Normally on characteristics of applied JFETs extends the list of possible fault modes. The protection system should take into account also a scenario when up to six transistors T 1 -T 6 is in the on state because of the gate drive(s) supply loss. As the output circuit is self-protected by diodes, the only danger is short circuit of the DC side supplied from the DC-source. Furthermore speed of the Materials Science Forum Vols. 679-680 (2011) pp 750-753 Online available since 2011/Mar/28 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.679-680.750 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 132.239.1.230, University of California, San Diego, La Jolla, USA-14/09/14,23:38:50)