IGBT Gate Driver with Soft Turn-off Function
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IGBT:n hilaohjain hidastetun sammutuksen toiminnolla
The purpose of this thesis research was to develop a new gate driver circuit for IGBT semiconductor switches that are used in frequency converters. Frequency converters are widely used in industry and electrical systems of buildings, including elevators and es-calators, for energy efficient speed control of electric motors. The thesis is a part of larg-er research entity done in Danfoss Drives in order to enhance the short circuit protection of frequency converters. The research was based on existing simulations of the driver, done by another development engineer of Danfoss. The reference material used as back-ground information for the research consisted mainly of academic research articles and the fundamental literature regarding electric converters. The study consisted of performing further simulations for the driver circuit, building a prototype and performing measurements for it in a high voltage testing environment. The testing environment modelled the main circuit of a frequency converter. The target on the driver development was to achieve short delays, high gate currents and fast out-put voltage transitions, thus providing flexibility for the application-specific adjusting of the driver. The second section of the research consisted of developing an additional soft turn-off circuitry for the gate driver. Soft turn-off can be used for decreasing the turn-off voltage overshoots over the IGBT switch. Overshoots are caused by turning the IGBT off when a short circuit has appeared in the output of the converter. When using normal turn-off switching procedure for high currents, the momentary overvoltage can exceed the tolerance of the IGBT and cause damage to the converter. The theoretical section of the thesis includes introducing the main circuit of a frequency converter, the structure and controlling method of an IGBT and its behavior in fault situations. Also the general requirements for a gate driver are presented. The section considering the practical research focuses on the technical solutions of the developed driver and measurement results regarding the driver and the driven IGBT. Additionally, the behavior of the IGBT and the main circuit is studied with different configurations of soft turn-off circuit. For comparability of the driver’s performance, an existing driver is introduced and measured for reference. The developed gate driver reached the targets for the gate current and delays. It re-duced the delays caused by the driver by more than 50 % when compared to the refer-ence driver. In turn, chosen gate power supply voltages turned out to be rather low. Also the power rating of the IGBT module in the test setup was significantly lower than what the driver is designed for. The soft turn-off was reducing the turn-off overvoltage even more than expected. According to measurements performed with the final configura-tion, the overshoots were 82 % lower than when using the normal turn-off.