PhD Oral Examination of Subhas Chandra Das (ERP)

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Abstract

Insulated gate bipolar transistors (IGBTs) are the dominant power semiconductor devices in high power applications, such as, locomotive traction and megawatt-level renewable energy systems. Power electronic converters in such applications are expected to have a long-life span of about 20-30 years. Hence, efficiency and reliability of these converters are very important. IGBT switching behavior has a direct influence on both power conversion efficiency and system reliability.

The various switching characteristics parameters of IGBTs, which are available in the respective device datasheets, are limited to certain operating conditions. For an example, the switching characteristic parameters are available for only one or two DC link voltages; however, in applications such as diesel-electric locomotives, IGBTs have to operate over a wide range of the DC link voltages. Similarly, the characteristic parameters are available at only one or two junction temperatures (e.g. 25 oC and 125 oC); but, the IGBTs in traction and wind energy systems have to operate over wide range of temperatures including sub-zero ambient temperatures.

In this work, switching behavior of IGBTs of four different makes are studied experimentally over a wide range of operating conditions. The load current is considered upto 1.667p.u., where 1 p.u corresponds to the rated current of the IGBTs. The range of DC link voltage considered is from 0.571 p.u. to 1.321p.u., where 1.0 p.u. is the nominal voltage of the application. The junction temperature range is considered from -35 oC to +125 oC. The following are the major highlights of the research work:

1. Generation of experimental data on switching behavior of IGBTs over wide range of operating conditions as mentioned above.

2. The experimental data, which are generated, complement the technical information available in device datasheets.

3. The experimental investigation are carried out on four traction-grade IGBTs of different makes and of comparable ratings to ensure that the findings of the study are applicable to reasonable cross-section of the available commercial devices.

4. Experimental study on the switching behavior of an IGBT converter leg, having top and bottom devices of two different makes, and its comparison with the switching behavior of a converter leg, having complementary devices of the same make.

5. Experimental study of the rise and fall times of the device switching voltages and currents, both during turn-on and turn-off, over the complete range of operating conditions.

6. Evaluation of turn-on and turn-off switching energy losses as functions of load current, DC link voltages and junction temperatures, which are valid over the complete operating range.

7. Experimental study of reverse recovery characteristics of anti-parallel diode of IGBTs with varying DC link voltage, load current and junction temperatures.

8. Experimental investigation on the effect of variations in DC link voltage, load current and junction temperatures on device peak stress parameters, namely, peak device voltage, peak device current, peak rate of change of device voltage, and peak rate of change of device current.

9. Experimental study of sub-intervals of the turn-on switching delay, turn-off switching delays and parameters related to the switching delay intervals over the complete operating range.

10. Correlation of the various turn-on and turn-off switching parameters with junction temperatures based on the experimental data generated.

11. Study of the consistency of the above correlations across different traction-grade devices of comparable ratings and different makes.

12. Critical review of various thermo-sensitive electrical parameters (TSEPs) already reported in literature.

13. Identification of new TSEPs that can be obtained from the measured gate-emitter voltage during switching delay times.

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