Diagnostic characterization of high-power IGBTs with field instrumentation: Pitfalls in using curve tracers only and improvements by oscilloscopes

The exigency of characterizing high-power IGBTs directly on-site increasingly emerges in field installations hosting multilevel converters that employ such devices by the dozen, when not by hundreds, as it is already the reality for modern modular voltage-source HVDC and FACTS systems. Nonetheless, differently from the actualities in permanent laboratories, the typology of the instrumentation rapidly obtainable on the field is often rather restricted by the aspects of logistic, necessary timing of intervention and ruggedness against the usually harsh environments. Commonly, the only specimens realistically expectable at short notice on a field installation are portable curve tracers, portable field oscilloscopes with non-isolated channels, passive voltage probes, current probes based on Rogowski coils, as well as portable multimeters and variable DC power supplies. The paper describes a measurement technique devised to diagnose and characterize high-power IGBTs by employing solely such a limited set of instruments. The severely erroneous conclusions that possibly descend from the utilization of curve tracers only are highlighted and it is shown how, conversely, the additional presence of properly connected oscilloscopes and probes becomes mandatory in order to attain a proper characterization. The role of the curve tracers should be strictly limited to mere stimuli generators only, for their compensation techniques against the systematic measurement errors occurring with signal and low-power transistors, rarely suffice in case of high-power devices. Additionally, the equivalent differential capacitance between the Collector and Emitter terminals of a high-power IGBT-Diode module is characterized too. It is often insufficiently documented in the datasheets despite its evolution being important for the dynamics of the commutation. Experimental results from an ABB 5SNA 1200G450300 IGBT module (4.5 kV, 1.2 kA) are presented.