Manufacturers usually try to fully describe the areas of reliable performance for the power semiconductor devices they produce by considering the structural and technological features. However, it is impossible to take into account all features under different conditions, and sometimes a sequence of reliable conditions can cause device failure.
For example, a high-voltage thyristor was used in an inverter that offered durable service at low anode current (angle of ignition is 180°). Later, the angle was abruptly decreased and anode current increased rapidly. Its average and peak values, however, did not exceed the permissible limit. Low current operation mode also did not exceed the limits despite the fact that only the auxiliary thyristor was switching, and its overheating did not lead to overriding of the maximum admissible temperature.
Nevertheless, the sequence of these two reliable modes led to failures of a systematic and massive character. The destruction was in the auxiliary thyristor cathode.
The reason for failure was that during operation in low current mode, the auxiliary thyristor was locally heated, though the main thyristor was still cold. After switching to high current mode, due to the difference in temperature of the main and auxiliary elements, the auxiliary thyristor was still in operation mode after switching of the main thyristor, and the current level of the auxiliary thyristor exceeded the safe limit. This overload current led to overheating of the auxiliary element, followed by a thermal breakdown.
Ignoring certain features of such a device can lead to the destruction of the thyristor. These features include the level of minimum anode current necessary to switch the main thyristor and on-state temperature characteristics of the current-voltage diagram of the main and auxiliary thyristors. The user did not have any information about the above mentioned characteristics, because this information is not included in the standard documentation, and the manufacturer didn’t have complete information about the features of the end user application.
It is clear that tracing the reasons of failure caused by lack of information of specific features of the used semiconductors, which are determined by their structure and production technology, is quite a difficult task that cannot be solved without close cooperation between the manufacturer and end user.
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