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Problems associated with the short circuit tolerance of power control circuits can be solved successfully using IGBT's as power switches even when the intrinsic short circuit performance of these devices is modest. The protection circuit of a IGBT with modest short circuit capability is more critical than the protection circuit of a rugged IGBT. On the other hand, IGBT's with the best short circuit performance have the higher operating losses due to their higher saturation voltage. This note demonstrates how the IGBT's and the protection circuit were optimized to get the best trade-off between efficiency, ruggedness, and reliability of the application.

This paper analyzes an anomalous failure mechanism detected on last generation low voltage power metal oxide semiconductor (MOS) devices at low drain current. Such a behavior, apparently due to a kind of second breakdown phenomenon, has been scarcely considered in literature, as well as in manufacturer data sheets, although extensive experimental tests show that it is a common feature of modern low voltage metal oxide semiconductor field effect transistor (MOSFET) devices. The paper starts by analyzing some failures, systematically observed on low voltage power MOSFET devices, inside the theoretical forward biased safe operating area. Such failures are then related to an unexpected thermal instability of the considered devices. Experimental tests have shown that in the considered devices the temperature coefficient is positive for a very wide drain current range, also including the maximum value. Such a feature causes hot spot phenomena in the devices, as confirmed by microscope inspection of the failed devices. Finally, it is theoretically demonstrated that the thermal instability is a side effect of the progressive die size and process scaling down. As a result, latest power MOSFET's, albeit more efficient and compact, are less robust than older devices at low drain currents, thus requiring specific circuit design techniques. Index Terms--Average temperature, bipolar devices, FBSOA, MOSFET, thermal instability, threshold voltage.