Safe Operating Area (SOA)
Defines the region in which a power semiconductor can operate without damage in terms of the voltage, current and duration triple. Data sheets give it as a graph with logarithmic axes; as the pulse duration gets shorter the permitted current rises, while in continuous operation the limit narrows noticeably.
The SOA graph is the intersection of four limits: maximum current, maximum voltage, the limit coming from channel resistance, and the thermal limit. On the graph the current permitted for 10 µs pulses can be tens of times the continuous rating, because the heat produced in a short pulse does not even get the chance to spread through the die.
The dangerous cases are those where the switch stays in the transition region for a long time. Motor stall, a short circuit, or a weakened gate drive lead to this condition; current is high, the voltage across the device is high and the duration is long. With such a combination the SOA limit can be exceeded in milliseconds rather than seconds.
For this reason the protection layer in a driver cannot be thermal alone: the temperature sensor sits a few millimetres away from the die and by the time it notices the event the damage may already be done. Fast current sensing and current limiting are therefore necessary. On the user side the equivalent is this: running a motor continuously against its mechanical end stop is a real risk for the electronics when the current threshold is not set correctly.
The context in which this term is used: DC Motor Driver Selection Guide