Protection
The most common form of SOA protection used with bipolar junction transistors senses the collector-emitter current with a low-value series resistor; the voltage across this resistor is applied to a small auxiliary transistor that progressively 'steals' base current from the power device as it passes excess collector current.
This approach is effective but not bullet-proof. In practice it is very difficult to design a protection circuit that will work under all conditions, and it is left up to the design engineer to weigh the likely fault conditions against the complexity and cost of the protection.
Read more about this topic: Safe Operating Area
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