the collector voltage increases too quickly. Since the RBSOA is associated with a very brief turn-off process, it is not constrained by the continuous power dissipation limit. The ordinary safe operating area (when the device is in the on state) may be referred to as the Forward bias safe operating area (or FBSOA) when it is possible to confuse it with the RBSOA. 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. Another style of protection is to measure the temperature of the outside of the transistor, as an estimate of junction temperature, and reduce drive to the device or switch it off if the temperature is too high. If multiple transistors are used in parallel, only a few need to be monitored for case temperature to protect all parallel devices. 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. See also Derating References H. A. Schafft, J. C. French, Secondary Breakdown in Transistors, IRE Trans. Electron Devices ED-9, 129-136 (1962). online Michaël Bairanzade, Understanding Power Transistors Breakdown Parameters, OnSemi application node AN1628/D online Apex technical document on operating power opamps within SOA Power electronics Electronic engineering