In contrast to circuits utilizing the MOSFET as a fast switch, SOA is more important for hot swap circuits, which provide inrush current control, current limiting, and circuit breaker functions. This figure shows a board-resident LTC4233 controlling a 12V supply. When the board is plugged in to the backplane supply, the LTC4233 waits for the connector contact bounce to finish before soft-starting the internal MOSFET gate. The output voltage follows and reaches 12V in 40ms. For this inrush period, 200mA of capacitor charging current flows through the MOSFET, while its drain-to-source voltage ramps down from 12V to almost 0V. In other words, the MOSFET dissipates power for a significant period of time. Similarly, current limiting and input voltage steps also stress the MOSFET by putting both current and voltage across it. In these situations, a hot swap MOSFET is required to handle power dissipation from a few to tens of milliseconds and attention needs to be paid to its SOA performance.