In hot-swap applications, if there is a large overcurrent fault, the protection integrated circuit (IC) will quickly cut off the current to protect nearby components from damage. This rapid turn-off of current-possibly from 50 A (overcurrent) to 0 A (turn-off for protection) will occur within tens of nanoseconds and cause large current transients (di/dt) as shown in below equation.
This current will be trapped as energy inside of input trace or input wire inductance. Although the trace inductance may be very low about 10nH, it still generates a surge on the input of the hot swap controller according to the below equation.
The -50V surge will be in series with the input power supply and will effectively generate a positive voltage spike on the input rail, which usually exceeds the rated voltage of the hot-swap controller or MOSFET drain-source voltage (VDS) (see below figure). To prevent this voltage surge from happening, you can place a TVS at the input to transfer the energy directly from the inductor to the ground. The best placement of the TVS will be after any series inductance on the input (such as after a fuse).