Is a 9.6µs over-voltage transient long enough to fry my circuit (Details inside)?
Just throwing this out there:
Typical FET response is measured in nanoseconds.







simulate this circuit &ndash; Schematic created using CircuitLab

Vshutdown &gt; VzZD1 + VgthFET1

If VzZD1 = 50V and VgthFET1 = 3V then Vshutdown &gt; 53V


As shown, FET1 may (will try to) destroy itself to protect the load.
Determine resistor sizes as follows:

R1 protects FET1  
(R2 &gt; R1) ensures lower-impedance path through FET than load


Size R2 according to Ohm's law with Vout "grounded":

E = I * R
Edrop = Vin - Vout = 12V - 0V = 12V
R = Edrop / I
R = 12V / 1.4A = 8.57ohm


If Vzd1 = 12V and VgthFET1 = 3V, then Vshutoff = 15V:

Ir2 = E / R
Ir2 = 15V / 8.57ohm = 1.75A


FET1 will pass Vin - Vzd1 - VgthFET1 when active.
If R1 = 0 and Vin = 50V:  

VdsFET1 = 50V - 12V - 3V = 35V
I = E / R
I = 35V / 25mOhm = 1400A //Zap!


If R1 = 4 ohm ("less" than R2) and Vin = 50V:

VdsFET1 = 50V - 12V - 3V = 35V
I = E / R
I = 35V / (4 + 25mOhm) = 8.70A
EdropR1 = 8.70A * 4Ohm = 34.8V
EdropFET = 8.70A * 0.025Ohm = 0.2V
P = I * E
PFET = 8.70A * 0.2V = 1.74W
PR1 = 8.70A * 34.8V = 302.76W //Warm