I have a supply rail that is 15 V nominally +/- 5%. So the highest nominal voltage is 15.75 V. Due to external fault currents of up to 200 mA, the voltage on this rail can rise anomalously. The fault currents are cut out by additional circuitry after several 100 µs, but during this time, clamping diodes dump the fault current into the supply rail.
The devices on this rail have a lowest absolute maximum rating of 18 V. I have figured that the window between 15.75 V and 18 V is too tight for a simple limiter based on Zeners/TVS diodes. So I thought about using a cheap generic shunt regulator such as the TL431 and came up with the following circuit (which limits approximately to 16.5 V as set by R1 and R2):
I would like to ask if this circuit would work as expected, as I have never used such shunt regulators before and don't know their pitfalls.
Specifically, since the reference pin voltage is below 2.5 V during normal operation, the TL431 is not in regulation. When an overvoltage strikes, would the regulator turn on quickly enough as in the simulation to limit the rail voltage? Also the datasheet mentions regions of instability due to cathode-to-anode capacitance (e.g. page 17 and 18). As the TL431 is not in regulation, could that region change and cause instability also in my circuit?