help understanding this reverse polarity protection circuit

Going through LT4363 evaluation kit DC2062A-B (schematic here), I came across a reverse protection circuitry that protects downstream components from reverse transients and reverse DC voltage:

In short, this circuit protects the downstream components by shorting the negative voltage when present at INPUt to DGATE and thus turning off Q2. when the voltage at INPUT is positive, Dgate gets biased via D2.

from DC2062A demo manual:

The DC2062A features reverse protection circuitry that protects downstream components from reverse transients of up to –150V. This number is limited by the BVDSS of Q2. The reverse protection circuitry also protects against reverse DC voltage of up to –30V.

My questions are:

1. Why is the reverse DC voltage protection limited to only -30V? I simulated the circuit at input -100V and couldn't find what components are stressed or damaged at this voltage. So where did the number (-30V) come from? Am I missing something here? I want to know this as I would like to make the circuit block negative voltages upto -100V.

2. A similar circuit is implemented in LTC4366 DEMO circuit DC1850A-A but with a -100V reverse DC voltage protection:

What are the pros/cons of using zener diode and capacitor in the first circuit over using the simpler solution of the second circuit? I'm guessing that the capacitor is used as a tank help turn off the mosfet Q2 quicker when the input voltage is suddenly switched from a posotive voltage to a negative one. I'm not sure.

• In your first schematics have you consider the power dissipation in R10 at -100V?
– G36
Dec 26, 2018 at 11:37
• at 100V the power dissipation is ~1W. at 50V is it ~0.25W which is the standard power dissipation of a 1206 (with proper ambient temperature and good enough PCB pads). So maybe R10 is the limiting factor
– fhlb
Dec 26, 2018 at 14:56
• If we look the full schematic I would say RLED4 (3.3k) is the limiting factor and R10 also. But to sure ask about it to LT/AD engineers.
– G36
Dec 26, 2018 at 15:14