Electronic – 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:

reverse protection circuit

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:

reverse protection DC1850

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.

Best Answer

the -100V is a 5A inductive pulse, whereas DC survival spec is only -30V +50V.

The reason for this -30V is in case a alternator charged 24V vehicle is used to jumper cable another with the cables reversed. (momentarily)

The 2nd simple circuit is missing parts and will not survive all the test conditions for ISO-16750-2 tests 1 thru 5b . Keep this date in mind when reviewing solution documents.

For more details, consult with ISO-16750-2 or the older (2010) ISO7637-2 test levels on each pulse and Steady state conditions. THere are different classes III and IV with pulse levels up to -600V for 24V system.