The circuit in question uses the LT4356:
So, I'm trying to get a grip on how this circuit works precisely. I've prototyped the pictured schematic minus everything to the left of D2. So, essentially, no reverse polarity protection... it's just the main IC controlling Q1 to allow power through. That part I understand just fine.
When adding the reverse polarity protection via all that extra stuff before D2, everything get super fuzzy for me. I know Q1 and Q2, based on their datasheet, have body diodes. Thus, I know that when everything is hooked up correctly, Q2 will let power flow and everything past it should work normally. Conversely, I know when polarity is reversed, the body diode will block power flow.
After that, I'm pretty confused. I can roughly reason the operation of D1, Q3, R7 and D3 as a group, but together with the rest of the circuit.... I'm confused on how they work / help the circuit in either a normal or reverse polarity situation.
Anybody able to shed a little light on this for me? :)
EDIT: I realized, after some more graph paper and highlighter action, that when there is normal polarity that Vbase will be the same as Vemitter, so Q3 should effectively be off.
I'm still sort of unclear, though... when Q3 starts conducting during a reverse polarity situation... is all it's doing is pulling the gate for Q1/Q2 to ground to stop them from turning on? If so, why even bother with Q3? Q1 would have its body diode to allow enough current to pass to turn things on normally, and otherwise, it would block reverse polarity. Confused on the benefit of Q3 still. :(