I have designed the attached power selector circuit, it has a 12V input and a 15V input, and should prioritize the later.
It operates in three different scenarios:
1- 12V source R76+R79 divider polarizes the Q25 BJT, driving the gate of the the Q9 P-MOS low, sucessfully generating -12V Vgs and opening the channel for current to pass. That way, I can switch this line without having the voltage drop on the intrinsic diode from Q9A.
2- 15V source R80+R84 divider acts in the same way as mode of operation #1, instead driving the gate of the MOSFET Q9B through BJT Q28.
3- Both Sources The 15V supply has priority, so when it is connected, it drives not only Q28, but also Q27 - This transistor brings the base of Q25 to GND, effectively bringing the Collector of Q25 to 12V, so that Q9A is not in conduction mode anymore.
What I'm observing, is that even when I'm in mode of operation #1, it seems that Q27 gets polarized in some way, as if the 15V supply were present. That closes the channel on Q9A and all the current supplied by the 12V source has to go through the intrinsic Diode, thus heating Q9A by a considerable ammount (we are talking in the 7 Amps figure here, so a diode drop is quite significant in terms of dissipated power).
I designed this circuit with the intention of having no diode drops, and have fallen exactly on that scenario... The question is: Why is Q27 still driving the gate of Q25 to GND if I haven't fed the 15V input?