Doing some reverse engineering to a PCB, I drew this part of the circuit. Looking into the datasheets, I found out the comparator (LMC6772) is able to drive the SCR (S6S2) by itself. But the designer used a PNP transistor in between. I see that the comparator is fed with single power supply, so I assume the PNP is to do some kind of shift as the SCR is referenced to -13V. Why would the designer do this? It's like the current from the coil is sinking to -13V instead of the ground, but why? If you need more details about the circuit I can expand the drawing, but that's the basic concept of my question.
Below there's a link which is a follow up of this thread. Some notes:
TR1 in the old thread is U5 in this thread.
V+ in this thread is the point between R2 and R3 in the old thread.
L1 in this thread is the coil in parallel with D9 in the old thread.
R20 in this thread is (R2 + Rds (FET)) in the old thread.
In this thread, I just drew the circuit for simplicity, since in the old thread the path from R3 is bypassed when the SCR is fired.