I am working on a data logger design. I need to incorporate two power sources, one internal non-rechargeable battery (9V) and one external power source (12V-24V range).
I've looked at some online resources and reference designs. Here is a simple two P-MOSFET OR design:
Power Multiplexer Prevents Cross-Current Conduction POWER DESIGN By Scot Lester, Applications Engineer, Portable Power, Texas Instruments, Dallas
I'd like to base my design on this one instead of the much more complicated design in later part of this reference. Since the internal battery is non-rechargeable and lower in voltage, I don't think this is safe for my situation. if Q2's gate is pulled low, then a higher main WILL charge the non-rechargeable battery and possibly cause fire.
What if I connect Q2's gate to main instead of pulling it low? This way since main (external) is always higher than internal battery (aux), the selector always prefers external, which is what I'd like to do, to preserve internal battery as much as I can. I can also put fuses inline with main and aux. Will this be a sensible solution? I imagine I use a 100Kohm resistor inline with gates on both MOSFETS and the main's MOSEFT is always pulled low.
One rare situation would be that the external battery is run down to below 12V (unlikely if I have a legit Lithium battery solar charger in between), the internal battery will charge the external battery. Will this happen?