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Zero999:
Perhaps I'm confused? No schematics have been posted of what the original poster meant by two MOSFETs.


--- Quote from: OM222O on April 20, 2019, 10:29:57 pm ---I understand how the power oring chips works but even the cheapest one on mouser (MAX40200) costs about 0.70$ ... I can get 2 dual fets (any fet is easily rated for 5A) for almost half of that, but those chips are also a viable option. thanks!

--- End quote ---
How would you intend to solve this, without one of those ICs?

Because the MOSFETs conduct in both directions, the voltage on one input can back-feed to the other input, which isn't connected. The direction of the current needs to be sensed, in order to decide which MOSFET is activated, and is what these ideal diode controllers do. It's possible to do it with and op-amp, but is far easier to just use a dedicated IC.
madires:

--- Quote from: OM222O on April 20, 2019, 10:29:57 pm ---I understand how the power oring chips works but even the cheapest one on mouser (MAX40200) costs about 0.70$ ... I can get 2 dual fets (any fet is easily rated for 5A) for almost half of that, but those chips are also a viable option. thanks!

--- End quote ---

It's either an ORing controller plus MOSFET or a Schottky diode with larger power loss (less battery runtime). Choose one! ;)
OM222O:
as per the application note by TI, if you connect two P channel fets with common source config (or N channels with common drain) there will be 2 body diodes in opposite directions, effectively blocking the back feed. their gates are also connected so they turn on / off together. dual fets with common drain / source configs are pretty cheap on mouser.
SeanB:
Dual 12A Schottky rectifier in a TO220 package, with power from battery on one leg and 12V on the other. 0.4V drop, and low enough leakage at room temperature to not worry, plus with 5A only a small TO220 PCB mount heatsink needed. 2W of power worst case, and the highest voltage source supplies the load, no dropout when changing. Common item, used in every PC power supply, and available from multiple vendors.
OM222O:
as I mentioned, I don't want to deal with diodes because of huge wastes of power and voltage drop (especially from battery) as I said the cheapest option seems to be the dual fet approach.
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