Okay.. got it. It needed another rough comparator:
- charge3.jpg (26.18 KiB) Viewed 584 times
To work through the reasoning, the basic power control module looks like this:
- control-1.jpg (13.81 KiB) Viewed 584 times
If V1 is higher than V2, the comparator turns the P-mosfet on and the output voltage equals V1. Replacing the comparator with a mosfet looks like this:
- control-2.jpg (12.26 KiB) Viewed 584 times
It does roughly the same thing, but now there's a DC offset between V1 and V2.
The basic rule for mosfets is that when the voltage between gate and source rises above a certain level (called Vth), the channel becomes conductive. The more Vgs exceeds Vth, the more conductive the channel becomes. For the circuit above, both the P-mosfet and N-mosfet are controlled by the difference between V1 and V2, so the circuit will work best when V1-V2 is significantly higher than Vth for both mosfets.
I drew the resistor above the N-mosfet connected to a general 'high' voltage because it doesn't specifically need to be connected to either V1 or V2. It just needs to be high enough to keep the P-mosfet shut off when the N-mosfet is closed.
Taking those ideas back to the circuit above, it's easier to see the 'comparator' connections.
N1 compares the 7v supply connection to a 6v reference voltage derived from the 12v rail. The 1M-1M voltage divider pulls the 12v input line to GND when the 12v supply isn't connected, so in that condition N1 will be shut off no matter what's happening on the 7v rail. If both the 12v and 7v supplies are connected, N1 compares the 7v input to a 6v reference voltage. That still keeps N1 shut off, so P2 also remains shut off. If the 12v supply is connected and the 7v supply isn't, N1 comes on, putting N1's 1M drain resistor in series with the 100k source resistor. That pulls the gate of P2 about 11v lower than its source, which connects the 12v supply to U2.
N2 compares the 12v input to GND. If the 12v supply isn't connected, N2 will be shut off, and so will P3. Shutting off P3 is important because we want to keep P1 from pulling its own gate high when it turns on (the problem in the previous versions). I connected the N2's drain resistor to the input of U1, because that node carries the highest voltage connected to the circuit at any given moment.
With N2/P3 isolating P1's drain from its gate, P1 turns on when the 7v supply is connected but the 12v supply isn't, and shuts off when the 12v supply is connected.. what I wanted before but didn't guarantee.
I took the time to do a proper circuit analysis this time, and think I found most of the gotchas. There will be some minor misbehavior during the rising and falling edges when you connect/disconnect one of the power supplies becase there's a range of voltages where both P1 and P3 are open at the same time. That would be a problem if you planned to keep both supply voltages in that range for a long time, but in practice the circuit should blast through that range in a few microseconds. The 100uF cap should be able to absorb any spikes caused along the way.
