Power source switching/isolation circuit

Question

I'm currently designing device which will be powered from external PSU but also will have backup power system from battery. I'm trying to solve my problem. When external power source is down (5V - USB), I'm switching to battery source. Battery powers my 3,3V power line through LDO regulator. At the same time boost converter turns on and powers my 5V bus. I have problem that external 5V source (USB) is connected directly to my main 5V bus, same as my boost converter. Where is problem? Charging circuit is also powered from 5V bus (USB), that means battery will charge itself, it'll drain quickly.

I want to note that only one source will work at the same time, there are additional switching circuits which will take care of it. There is no possibility that boost converter will be working when external source is connected.

I came with such idea like on picture above. However I'm bit suspicious, will it work? N-channel mosfet is "inverted" and I'm unsure how it will behave in real life scenario. I tried it with simple simulation and it somehow work, but it's just simulation... Any advices are welcome. I'd like to keep it simple as possible. If 5V bus wouldn't be critical, I'd probably just use diode, but I need to keep it as close 5V as possible at this stage of design.

Falstad:* (n-channel) (p-channel) *when top source is 5V, right source is 0 and vice versa

Also piece of main schematic I'm working on

I was also thinking about doing same thing with NPN transistor, but I'm unsure because of power losses and current ratings (this part of bus consumes up to 200-250mA),

In tl;dr I want to avoid powering a piece of power bus, isolate it.

Answer 1

Edit: After some thought, I have decided that I do not like the solution below for the following reason.

Under the assumption that the circuit acts ideally as a diode, and under the assumption that the battery boost circuit outputs exactly 5V, there still seems to be a 50-50 chance that the external supply is greater than the battery boost 5V vs being below the battery boost 5V.

That means that some times, the load will draw current from battery, and some times from the external supply.

Although the load will work when the power is drawn from the battery, the battery will be drained, and if the external supply should become unavailable, the drained battery will be of no use.

What I personally would like from a circuit that may use either a battery or external supply for power is

1. if the external supply is available, it is used to power the load, even if it provides a lower voltage than the boosted battery voltage.

2. if the external supply is available, and has excess capacity after powering the load, it will additionally charge the battery with this excess capacity.

3. if the external supply is available, but it does not have the capacity to supply both the load and charge the battery at the same time, then it will supply the load rather than charge the battery.

These may not be the same objectives as you, but they seem reasonable to me.

The down side of these objectives is that I cannot think of a circuit that achieves them with the same efficiency as an "ideal diode". The best I can think of involves a loss of power between the external supply and the load, possibly in the 5-20% range (80-95% efficiency). If my load does not require the amount of current that my external supply is capable of providing, I wouldn't mind that loss. However, if my load could only barely run with a standard usb supply, I might not be happy with such a loss.

I may offer an alternative that assumes that a 20% power loss from external supply to load is acceptable.

Original: Try this (partial) circuit to isolate the power going into the charger, battery, and boost converter from the +5V rail.

(The switch is there to simulate that the external power supply might be disconnected. It is not needed in your circuit).

^{simulate this circuit – Schematic created using CircuitLab}