I'm currently working on a battery powered IoT device. The situation is the following:
I have a supercapacitor connected to my battery via a network that boosts the 3,6V battery to 4,5V and limits it to 40mA to charge the supercap. This supercap is then connected to a buck/boost that provides 3,8V for my circuit.
I want to add a USB connector so I can update the firmware on one of my modules. In order to do that I need to connect the 5V USB line to the buck/boost so my module gets power. But I do no want my 5V USB to charge the supercap that is on that same line. So what I need is a circuit that decouples the 4,5V supercap and enables the 5V USB if the USB is connected.
Since this is a battery powered project, I want it to be as efficient as possible. I have had a couple of ideas so far, but all of them had flaws:

Idea 1:

This circuit uses a P-Channel MOSFET so disable the 4V5 line. Downside is that the supercap can still be charged due to the internal diode. Another downside is that p-channel FETS generally have a high Rdson. I've considdered using a JFET to prevent the supercap from being charged, but the added resistance on those isn't practical.
Idea 2:

This uses a simple diode with a low voltage drop. Yet this still consumes extra power and adds a voltage drop.

Do any of you perhaps know a circuit that has a low voltage drop but still meets my requirements?

~Michael

 

If the USB will only be connected very occasionally, why not use a mechanical switch to isolate the other supply?

 

If the USB will only be connected very occasionally, why not use a mechanical switch to isolate the other supply?

That sounds like a great idea. I kinda feel bad I did not think if it myself. Sometimes we overthink the simple stuff. Thanks for the tip!