Hi,

I recently came across this article on how to modify MCP73831 circuit to make load sharing possible with a simple P-channel MOSFET. I want to clarify the direction of the P-channel MOSFET placed in the below circuitry.

Question: Isn't Q1's drain and source connections supposed to be reversed? because in this mode, even through MOSFET is off, the body diode conducts right?? Isn't that wrong? Please correct me

 

I see two conditions that system may be used in: 1> The load drawing current while the USB connection is supplying power, and 2>the load drawing current when the USB connection is not supplying power. My guess is that the added MOSFET is somehow supposed to disconnect the battery from the load when the USB is providing power. The actual mechanism of that working is not clear to me. I have not read the article, which might explain the theory of how it should work.

 

It looks incorrect to me, but I may be missing something.

 

I see two conditions that system may be used in: 1> The load drawing current while the USB connection is supplying power, and 2>the load drawing current when the USB connection is not supplying power. My guess is that the added MOSFET is somehow supposed to disconnect the battery from the load when the USB is providing power. The actual mechanism of that working is not clear to me. I have not read the article, which might explain the theory of how it should work.

That's right, MOSFET controls the connection of battery in the presence and absence of the main voltage. My doubt is particularly about the MOSFET direction, I am assuming the MOSFET conducts even when it's OFF through body diode right?

 

It looks incorrect to me, but I may be missing something.

Apparently MCHP's appnote also suggests the same, I couldn't understand why.

 

I rather much doubt that the circuit actually works that way. So now the questions: What is the actual source of this article, and who is the author? AND WHERE is the source of the article?
Some on-line sources are not very technically competent. But they write quite well.

 

Hi,
The circuit diagram being used is from this App PDF.
E

 

Hi,
The circuit diagram being used is from this App PDF.
E

OK, that is a trustworthy source! Thank You!!

 

here's the explanation I got from some more research, the argument is if we reverse the MOSFET's drain and source then the USB voltage would be conducted through the body diode and charge the battery incorrectly which is unsafe, so we can reverse the MOSFET even if it comes at the cost of battery discharging. I have raised a support ticket with microchip will update the latest here.

 

Makes sense, leave it to me to miss an unobvious current path...seems to be my superpower.

I guess the solution of using a diode in-between the battery and the MOSFET would lose too much battery voltage.

 

I guess the solution of using a diode in-between the battery and the MOSFET would lose too much battery voltage.

Ahhh yes using diode to isolate battery isn't a great way and I read somewhere using MOSFETs are the control circuit are called "Ideal diodes" which makes sense as they drop negligible voltage