Switch off circuit while charging LIPO

Thread Starter

rjjenkins

Joined Apr 16, 2011
179
Hello

There is a problem that seems well recognised, that it can be difficult to charge a LIPO battery while that battery is still powering a load. A solution to this, using the MCP73831 as the charging chip, is described here: http://blog.zakkemble.net/a-lithium-battery-charger-with-load-sharing

The circuit there uses a P channel mosfet to switch off power to the load while the battery is charging, and instead power the load from the USB port, via a Schottky diode.

Since I do not need the load to be switched on while the battery is charging, it is sufficient simply to cut power to the load while there is a voltage present from the USB, therefore it seems to me I do not need the diode. Also, I am in fact charging from a wireless charger which has an output of 4.2V rather than the 5V from a USB.

So I am hoping that this schematic will work
upload_2019-7-22_10-10-46.png

Apart from - will this work, my small questions are: presumably R1 can be any reasonably high value, eg 100k, enough to pull the gate low when there is no voltage from the charger, but small enough to not cause a voltage drop that would stop the MCP73831?
Also, is that capacitor really necessary, given that there is already one on the other side of the battery
 

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Thread Starter

rjjenkins

Joined Apr 16, 2011
179
Do you? P channel mosfets really confuse me. I’ve copied the schematic from the blog I linked to, and he seems pretty sure it’s this way round.
 

AlbertHall

Joined Jun 4, 2014
11,524
Perhaps wait for someone who knows what they are talking about.
To me it seems that the battery will always be feeding the load through the diode in Q1, whether or not the MOSFET is switched on.
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
179
I've gone through the guy's comments, and someone has asked the same question - and the answer he gives is the same as yours: reverse the Mosfet

upload_2019-7-24_18-39-6.png
 
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