Electronics > Beginners
Please review my lipo charger and step down schematics
Peabody:
Happy New Year!
First, it seems you have the P-channel mosfet oriented backwards. You have it oriented in the typical switch mode, but for this purpose it needs to be the opposite way around - with BAT+ feeding the drain, and the source at Vout. The reason is the body diode, which needs to be forward biased from the battery toward the load. The body diode needs to block current from flowing from USB/D1 back into the battery. The mosfet will conduct about the same in either direction if it's turned on, but in this case the direction of the body diode is controlling. Compare your drawing to the gerdmuller drawing.
The purpose of the resistor is to make sure the mosfet gate is turned ON when USB is NOT plugged in, so the battery can supply current, but turned OFF when USB IS plugged in. You want USB to directly power the load and charge the battery, independently, and you need the mosfet to be OFF then. The body diode will not pass current from the battery because the 4.2V maximum voltage there is lower than the USB voltage. You may have to adjust the value of the resistor. A full explanation can be found here:
https://blog.zakkemble.net/a-lithium-battery-charger-with-load-sharing/
And yes, a Schottky is used because the forward voltage drop is lower.
Kasper:
Happy New Decade!
re PMOS orientation: it is helpful to include the body diode in the schematic symbol so it is easier to read and harder to screw up.
online1234:
Thanks for the explanation! I was confused by the name of the terminals, thinking just to connect them according to the name.
Trying to figure out the value of the resistor, but regrettably, I don't have the basic electronic knowledge. My brain is fired only just look at those equations. :palm:
I go with 100k as per the example for now. :P
online1234:
--- Quote from: Kasper on January 01, 2020, 08:16:30 pm ---Happy New Decade!
re PMOS orientation: it is helpful to include the body diode in the schematic symbol so it is easier to read and harder to screw up.
--- End quote ---
I downloaded the symbol from somewhere.
After more research, I redrew the symbol and it makes much clearer on how it works!
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