Is this MOSFET correct?!

General Discussions
1

I have checked schematics from multiple companies and they all share the same lipo charging circuit, which I think it is wrong! Can someone tell me how I am wrong here please!

Here is how a P-MOSFET should work

image

It is used in lipo circuits, I am guessing, to power up the system from battery when there is no USB power. so lipo (VBAT) to system (VSYS) but this is backwards here

and here

and here

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What I think, it worked because the internal protection diode that is built inside the transistor allowed the VBAT to flow into the system but this is not what the FET suppose to do!

Please tell me I am crazy here and not the entire world is copy pasting wrong design.

2

both.

1.you are right for FET and you are not crazy.
2.copy past done because this solution work and do job ,even if not proper way.

3

Why would you want to implement a charring circuit? :smirk:

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4

fixed, thanks :wink:

5

new FEZ planned ?

6

Did not say yes :grin:

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7

If they are using the diode to allow the battery voltage to pass through then surely 2 things to consider. 1, what is the current capacity in this state and 2, what is there to stop the diode conducting in when the USB power is active? The battery voltage will be lower than the USB voltage but surely some current will still be flowing to the output.

I need to set this up in Altium and do a simulation as it does seem a rather strange way to do this.

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