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In the below image of a DW01 protection IC they use a dual channel N-type mosfet to switch the battery negative. What confuses me is the symbol shows them opposite. I'm guessing they can conduct from drain to source and vice versa? A dual channel mosfet is already internally connected drain to drain, so that is why they appear this way?

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MOSFETs can conduct in either direction but can only block in one direction because of the body diode.

The body diode is a characteristic of the MOSFET construction and not something that is deliberately created for any particular purpose.

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  • \$\begingroup\$ If one were using individual MOSFET's, would they need to be oriented drain to drain as in a dual package? Is the dual package generally for paralleling them up so as to reduce the resistance and increase the max current they can handle? \$\endgroup\$
    – Cam_ _D
    Commented Jan 30, 2023 at 6:09
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    \$\begingroup\$ Either way (source to source or drain to drain) is theoretically workable if blocking in both direction is the only requirement. In this case, there are other requirements to make the battery behave sensibly and the two gates are independently controlled- see the datasheet of the FS312. Obviously the resistance of two MOSFETs in series is double that of one, all other things being equal. I would not be surprised if the dual MOSFET is manufactured for this particular application- the volumes must be astronomical. \$\endgroup\$ Commented Jan 30, 2023 at 6:22
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    \$\begingroup\$ Internal drain to drain connection is not otherwise a particularly useful configuration for an N-channel MOSFET pair, IMHO. \$\endgroup\$ Commented Jan 30, 2023 at 6:25
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I'm guessing they can conduct from drain to source and vice versa?

Yes. When a power MOSFET is fully enhanced ("on"), current can move in either direction with equal facility.

A consequence of the FET construction is a parasitic diode, called the "body diode". Usually this is a zener diode with a breakdown voltage that is greater than the drain-source voltage rating of the FET.

When the FET is off, the diode still is there, and will pass current in one direction just like a power rectifier. To prevent this, you need a second diode connected "back-to-back). This can be a actual rectifier diode, but then that diode's forward voltage drop will be in the circuit when things are on. By using a second FET, the two diodes prevent current when things are off, and each FET shorts out its own diode when things are on for much better efficiency.

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The dual mosfet scheme uses N channel which are cheaper and easier to find and have less on resistance for the same die size. The series scheme allows charging and discharging to be stopped if there is a problem .If you had just one fet the on resistance is half but the body diode would conduct in the reverse direction so you could not stop charging and discharging .

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