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Can this MOSFET pass current in 2 directions? I want to use it as switch but VCC and GND may be changed by the user. I'm worried about the diode inside.

enter image description here

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  • \$\begingroup\$ (what do you think the diode symbol in the circle means?) \$\endgroup\$ Jan 3, 2022 at 7:18
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    \$\begingroup\$ When the MOSFET is on, it is on in both directions. When it is OFF, it is like a diode. \$\endgroup\$
    – mkeith
    Jan 3, 2022 at 7:22

2 Answers 2

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The MOSFET is a controllable switch only in one direction.

In the other direction, the diode will always conduct.

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    \$\begingroup\$ I just want to add a clarification here. What you say is true of a discrete MOSFET where the manufacturer has connected the source to the body. In VLSI design a MOSFET is a four-terminal device and can be used as a bidirectional switch. \$\endgroup\$ Jan 3, 2022 at 14:40
  • \$\begingroup\$ If the gate is sufficiently biased, the channel can 'short' the diode for lower losses. Yes, you cannot ever turn the switch 100% 'off' but your point that the diode will 'always' conduct is not true. Many high efficiency SMPS units use N-channel MOSFETs in this manner for output OR-ing. The gate is bootstrapped to a sufficient voltage to allow channel conduction when the power supply is operating (to prevent burning tens of watts in the body diodes) and when fault conditions are detected the gate is pulled low, the MOSFET switches off and the diode is reverse-biased (i.e. blocking). \$\endgroup\$ Jan 4, 2022 at 14:32
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When a MOSFET is "on", it will conduct in both directions. When a MOSFET is "off", it will still conduct if the body diode is forward biased.

A way to avoid this behavior is to use two MOSFETs in series with sources and gates tied together thus:

schematic

simulate this circuit – Schematic created using CircuitLab

When configured in this manner, when the MOSFETs are "off", the only current through them is the body diode leakage current. However, when they are "on", they have the typical low resistance of an "on" MOSFET (times 2 unfortunately, but often that is acceptable).

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