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I am using transistor NXP PSMNOR9 http://www.nxp.com/documents/data_sheet/PSMN0R9-25YLC.pdf

and I have it in an OR power controller circuit using a TI TPS2413 http://www.ti.com/lit/ds/symlink/tps2413.pdf - Page 18

When I apply 5V to the source and have the gate grounded then I am getting 4.5V on the drain.

I also have the transistor by itself simply hooked up to a power supply with 5V on the Source and ground on the Gate and the Drain is at 4.5V.

My prototype PCB works the way that we intended, but we wanted to add a way to drop out one of the multiple power supplies by forcing the gate low. This looks like it will still work by doing this. However, I was under the impression that bringing the gate voltage to 0V would thus bring the drain to 0V.

I understand this is an unconventional use of a MOSFET, and I can not find any information on why it is performing this way in this use.

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    \$\begingroup\$ Two words: body diode. Look at the first figure in the TI datasheet carefully. \$\endgroup\$ – The Photon Jan 5 '16 at 18:18
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    \$\begingroup\$ The N channel MOSFET works Drain -> Source and not Source -> Drain. There is an internal body diode that is forward voltaged Source to drain that will always remain on. Flip the MOSFET and see what happens. \$\endgroup\$ – vini_i Jan 5 '16 at 18:19
  • \$\begingroup\$ Thanks guys, that makes a lot of sense now. The 0.5V drop is from the body diode. The way that the circuit works it has to be this way to prevent reverse current to a power supply. I just wanted to understand what was going on. \$\endgroup\$ – tooclosetosee Jan 5 '16 at 18:59
  • \$\begingroup\$ Also to note is that you can reduce the voltage drop and therefore power loss by turning the FET on when the diode would normally conduct anyway. This isn't always needed, but it can sometimes make a big difference in overall efficiency. \$\endgroup\$ – AaronD Jan 5 '16 at 22:38
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Your circuit cannot block forward currents because of the MOSFET body diode. Notice how it is shown in the application diagram from your TI datasheet: enter image description here

Even if you pull the gate voltage to 0, the body diode will still conduct, however it will drop some tenths of a volt, and so consume power. The reason to use the TPS2412 is that when the TPS2412 applies a high voltage to the FET's gate, it creates a conducting channel through the MOSFET, reducing the power consumed by the FET.

On the other hand when the "voltage source" is not available, the TPS2412 can still prevent reverse current flows from the "common rail" by pulling the gate low.

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