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Reverse voltage protection: shouldn't the mosfet be connected in another way?
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Moriambar:
Hi.

I just watched on reverse voltage protection.

There's something I don't get: why is the drain tied up to 12V? I thought that the source should be tied to Vcc and the drain to the circuit in a P-fet. Am I getting something wrong?

Cheers
exe:
I always struggle to put fet right. So, I normally use a simulator to confirm circuit behavior.

This circuit is called "mosfet ideal diode" and matches google images. So, I guess, it is correct :).

The circuit works because mosfet doesn't care which way current flows.

But if you put it "the right way" the intrinsic diode will start conducting.
Zero999:
A field effect transistor's drain and source are theoretically interchangeable, however in practise, MOSFETs always have a parasitic body diode, which is always connected cathode to source in a P channel device or cathode to drain for an N-channel device.

In this circuit, the drain is acting as the source and the source as the drain. When the power is connected up in the correct polarity, the MOSFET turns on, bypassing the internal body diode, dropping a  much lower voltage, than the usual 0.6V to 1V we'd normally expect from a silicon diode. When the polarity is reversed, the MOSFET turns off and the diode is also reverse biased, so blocks the current. If the MOSFET were connected up the other way, it would not block the current, when the polarity is reversed.
Moriambar:
thank you. I forgot about the body diode
Kasper:
Never forget the body diode ;)

I always include body diode in schematics.
1: as a reminder to everyone that it exists.
2: to make sure I orient it properly.

TI has a good explaination of reverse battery protection here:
http://www.ti.com/lit/an/slva139/slva139.pdf
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