Reverse voltage protection: shouldn't the mosfet be connected in another way? - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: March 21, 2019, 10:30:33 am
content: Hi.I just watchedon 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

author: exe
author_rank: Unknown
timestamp: March 21, 2019, 10:47:30 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2019, 10:52:54 am
content: 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.

author: Moriambar
author_rank: Unknown
timestamp: March 21, 2019, 11:53:38 am
content: thank you. I forgot about the body diode

author: Kasper
author_rank: Frequent Contributor
timestamp: March 21, 2019, 03:43:55 pm
content: Never forget the body diodeI 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

author: Moriambar
author_rank: Unknown
timestamp: March 21, 2019, 03:56:50 pm
content: Quote from: Kasper on March 21, 2019, 03:43:55 pmyou are right. Awesome link!

author: exe
author_rank: Unknown
timestamp: March 21, 2019, 04:36:27 pm
content: Quote from: Kasper on March 21, 2019, 03:43:55 pmSame here. Without it symbol is quite confusing for me. I also don't really distinguish n-fet and p-fet, only remember it's the opposite to npn/pnp.

author: David Hess
author_rank: Super Contributor
timestamp: March 22, 2019, 12:12:42 am
content: Quote from: Zero999 on March 21, 2019, 10:52:54 amIn practice, MOSFETs always have *two* parasitic body diodes; one of which may or may not be shorted, leaving the other between the source and drain. Four terminal MOSFETs with a separate substrate connection illustrate this perfectly:https://www.microchip.com/wwwproducts/en/MIC94030For an n-channel MOSFET, the substrate connection must be to the most negative voltage or the substrate diodes can be forward biased. For a p-channel MOSFET, the substrate connection must be the most positive voltage.Quote from: Kasper on March 21, 2019, 03:43:55 pmI always include the body diode in my schematic symbol. Some people say this should not be necessary because a diode is already marked at the center of the symbol but that diode is consistent with the emitter of the parasitic bipolar transistor.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2019, 09:33:42 am
content: Quote from: Moriambar on March 21, 2019, 11:53:38 amIt's also why MOSFET power switches often have two transistors connected back-to-back, to prevent back-feeding.Quote from: David Hess on March 22, 2019, 12:12:42 amThat's a useful part. There's also the Si3831DV, which has a proprietary biasing circuit to avoid the requirement for two back-to-back MOSFETs.https://www.vishay.com/docs/70785/70785.pdf