N00b MOSFET question - Page 1
Forum section: electronics / beginners

author: jeringa
author_rank: Newbie
timestamp: April 11, 2020, 05:50:54 am
content: On a board I'm working on. I wanted to be able to protect the power supply side from the USB power side... I thought the following would work to stop power from backflowing into the USB when on the Regulated supply... I was wrong... But why? I've seen this circuit as an example for battery protection.When I built it up; I could see ~5v on the Drain. Gate buzzed out as connected to GND. Nothing on the source pinI've attached the FET circuit I was trying to use as an attachment.Could some kind soul point out the error of my ways?

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2020, 06:48:51 am
content: The source floats up due to minuscule leakage current through the MOSFET from the drain until there's enough Vgs to start turning it on. Try it with 100K from source to ground, and you should no longer have any voltage on the source, so long as it wasn't plugged in to a USB host when external power was initially applied.However to achieve the desired result of preventing back-powering USB Vbus, you need to take its gate high when external power is applied. e.g. add a 10K resistor between the gate and ground, then pull it up via a diode from some point that only gets external power.

author: t1d
author_rank: Super Contributor
timestamp: April 11, 2020, 12:33:35 pm
content: Quote from: Ian.M on April 11, 2020, 06:48:51 amGood information, Ian. These are things I would not have thought of.Jeringa, remember to watch out for the orientation of MOSFET-type pins... These are ICs, which include multiple parts... Meaning, the direction of the integrated diode is important, as you will have power on both sides of it.

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 11, 2020, 02:34:36 pm
content: Case 1: 5V on pin 3, float pin 2Body diode will lift pin 2 until the channel is inverted pulling pin 2 to 5VCase 2: 5V on pin 2, float pin 3Channel is inverted pulling pin 3 to 5VAs Ian said, this is not the circuit you want to achieve the goals stated.

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2020, 06:01:54 pm
content: Whoops - I mixed up source and drain and as a result, posted a load of baloney in my first paragraph. Please go with Wimberleytech's explanation of the problem, *NOT* mine!The second half of my post is mostly correct - the gate (with a pull-down resistor) needs to be tapped off from somewhere in the ext DC in circuit, and the diode I mentioned, needs to be in the ext DC in circuit after that point, not in the feed to the gate, so any leakage back through 5V regulators etc. when Vcc is coming from USB Vbus cant turn the MOSFET off, which would cause excessive voltage drop. If you need to use 5V EXT DC in rather than a higher voltage and regulating it down on board, and need a full 5V Vcc, you'll need to use an ideal diode controller and another MOSFET in the ext DC in circuit.

author: jeringa
author_rank: Newbie
timestamp: April 11, 2020, 11:31:48 pm
content: Awesome guys! Cheers very much

author: langwadt
author_rank: Super Contributor
timestamp: April 12, 2020, 12:04:13 am
content: this is(was?) used on the raspberry pihttps://github.com/xioTechnologies/Ideal-Diode/blob/master/README.md

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 12, 2020, 02:32:27 pm
content: Quote from: langwadt on April 12, 2020, 12:04:13 amClever design!