P-channel Mosfets review  - Page 1
Forum section: electronics / beginners

author: Ryan_!
author_rank: Newbie
timestamp: May 31, 2022, 02:41:43 am
content: Hi,I am a hobby maker and am designing a board never used P-channel Mosfets before. Hoping if someone could take a look at my design and verify it for me. I am using the same n-channel Mosfets else were to keep part count down reusing here. Also, Can anyone recommend a paid PCB review service before I get it made?Thanks

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 31, 2022, 04:20:21 am
content: Looks OK to me FET wise. But I don't know why you'd need the 1k resistors unless it's to deal with gate capacitance. You could also save a bit of power by increasing the 10K resistors to a higher value if switching speed isn't much of an issue.

author: Peabody
author_rank: Super Contributor
timestamp: May 31, 2022, 03:42:37 pm
content: It's a technical quibble, and probably won't make any difference in operation, but you have both gates at the midpoints of what are essentially voltage dividers. Move the source resistors to the other side of the gate resistors.

author: ledtester
author_rank: Super Contributor
timestamp: May 31, 2022, 04:34:52 pm
content: Quote from: Peabody on May 31, 2022, 03:42:37 pmWell, the 1K resistors could be reduced quite a bit (like to 100R) and that would skew the voltage dividers a lot more.Still, it is interesting that practically every MOSFET circuit you find uses the voltage divider configuration.

author: gamalot
author_rank: Super Contributor
timestamp: May 31, 2022, 05:01:35 pm
content: As others have said, those two 1K resistors are not necessary here, and those pull up/down resistors can be much higher values than 10K, I'm using 560K in my similar design , of course you can just ignore this advice if it's not a low power design.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2022, 05:16:28 pm
content: The AO3400A is overkill. Use a BJT with a 10k base resistor and get rid of the pull-down.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 31, 2022, 06:30:09 pm
content: If the controller also works at 5V, you can skip the N-ch Fet

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2022, 06:39:49 pm
content: Quote from: DavidAlfa on May 31, 2022, 06:30:09 pmThe schematic attached to the original post says 3.3V, so no.If it were 5V, you would be right, but the logic would be inverted.

author: Ryan_!
author_rank: Newbie
timestamp: May 31, 2022, 11:17:35 pm
content: Thank you for your helpI have removed the P-channel gate resistor and increased the P-up/ P-dn resistors to 200kOn the N channel I have left it as I am under the impression that you should limit the current to the gate from the micro controller? I am using a pocket beagle so a dead pin would be an expensive problem.I have included my 'Reference' MOSFET circuit as well

author: ledtester
author_rank: Super Contributor
timestamp: June 01, 2022, 01:29:24 am
content: Quote from: Ryan_! on May 31, 2022, 11:17:35 pmIf in doubt I would leave a footprint for it when laying out your PCB. You can always populate it with a 0 ohm resistor.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 01, 2022, 03:05:27 am
content: Increase the turn-off resistors to 220K and the gate resistors to 10K.Should work in the same way for on/off functionality, unless you're using PWM.The increased gate resistor prevents any kind of damage on the pocket beagle unless it sees really huge voltage.There's no problem on driving the P-ch gate without resistor.The capacitance is very low and the n-fet won't turn on in 1ns, the rising edge will smooth the initial current, and by the time it's fully on the gate will barely hold any charge.It would be a problem when switching very high frequencies, causing resonances, ringing... But not an issue for simple on/off.But be clever and put a 10R resistor there, won't harm anything and will allow easy modification for whatever that may happen in the future.

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2022, 08:16:11 am
content: Quote from: Ryan_! on May 31, 2022, 11:17:35 pmThe P-FET Pull low for on state, needs to have >4.5V on the gate to turn off. The AO3401A has a minimum threshold voltage of -0.5V, which means it might start to turn on, if the gate is pulled 0.5V below the source, which would be 4.5V, as the source is at 5V in this case. In other words, it requires a 5V control signal and won't turn off properly with a 3.3V signal.