[SOLVED] P-Channel mosfet voltage loss between D-S - Page 1
Forum section: electronics / beginners

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 10:16:18 am
content: I'm busy with a project to switch 5V (0.5A) using a p-channel mosfet (NDP6020) and it's kind of working although I'm getting a unexpected voltage drop across the D-S of the mosfet.https://www.onsemi.com/pdf/datasheet/ndp6020p-d.pdfThe mosfet is being driven by a NPN transistor and I'm using a 3.3V atmega88 microcontroller to switch the mosfet on/offWith the output of the Atmega88 at 3.3V this turns on Q2 which puts the voltage of gate of the mosfet low enough to turn it on.I would expect 4.98V at the drain. I though it might be as there was no load on the drain but have added in a 1k resistor from drain to GND and still seeing 4.63VClearly there is some issue with the circuit or my understanding of mosfet is wrong.I've taken voltage readings and added them to the schematic.Where am I going wrong?

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 27, 2024, 01:25:09 pm
content: Did I read it right you loaded the FET with 1Ohm? Then around 5A I_DS are expected, and a voltage drop of 350mV with a RDS_on of 50mOhm + some mOhm of PCB/solder/voltage source resistance seems not unplausible.Can you measure the I_DS?

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 01:28:58 pm
content: Sorry, correction. loaded with a 1k resistor.

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 27, 2024, 01:33:56 pm
content: Did you incidentally swap S and D? Is the switching working in general?

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 01:45:33 pm
content: I've rechecked and orientation is correct, but it's not switching off eitherTried swapping the mosfet - same issue.Have updated the schematic with voltages when the output the uC is LOW.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: June 27, 2024, 01:49:39 pm
content: Are you absolutely, positively sure you haven't got the MOSFET the wrong way round? A voltage drop of 0.35V sounds like what you might see for the VFof the body diode passing current. Especially if it "won't turn off".

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 01:55:34 pm
content: 100% sure. Rechecked footprint and soldered in the right way.Unless I've got some dodgy mosfets but i have tested them with a component tester and the pinouts match.

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 27, 2024, 01:58:40 pm
content: Do you have a transistor tester?Many MOSFETs from dubious sources are fake, sometimes they just relabel anything in a TO220-housing.

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 02:06:51 pm
content: Transistor tester passes and matches the footprint.

author: wraper
author_rank: Unknown
timestamp: June 27, 2024, 02:08:58 pm
content: NDP6020 is N-channel and you tester shows that toohttps://www.farnell.com/datasheets/59594.pdf, NDP6020Pis P-channel. Also this is an obsolete part which you should not buy for anything other than repairs, still I would get something different for that as you'll likely get fakes from places where they are available.

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 02:14:46 pm
content: Aaggghhh. How many mistakes can I make on this projectIt's supposed to be a P-channel but during my search for a mosfet I select N-channel instead.Will look into making some hardware hack to use a N-channel (Or probably better just buy the correct mosfet)

author: wraper
author_rank: Unknown
timestamp: June 27, 2024, 02:17:22 pm
content: Quote from: gmc on June 27, 2024, 02:14:46 pmN-channel won't work without about 8V or higher supply to drive its gate unless you put it on the low side.

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 02:19:37 pm
content: Scratch the above. Another doh moment.I bought a NDP6020P (So at least I'm not losing my mind) which as you said should be a p-channel mosfet.It's testing as a N-channel so I've been sold a fake as you suggested

author: wraper
author_rank: Unknown
timestamp: June 27, 2024, 02:22:47 pm
content: What's actually written on the part? I see listings on aliexpress use both part numbers in the title. It's very rare that suffix means different channel type, so easy to get confused even for experienced engineer.

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 02:25:34 pm
content: NDP6020P

author: wraper
author_rank: Unknown
timestamp: June 27, 2024, 02:26:33 pm
content: Then must be fake, counterfeiters looked in the wrong datasheet

author: gmc
author_rank: Regular Contributor
timestamp: June 27, 2024, 02:28:08 pm
content: Thanks for the help. At least that explains it. Will source a more modern part while from a better source now.

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 27, 2024, 02:38:03 pm
content: Quote from: wraper on June 27, 2024, 02:26:33 pmOr the P-type is 1cent more expensive... This way the component will at least not work from the beginning instead of failing during usage.People who organize this should get circumcised with the solder iron.