P channel MOSFET "switch" - Page 1
Forum section: electronics / projects

author: tchiwam
author_rank: Regular Contributor
timestamp: March 20, 2021, 09:39:14 pm
content: I am looking for a neat way to power myself down with a P-ChannelBattery power is 3.7VVCC after the DC-DC is 3.3VMosfet located just before the DC-DCMax sustained power is 1A, Inrush 1.6AG to 0V powers the unit upG to >3V powers the unit down unit I press the power on buttonTrying to get the smallest stand by power as possible.Any suggestions ?I do have a reflow station, doesn't need to be that easy to solder...

author: MarkF
author_rank: Super Contributor
timestamp: March 20, 2021, 10:21:36 pm
content: Something like this?Just add a switch across the NPN transistor to pull the MOSFET gate low and turn 'ON' your circuit.The MCU pin D2 will maintain power and enable it to turn itself 'OFF'.

author: tchiwam
author_rank: Regular Contributor
timestamp: March 21, 2021, 12:30:59 am
content: Pretty much that, Instead I use an N-Channel instead of the 2N3904.I tried with a Si2323DS , this is currently not working when Vin goes above 2.2V. Bummer since the battery is 3.7V.That is even if I hold the gate open or Vin tied to Vin.

author: MarkF
author_rank: Super Contributor
timestamp: March 21, 2021, 01:59:48 am
content: If you use a N-channel MOSFET instead of the NPN transistor, is the MCU output voltage high enough to fully turn it 'ON'?I would guess for a 3.3V MCU the answer is no.

author: rmaranhao
author_rank: Newbie
timestamp: March 21, 2021, 02:15:37 am
content: Quote from: MarkF on March 21, 2021, 01:59:48 amDepending on the mosfet, sure. You can get thresholds way below that with small currents like that (370~400uA ish?).@tchiwam is your pass mosfet connected correctly? Isn't it conducting through its body diode?

author: tchiwam
author_rank: Regular Contributor
timestamp: March 21, 2021, 02:33:07 am
content: I am currently driving the gate manually between 0V at 3.3V Si2323DSD(3) to S(2) 425mVS(2) to D(3) OpenG(1) to D(3) or S(2) OpenD Battery sideS Load sideG a jumper for nowEdit:Battery side (bench PSU from 2 to 3.9VDCLoad side 3.3VDC

author: tchiwam
author_rank: Regular Contributor
timestamp: March 21, 2021, 03:16:19 pm
content: Yup, I hate flipping SOT-23 upside down and bodge solder them at 60deg...

author: tchiwam
author_rank: Regular Contributor
timestamp: March 21, 2021, 06:53:37 pm
content: 2303 are also P-Channel MOSFET ... I need N-Channel...Remember not to copy and paste .... Si2312BDS are in order, let's see...

author: Mecanix
author_rank: Frequent Contributor
timestamp: March 23, 2021, 11:30:59 am
content: N-Chan, High-side. See if that works, for you.

author: Peabody
author_rank: Super Contributor
timestamp: March 23, 2021, 02:23:23 pm
content: I'm having trouble understanding where things stand at this point. Do you understand that your configuration in reply #5 is backwards? For P-channel, the source should be connected to the battery and the drain connected to the load. Anyway, here's a two-mosfet circuit where the pushbutton turns on the power, but can also be used for further input after the circuit is on.

author: tchiwam
author_rank: Regular Contributor
timestamp: March 23, 2021, 09:50:04 pm
content: Yep, it was plugged backward SD-DS and the footprint was wrong too. So I ended up flipping the legs top side and soldering the SOT 23 60 deg rotated on the PCB.Works now, Now looking for the lowest possible stand-by power possible.Thanks for the sanity check.

author: tchiwam
author_rank: Regular Contributor
timestamp: April 04, 2021, 02:33:35 pm
content: Hmmm, next challenge ... Make sure the N channel will not try to turn on with a Gate between 1.6 and 2.5V. My 2nd power up logic is 1.8V driven ...