P-Channel  VS  N-Channel with driver IC - Page 1
Forum section: electronics / beginners

author: BSYS
author_rank: Newbie
timestamp: February 08, 2022, 09:22:10 pm
content: Hello everyone,I am seeking help here because i am working on a circuit that runs on 3V, and i am trying to switch a load on the high side.The load runs on 3.00v and draws between 100 and 200mA.The signal ( Logic High and Low, 25mA max ) is comming from a microcontroller, and the two solutions i came accros for high side switching are using a P-Channel Mosfet ( connect the gate to the microcontroller and a pull up resistor ), or using an N-Channel Mosfet in conjuction with a High-side Mosfet Driver IC ( with an internal charge pump ).Due to the low tension and consuption of this Load, I'm having a hard time choosing the best solution for this particular case.The main concerns are Reliability and power consuption. ( The device runs on a battery, and cost is not a problem as this is a one-off project ).Your help and insights are greatly appreciated.Thank you !

author: Manul
author_rank: Super Contributor
timestamp: February 08, 2022, 10:40:23 pm
content: Judging from what you wrote - definitely a P-ch mosfet. Add pullup, 10-100k. Add gate series resistor around 100R. Choose a mosfet with a suitable gate threshold voltage - around 1-1.5V maybe. Remember - you want mosfet to be fully on at the 3V gate drive (hint: look datasheet graph). If 3V might be less then 3V judge accordingly. For such low current there is no need for a very low Rds(on) 100-500mOhm should work nicely, so no need to put a "monster" there.

author: BSYS
author_rank: Newbie
timestamp: February 09, 2022, 12:40:59 am
content: Thank you very much for your insight.I made some researches and i found a P-Channel Mosfet that might be perfect :https://www.mouser.fr/ProductDetail/Vishay-Siliconix/SI2333CDS-T1-BE3?qs=CiayqK2gdcLKByimzq%252BdDA%3D%3DVgh(th) is of -1v, so it should be fully open if the Gate is pulled to ground.For the pull-up resistance i am planning to go with 50kOhms, and for the series resistance you recommanded I calculated a 300 Ohms resistor to limit the current to 10mA at the gate ( The microcontroller can output max 25mA safely ).Please let me know if one of my choices here are incorect.Thanks again for the help.

author: magic
author_rank: Super Contributor
timestamp: February 09, 2022, 10:11:44 am
content: Vgh(th) is where it barely starts to conduct some microamps.The truly relevant spec is RDS(on) at -3V or at -2.5V or -1.8V or something similar, tested with load current of at least 200mA.From there you know how much voltage the FET will drop while conducting your 200mA.

author: Manul
author_rank: Super Contributor
timestamp: February 09, 2022, 11:17:11 pm
content: Quote from: BSYS on February 09, 2022, 12:40:59 amYep, pretty much ok. Design choice also depends on what kind of load you want to switch. If you switch inductive load, you should add a clamping diode. If your load contains significant capacitance, you should slow down the switching speed perhaps to avoid very high peak currents. If you want high speed - your choice of mosfet is not ideal, because it has quite high gate charge (it will switch somewhere in the range of 200-300ns with the values you provided). It is also recommended to add bypass capacitor near the switching mosfet, especially if the load is somewhat capacitive or if switching is high speed.

author: langwadt
author_rank: Super Contributor
timestamp: February 09, 2022, 11:32:59 pm
content: for that voltage and current, an USB power switch would be a simple and cheap solution, and usually include some protectionjust one example:https://lcsc.com/product-detail/Power-Distribution-Switches_GMT-Global-Mixed-mode-Tech-G5243AT11U_C356795.html