| Electronics > Projects, Designs, and Technical Stuff |
| Reverse bias protection with p-Mosfet for 4xAA cells |
| (1/1) |
| alexg:
Hi all, due to out of stock on part I am currently using, this one: https://www.onsemi.com/pub/Collateral/FR014H5JZ-D.pdf, I need to replace it with p-Mosfet, I am trying to choose the right candidates, can you please take a look into these few and let me know if you see any issues? Project parameters: Power supply - 4xAA cells in series, so only rev. bias protection necessary if from user error. With fully charged batteries max reverse votlage is -7V With almost discharged batteries min rev voltage is -3.6V Current consumption is well under 100mA Only peak current consumption is during startup when caps are discharges (bulk cap and voltage regulators) but I suppose that will Be limited by AAs internal resistance ,etc, so about 2A peak only during startup. So if I understand correctly I need to find p-mosfet that meets following criteria: 1. Vds +-7V or more 3. Rds 50mOhm or less to be sure there is no voltage drop in the system 3. Vgs (-3.6-0.6=-3V) or less, to be able to turn on with almost depleted AA batteries 4. Vgs max be at least 7V I am thinking about these 4 specimens, what you think? http://www.ti.com/lit/ds/symlink/csd25310q2.pdf https://www.fairchildsemi.com/datasheets/FD/FDMA520PZ.pdf https://toshiba.semicon-storage.com/info/docget.jsp?did=481&prodName=SSM3J130TU http://www.vishay.com/docs/72024/72024.pdf Thank you in advance. |
| ignilux:
Any of those should work. I'd go with the lowest Rds,on specified at the Vgs you'll be in. Looks like the TI wins there. Furthermore, traditionally N-channel mosfets have lower Rds,on vs P-channel mosfets. There are a lot more to choose from, too. In a battery-powered application like this you can simply place it in the return path to the negative terminal, and connect the gate to the positive terminal. Edited for lack of reading, carry on. |
| ignilux:
Haha yes, I edited my comment before I saw your response. |
| TimNJ:
I would recommend using an N-channel MOSFET. It's no harder than using a P-channel. Connect the source to your circuit's local "ground", and drain to the negative terminal of the battery holder. Pick the lowest RDS(ON) FET you can get. Interestingly, in a switching application, operating in kHz or MHz region, the lowest RDS(ON) FET is not always preferred because low RDS(ON) FETs also have high gate charge and input/output capacitance. When you switch a device on and off thousands or millions times a second, each switching event has its associated switching loss (due to Qg or Ciss/Coss). But, here, there is no switching loss, because once it's on..it's on! So gate charge/capacitance isn't really a problem. |
| Navigation |
| Message Index |