Question about FET/Switch

[Dinnin]

I am designing a circuit with a PIC18F/24F not sure which yet, to monitor the battery life of a LiFePO4 battery. Long story short I want a switch controlled by the PIC to turn on and off the circuit which may run up to 8 Amps. I don't need anything complex something simple just want to know if i can use a MOSFET. Would a simple transistor work? if so any one have a recommendation i.e part number I could use?

DDD

 

[RCinFLA]

You need to pick a device with low turn on gate voltage. If you are only switching a single Li-Ion cell there are MOSFET's with 0.05 ohm R-on with 2.5v of gate bias. These are what is used in mobile phones for battery supervisor circuit and charger circuit. You will have to put about four or more in parallel .

 

[Dinnin]

You need to pick a device with low turn on gate voltage. If you are only switching a single Li-Ion cell there are MOSFET's with 0.05 ohm R-on with 2.5v of gate bias. These are what is used in mobile phones for battery supervisor circuit and charger circuit. You will have to put about four or more in parallel .

This is my first project, sorry if my questions seem ignorant. I am looking at mouser and digikey and find the MOSFET s read "Resistance Drain-Source RDS (on): .05 ohms @ 10V" are you saying I should look for one that is 0.05 @ 2.5V if so any idea where i can buy them? Also not sure if it makes a difference but the battery pack is 96V I think they are used for electric bicycles, if it makes any difference.

DDD

 

[MikeMl]

You are switching ~100V, so when you look for a Power FET with a voltage rating that high, it severely limits the RdsON, and increases the gate drive voltage. You should read this thread... Note how the OP there is driving the gate of the NFet from a 3.3V port pin on a PIC.

 

[Dinnin]

Ok, after reading the thread you linked i came up with this MOSFET

I found a 512-IRL640A with (0.18 Ohm @ 5 V) with a breakdown of 200V. The Breakdown say 200V thats around 2x my supply voltage like you suggested and i think the 5V is low also like you suggested. It is also a TO220 like you said as well. Does this one look like it should work?

Also The thread you linked he had what i think is a driver for the FET will i need one of this if i plan to control the switch/FET with a PIC24f?

Thanks
DDD

 

[MikeMl]

FET should work. If the PIC is powered on 3.3V, you will need the driver. If it is powered off 5V, you might be able to make it work without.

 

[Dinnin]

Yes I believe the PIC datasheet says its power is 2.0-3.7V so i will need a driver. So my next question is how do i know what kind to get? There are high-side and low-side, not sure if that even matters i think what matters is the Category (Voltage-supply) does this mean voltage in vs the voltage it sends out, meaning if my PIC send 3.3V i need the driver to send 5V to the mosfet to turn it on?

Again sorry if the questions seem ignorant my first project and i am learning as i go.

DDD

 

[MikeMl]

Do you have any voltages higher than the Pic supply, like 5V, 10V? What powers the PIC supply?

 

[Dinnin]

Well the circuit is basically a 96V LiFePO4 battery connected to a resistor. The plan is to place the FET in the circuit and when a new battery is placed on you then press a key the switch starts the circuit and the PIC monitors the voltage as the battery dies. When the battery is almost dead The switch needs to be switched off automatically. So the highest voltage i have is 96V if thats what your asking. I am in the process of designing the virtual circuit just not finished yet, this is my first project so its a big learning curve.

Thanks
DDD

 

[Dinnin]

Do you have any voltages higher than the Pic supply, like 5V, 10V? What powers the PIC supply?

I think i misunderstood this question when you first asked it, i plan to use a battery pack for the pic/lcd power. Not sure on the exact voltage but will be something around 5V or 10V i am just in the begging designs, sorry if i am asking very amateur questions.

 

[MikeMl]

Ok, so even when the PIC is running on a 2.5V regulator, there is a source of 5V or 10V on the input side of the regulator? I can devise a NFet gate driver that starts from a 2.5V port pin, uses a pullup tied to 5V or 10V to create the proper gate drive for the NFet.

 

[Dinnin]

I think i understand what your asking, my plan was like i said use a triple A battery back build a circuit to drop it to 2.5-3.0V to power the PIC. So i would need a driver that the PIC could send signal to then turn the FET on to begin the charging, if i can find a driver that takes say 5V or 10V then i can just get a battery pack to match up. I think i am saying this all right, i just got some software to put the ciruit on a virtual layout. Thanks again for all your help i know i am really newb!

Do i build a driver or buy one?

 

[MikeMl]

Do i build a driver or buy one?

If there is source of 5 to 10V readily available, then it is fairly simple (NPN transistor, a couple of resistors) to drive the FET. If all you have is ~3V, then that charge pump driver chip in that other thread might work for you.

 

[Dinnin]

Ok so i found this driver i think it is exactly what i need it says it takes 2-3.6V in and outputs ~5V The model number is ST619LBN. I think its a driver, I think its what i need not 100% sure. I think its just a DC-DC converter i guess it has its own internal power i am not sure lol. Do you think this would work to turn the FET on?

DDD

 

[MikeMl]

The higher the gate voltage, the better. The one you posted only puts out 5V, which means you have to use a "logic-level" FET. If you use the one that puts out >8V, the FET is less critical, cheaper.

 

[Dinnin]

The mosfet I chose says its a "logic-level gate drive" I have a few of these already model # 512-IRL640A on mouser. It says 0.18 Ohms @ 5V if that 5V means its the on voltage will that driver be ok?