Turn On Load On and off using mosfet with 4V

[joshin joy]

Hai,

Can Anyone please help me!!

I am designing a 4V lead acid battery charger using a PIC MCU.

the charger section is ok what my problem is

i have used a IRFZ44 MOSFET to turn the LED load on and off.

I want turn off the load when the battery is discharged to ~3V.

the circuit diagram is attached.

while i turn on the MOSFET i have a voltage drop of more than 1V. ie when battrey voltage 4V i get only 3V for my load even when it is unloaded.

what should i do to decease the drop voltage??

Please help!! :-(

 

[esp1]

hi,
Using a Vgs of only 3 to 4 V will not fully turn on the FET, try a Vgs of 10v as per the datasheet.
E

 

[joshin joy]

changing the mosfet to a power transistor like TIP122 or TIP127 will help me. if yes how can i bias it at this voltage. i cant add another power rail to the circuit.

 

[FvM]

These are darlington transistors with a voltage drop hardly below 1V. A suitable MOSFET is clearly the better solution. Unfortunately you didn't mention the switched load current, but there are many "logic level" MOSFETs that fully switch on with 2 or 2.5 V Vgs.

 

[joshin joy]

Sir,
My Load aproximate 3.7V LED Pannel with current of max or less than 5A. so if i select a Mosfet with 2.5V or 4.5V Vgs then i will be able turn on the MOSFET directly from my MCU. am i correct??. Can you suggest any MOSFET with 2.5V or 4.5V Vgs.

Thankyou

 

[FvM]

All major manufacturers have online selection tools or product tables that allow to search for suitable types. Vishay Si4838DY is e.g. a SO8 type good for up to 20A. Even tiny Si2312CDS in SOT23 can work up to 6A, but runs a bit hot.

 

[Audioguru]

The little Vishay Si4838DY Mosfet will heat with 0.1W max with a 5A load and a 2.5V Vgs because its max voltage drop will be only 0.02V.