Cell protection module mosfet driver

Discussion in 'General Electronics Chat' started by cages, Oct 22, 2011.

  1. cages

    Thread Starter New Member

    Jul 3, 2010
    19
    0
    I would like to use a Micro-controller to control two N channel MOSFET to control the charging and discharging of a cell (reduced schematic attached).

    The discharge MOSFET (Q1) can be controlled directly from the PICs IO pins(Vgs=4V).
    The charge MOSFET (Q2) will need a special driver to pull the gate to its source voltage.
    Does anyone Know of any N channel MOSFET driving ICs that can drive either Q2 or both MOSFET or a simple drive circuit that can be implemented for the PIC to be able to Switch both MOSFETs on and off?

    Cages
     
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  2. SgtWookie

    Expert

    Jul 17, 2007
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    The way you have Q2 drawn, its' body diode would conduct whenever Q1 is turned on.

    You should normally orient an N-ch MOSFETs' drain towards the more positive portion of the circuit. Keeping them oriented drain upwards usually avoids that confusion, as schematics usually have more positive voltages towards the top, more negative towards the bottom.

    You will want Q1 as it is, and Q2 with its' source terminal connected to Q1's drain, and its' drain connected to the positive rail. I suggest that you use something like an incandescent lamp as a load; they work very well for a task like this.

    [eta]
    Hmm, that's not going to work, as when Q1 turns off, the source of Q2 will be floating.

    So, in that case, Q1 will need to be in the positive rail, and Q2s' drain connected after it, source to the negative rail. Q1 will need a high-side driver.

    You have not said what the maximum cell voltage range will be; until you let us know what that will be, it will be difficult to make a decent recommendation.
     
  3. cages

    Thread Starter New Member

    Jul 3, 2010
    19
    0
    Both MOSFETs I believe are arranged as they should. Q1 can be used to switch off discharging of the Cell(battery) when a load is attached across the + and negative -terminals.
    Q2 is used to switch off the charging of the Cell(battery) when a charger is attached across the + and - terminal.
    The battery voltage will be 7.2V.
    The method of protecting batteries using N MOSFETs is used by commercial IC. I have attached a block diagram from a Texas Instrument part that switches two MOSFETs (bq77910). The block diagram indicates the IC has special inbuilt drivers capable of driving the MOSFETs in this configuration.
    I am trying to implement a similar device using a Micro-controller (PIC16) and wondered if there are MOSFET drivers that can drive the MOSFETs as arranged?
    The MOSFETs are logic level MOSFETs and there is a 5V rail for the PIC16.

    Thanks cages
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I guess I don't understand why you're trying to replicate the functionality of that IC using a PIC and supporting components; as the cost of the supporting components and board space will likely exceed the cost of the IC by a considerable margin?

    We've been through something rather similar to this; another fellow wanted to monitor the voltages across individual cells in a large battery pack consisting of numerous cells. I think my final recommendation to him was to use SSR's (solid state relays) to switch between the various cells.

    Once you solve the problem of driving the low-side N-ch MOSFETs, you're going to immediately run into the problem of the voltage drop from one end of the pack to the other. You'll be limited to an absolute maximum of 5.5v, as that's as high as a PIC can go without damage. Also, the PIC has a maximum I/O pin current around 25mA, and a package current limitation that depends upon the PIC in question.

    Anyway, the functionality of the low-side FET drivers is covered in the datasheet:
    http://www.ti.com/lit/ds/symlink/bq77910.pdf
    beginning on page 17, right after the same schematic you posted in reply #3.
    I don't know offhand of drivers that mimic the functionality of the integrated driver solution.
     
  5. cages

    Thread Starter New Member

    Jul 3, 2010
    19
    0
    The IC does not do exactly what I would want to achieve. There are quite a few Battery Management\Cell Protection ICs available for various applications. There are also quite a few projects on the internet for RC models that use micros as custom battery protection devices and I am hoping to make something similar.

    I was hoping there might be a stand alone driver for this MOSFET configuration (as a bidirectional switch). I might have to find another solution.
    Thanks for your help.
    Cages
     
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