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Original question:

BQ24650EVM-639: BQ24650EVM-639:

BQ24650EVM-639: Reverse Battery Protection of EVM circuit?

MikeH
Guru 15580 points

Part Number: BQ24650EVM-639

Jeff,

In a previous post you gave a link to a white paper on reverse battery protection using load switches. I am confused on how this would work in the EVM circuit that both supplies and sinks current from the battery. Can you verify and clarify?

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0 posts

  • 0
    TI__Guru**** 179836 points

    Hi Mike,

    I think you will need back to back PFETs to protect against birdirectional current, similar to what is shown below.  The FETs resistances will increase battery charge time.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 360 points

    Hi Jeff,

    Thank you for your answer. What is the functionality of 10 KOhm resistor in the circuit?  

    Regards

    Ashok

  • 0 in reply to Ashok R41
    TI__Guru**** 179836 points

    Ashok,

    The 10kohm provides a pull up voltage for the 2 series PFETs so that they turn off as well as a current limit for the NFET connected at ground.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 360 points

    Hi Jeff,

    Thank you for the response. Both PFET will be in on state when the battery is connected in the right way. NFET will disconnect the battery if the battery is connected in reverse direction. The pull up voltage at gate will be closer to source voltage for PFET. PFET Vgs is usually < 0; If the voltage at gate and source is almost equal, Vgs ~ 0 and PFET will remain on. In which case the pull-up voltage will turn off the PFET?

    Regards

    Ashok

  • 0 in reply to Ashok R41
    TI__Guru**** 179836 points

    Ashok,

    For PFET, VGS must be negative for the PFET to turn on (i.e. source voltage > gate voltage by the FET's Vthreshold).  Pulling VG=VS through 10kohm will turn off the PFET.  When battery is connected the NFET pulls the PFET gate low, turning it on.

    Regards,

    Jeff