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BQ76952: CHG and DSG FET Drivers at Low side

Prabhu Selvaraj
Intellectual 305 points
Part Number: BQ76952

Hi,

We are building a BMS with BQ76952 with parallel configuration. Since we are building it for a high current (100A) application, we are opting for different configurations to get a optimal solution.

  1. Could we use the CHG and DSG FET drivers in the low side (i.e)., in series with the coulomb counter circuit by connecting the positive terminal of Battery straight away to the charger or load? What are the advantages and disadvantages of it ?
  2. How could we calculate or choose the continuous current rating of the PCHG and PDSG FETs ?
  3. Which is advantageous ? Using a single CHG Driver MoSFET or using 2 or more MOSFETS in parallel to carry up to carry 100 A ?

Thank you,

  • 0
    TI__Guru 54715 points

    Hi Prabhu,

    See the application note https://www.ti.com/lit/pdf/sluaa84 for some information on using low side FETs with the BQ76952.  

    1. Yes if using low side FETs the Battery+ is the PACK+ and connects to the load and charger.  Low side FETs are easier to drive on quickly with a supply that is in the range of the battery.  Low side FETs will break the ground path, so ground referenced communication will not work, use care to avoid leakage through communication lines, often isolated communication is needed.  Also check connections between the battery and charger, many chargers are referenced to the low side so signals which cross must be isolated.

    2. When using DDSG and DCHG the signals combine the precharge and charge times, and the predischarge and discharge times, so logic will need to separate these, see the application note.  Precharge current and predischarge current will be limited by the selected resistor, so assuming an operating condition will establish the maximum current based on the resistor selection.  Calculate the power in the FET and resistor is selecting components.  

    3. If you can use a single FET you have only 1 component to purchase and mount.  The challenges in a design may be that that FET will get hot although it can handle the current, so many times parallel FETs are used to spread the heat and make it easier to dissipate in the battery system design. The parallel FET will reduce the effective RDSON for the parallel combination resulting in less heating in the circuit, or allow using FETs with higher RDSON.  

  • 0 in reply to WM5295
    Intellectual 305 points

    Hi,

     It was clear. Could I know about the current sensing at High side feasible in BQ76952 for a 12 V/ 100 A inverter Application ?

    Thank you,

  • +1 in reply to Prabhu Selvaraj
    TI__Guru 54715 points

    Hi Prabhu,

    The BQ76952 cannot support high side current sensing.  The SRP, SRN input ABS MAX rating is the REG18 voltage plus 0.3V, the recommended input level is less than 0.75V to avoid a reset which will occur as the inputs approach the REG18 voltage.   There are some INA devices which can support high side current sense, see www.ti.com/currentsense but the BQ76952 is not intended to use their output.