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BQ76952: Reverse Charger Protection Circuit Query

Nick
Intellectual 672 points
Part Number: BQ76952

Hello,

I have a query on Reverse charger protection component selection.  I am referring to Figure 1-1 in SLUAA09A app note.

I am looking for rating of Q6 mosfet & R24. For 48V battery pack and 54 Charger, The Vds for Q6 can go up to 113V (48+11+54) .  The Voltage across R24 can also be 113 V ; considering 10K resistance- power dissipation across R24 will be 1.3W. Splitting R24 in two 5k each will give ~ 0.7Watt.

I am considering a case when Q5 is ON and charger is connected in reverse and Q6 switches ON before BMS could detect high current limit (OCD/SCD). As soon Q6 switches ON, Vgs of Q5 will be pulled down, while DSG pin still be high at 59V as BMS has not detected any fault. I am also assuming PACK+ can go down to -54V as charger reversely connected.

Is putting Rclamp on source of Q6  the solution to slow the Vgs pull down of Q5.

Please correct me if I am missing something. 

Thanks

  • 0
    TI__Mastermind 42060 points

    Hello Nitin,

    Your assumptions seem correct to me.

    Something to note, is that the ~113-V seen across the R24 resistor will be very brief (a few milliseconds or so). After Q6 is turned on, the charge-pump is likely to start getting discharged and eventually reach a voltage of around -0.7-V (due to internal diode). This means that assuming the reverse charger is left connected, the voltage seen across R24 will be closer to the charger voltage after a few milliseconds. Evaluate what is best for your design in this case. Your reasoning is correct at the instant that the reverse-charge is connected.

    Rclamp will slow down the turn-off switching speed of Q5. When Q6 is on, the Vgs of Q5 is pulled together and the gate-source would discharge through the Q6 path. With Rclamp added on this path, the gate-source voltage would discharge slower as the current is being limited by the Rds(on) of Q6 and Rclamp (if added). This is mostly used to prevent transient spikes due to inductances of the traces/system in the case where it may be a concern.

    The charge-pump of the IC is likely to be discharged through the reverse-polarity protection circuit, hence it may take a some tenths of milliseconds for the charge-pumps to be functional after a reverse charger is connected. How long it takes for the charge-pump to charge/discharge will depend on the charge-pump capacitor of the IC.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 672 points

    Please clarify how "The charge-pump of the IC is likely to be discharged through the reverse-polarity protection circuit". I don't see any path from C15 to Reverser protection circuit.

    Thanks

  • +1 in reply to Nick
    TI__Mastermind 42060 points

    Hello Nitin,

    See Figure 9-1. DSG Turn On Current Path in the Multiple FETs with the BQ769x2 application note and the explanation below it.

    The charge pump is used to drive the voltage of the DSG pin. When Q6 is enabled, it gives a path to the charge pump to discharge through R24 and Rclamp. How fast this occur will depend on the charge-pump capacitor CP1 (C15 in this example) and resistances in the path. 

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 672 points

    Luis, 

    Sorry to open old topic, but I think it is better to ask.

    When the CP1 discharges through R24 and Rclamp, DSG will come down to BAT+. I see in Figure 9-2, it drops to zero around 6.5 mA. So during reverse charge scenario, DSG will drop to VSS or above due to current load. Correct ?

  • +1 in reply to Nick
    TI__Mastermind 42060 points

    Hello Nitin,

    No worries! Although we usually recommend to open a new thread if a new question comes up. You can always create a new question from this thread and like this thread if needed.

    I believe you are correct. When the DSG FET goes below the BAT level, it will attempt to pull current through the BAT pin, which is limited as pointed out in the Figure 9-2 you mentioned. So DSG should drop to Vss during the reverse-charger scenario.

    Let us know if there's anything else Slight smile.

    Best Regards,

    Luis Hernandez Salomon