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How precharging function is working in BQ76940

Nikhilesh Mishra
Prodigy 135 points
Other Parts Discussed in Thread: BQ76940, BQ76920

I am unable to understand how precharging MOSFET is working.

As per below image

precharge MOSFET is connected in parallel of charge FET. The direction of body diode of charge FET is that it will always pass current which is originating from battery and passing through load. If we dont turn on the discharge FET there will be no current passing and if we turn on the discharge FET the current will pass through CHG as well as Pre-Charge FET body diode so it doesn't make sense having precharge FET.

Please help me understand where i am disconnected and appreciated the working logic of precharge FET.

Regards

Nikhilesh

  • 0
    TI__Guru 54715 points

    Your description seems correct.  If you are discharging the charge and precharge FETs are in parallel, both are diodes, the precharge FET (diode) has a resistor in series so it will carry little of the current.  Normally the charge FET will be on during discharge and provide a low resistance path so very little current flows through the precharge path.

    When you are charging, the discharge FET acts as a diode if it is off.  Charge current can flow through either the charge or discharge path depending on which is enabled.  When the cell voltage is very low, it typically should not be charged at a high rate.  If you have a smart charger, it may read the charge and discharge current from the gauge and recognize that the current is small while at low voltage.  If your charger is a CC-CV system which does not adjust to the gauge's reported charge current, the gauge can turn on the pre-charge path at low voltage.  For example with the bq76940 the system might be a 48V battery.  If the battery is deeply discharged to perhaps 30V, some cells may be below the threshold where they are safe to charge at high current.  If the charger is providing 48V, high current would flow if the charge FET was turned on, so the gauge may turn on the precharge FET.  The charger voltage remains at 48V with the voltage dropped across the precharge resistor (48 - 30V) and current is limited by the precharge resistor.  When the cell voltages rise sufficiently that the cells can accept the higher charge current the gauge turns on the charge FET and the charger voltage will typically pull down to the battery voltage. 

    Note that the schematic shown is from a bq76920 circuit, be sure to select transistors with sufficient voltage rating for a bq76940 design.

  • 0 in reply to WM5295
    Prodigy 135 points
    Thank you very much for the clarification.
    So the pre-charge function is to limit the charging current when battery voltage is very low. The schematic is very logical in that case. I was thinking that this function is provided to limit the inrush current when load is initially applied and because of capacitors on load there will be huge current to charge the capacitor. Some BMS probably provide that function as well. Providing that function (limiting the inrush current) will also be a good add-on I think.
    Thank you for the detailed clarification. Much appreciated.