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BQ77915: Working with BQ77915 & BQ24133 during over-voltage protection.

Part Number: BQ77915
Other Parts Discussed in Thread: BQ24133

I am working on a system that will charge the 3S Li-Po battery pack using the IC BQ24133, which has a maximum charging voltage of 12.6V for 3S configuration.

I have a protection circuit for the same 3S Li-Po battery pack, with balancer (internal) using BQ7791502, which has OVP of 12.6V (4.2V/cell)

In an ideal condition, both will work well.

So, my query is, what happens when BQ24133 gives a slightly higher output voltage to charge the batteries protected by BQ7791502. Lets say, the charger gave 12.62V for charging.

When this happen, BQ7791502 over voltage protection should turn on and switch off the CHG MOSFET. Discharge MOSFET will still be ON. (Table 9.6 from datasheet) But there is a body diode of MOSFET, which can still conduct current if needed, bypassing CHG MOSFET. The diode will need a little over 0.3V or better to conduct. Since the BQ24133 is at 12.62V and battery is at 12.59V or so, the diode will also not conduct.

Now, the CHG MOSFET is OFF, while DSG MOSFET is ON.

At present, since the CHG MOSFET is OFF, the pack cannot discharge.

  1. How does the CHG MOSFET turn ON again so that the pack can discharge ?
  2. Since the CHG MOSFET is OFF, will charger detect battery full as there is no current draw from the charger IC BQ24133 ? BQ24133 will learn that there is no current flow to the battery even if it is giving 12.62V
  3. Will CHG MOSFET turn on when this charging voltage is removed ?
  4. Will CHG MOSFET turn on only when a load is detected ? Will this be as the LD pin will be pulled up, and a pull down on the LD pin by a load will turn the CHG MOSFET back ON ?
  5. Is there an error in my understanding of how the system would work in this OV condition ?
  6. Should we be choosing a higher OVP IC (higher than 4.2V/cell) even if our cells can only be safely charged to 4.2V/cell, to avoid triggering this over voltage fault ?

Thanks a lot in advance.

  • Hi Jomon,

    Discharge MOSFET will still be ON. (Table 9.6 from datasheet) But there is a body diode of MOSFET, which can still conduct current if needed, bypassing CHG MOSFET

    When the CHG FET is off, even though the body diode of the DSG FET can still allow charge current to flow, the CHG FET will block that current from flowing to the battery.

    How does the CHG MOSFET turn ON again so that the pack can discharge ?

    The pack will still be able to discharge through the body diode of the CHG FET, and once the pack voltage has decreased below the recovery threshold, the CHG FET will turn back on. The device also has body diode protections, so if it recognizes a discharge current over a certain threshold it will also turn the CHG FET on until another fault occurs or the discharge current goes below the threshold.

    Since the CHG MOSFET is OFF, will charger detect battery full as there is no current draw from the charger IC BQ24133 ? BQ24133 will learn that there is no current flow to the battery even if it is giving 12.62V

    For questions specific to the function of the charger IC I recommend opening a new thread with those questions with the charger as the main part number so that the chargers team can see and answer them.

    Will CHG MOSFET turn on when this charging voltage is removed ?

    No, the CHG FET would stay off until either a discharge current is detected or if the cell voltage falls below the recovery threshold.

    Will CHG MOSFET turn on only when a load is detected ? Will this be as the LD pin will be pulled up, and a pull down on the LD pin by a load will turn the CHG MOSFET back ON ?

    The current protections will recover based on the load detection as described in the datasheet. The cell voltage protections will only recover when the cell voltage itself passes the recovery threshold. Attaching a load would eventually recover an OV fault because the load would eventually discharge the overvolted cell.

    Should we be choosing a higher OVP IC (higher than 4.2V/cell) even if our cells can only be safely charged to 4.2V/cell, to avoid triggering this over voltage fault ?

    If the cells can safely charge to a higher voltage (for example if 4.25V is still safe) then you could choose a slightly higher OVP threshold to give yourself some headroom on your charge cycle.

    Regards,

    Max Verboncoeur