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BQ76930 - Load causing CHG to go high

John Baillie
Prodigy 60 points
Other Parts Discussed in Thread: BQ76930, BQ78350

Hello,

I have separate discharge and charge paths in my circuit with the charge FET connected directly to the sense resistor. Without a load, the CHG and DSG FETs turn off correctly according to values programmed in BQ Studio. If a load is connected, CHG stays high and only goes low when the cell voltage exceeds the value defined as a permanent failure. I have the same circuitry around the CHG FET as in the EVm (zener diode, 1M pull down etc).

 

  • 0
    TI__Guru 54715 points

    There are 2 switching mechanisms in the  system, one is the bq76930 AFE direct control, the other is the bq78350 control through the serial interface.  When a single fault occurs typically only the FET for the appropriate current direction is switched.  The bq78350 assumes a series FET configuration and has a body diode protection implementation to keep the charge FET on in the case of  discharge current.  This may be keeping the charge FET on with continued discharge current. When you reach the permanent failure level it is the AFE which switches off the CHG FET.

    I will inquire with the gauge experts to see if there is a parameter setting which would get you the desired behavior.

  • 0 in reply to WM5295
    Prodigy 60 points

    Thank you for your reply,
    I don't understand why the CHG FET would be kept on - even in a series configuration, the body diode of the DSG FET would allow discharge current to pass if CHG FET is off.
    In my current test, it is true that the current measured by the BQ78350 is negative (discharge). I presume that if I use a charger and have positive current (charge) then the CHGFET would turn off. I will report the result here.

    Thanks,

    John Baillie

  • 0 in reply to John Baillie
    Prodigy 60 points

    I have just verified that if the net current is positive (current flowing into battery ie charging) then the overvoltage protection of the BQ78350 behaves as expected. To clarify: My initial test was performed with a (discharge) load of about 1A. I was increasing the cell voltages using a power supply with a resistor chain to emulate cell voltages. When the measured cell voltage exceeded the COV threshold, the CHG signal stayed high (expected to go low), however when I changed my test to include a charger so that the net current was positive (charging), the CHG signal went low as expected. This is logical since the cell voltages will not increase unless the current is flowing into the cells.

  • 0 in reply to John Baillie
    TI__Guru 54715 points

    Right, the gauge responds to the net current.  Positive current in the battery is charge current, so the battery current is (Charge current - Discharge current).  The gauge has 2 parameters which should control when the FET protection runs: “Chg current Threshold” and “Dsg Current Threshold”.  If charge current > Chg current Threshold than DSG FET is turned on to protect body diode, same way if discharge current > Dsg Current Threshold than CHG FET is turned on to protect body diode. If your currents are low adjustment may be helpful.

    If your load and charger can be connected at the same time you may want your system to have an independent control of the current path.  The battery switches are designed for protection and monitors current in 250ms intervals..  One situation to watch for is if the load has a switched load with a startup delay such as a MCU with reset. If the load has a short circuit with a charger attached, the short will pull down the charger and the battery causing a protection event.  The charger then recovers, the battery sees the charge current and turns on the DSG FET for body diode protection even though it is in SCD mode.  The load also wakes up from the charger voltage and after its delay turns on the shorted load again starting the cycle over.   Be sure to consider, design for and test any cyclic situations in your system.