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BQ76952: Pre-Discharge FET Not Turning ON

Naga Penmetsa
Prodigy 60 points
Part Number: BQ76952

Hello,

I am using BQ76952 for a battery management system and the Pre-discharge MOSFET is not turning ON as we are seeing huge inrush currents because of our Load. Attached picture is the schematic of the MOSFET circuit and also below are our AFE Register settings for Pre-discharge:

1) Settings: FET Options =>  0x3F, so PDSG_EN bit is Set

2) Settings:FET:Predischarge Timeout => 50, so pre-discharge Time is 500mSec

3) Settings:FET:Predischarge Stop Delta => 0, so LD pin pre-discharge function is disabled.

4) Dsg Current Threshold => 0x3E8, discharge current threshold set to 1000mA

Our Host MCU detects when a Load is connected to BMS and host is sending the following commands. First we send a command to turn on the Pre-Discharge FET and then send a command to enable both Discharge FET and charge FET. 

1) From host we set FET Control Register  to turn ON the pre-discharge FET and we wait for 5mSeconds (when we send this saw the FET Status bit for PDSG bit will be set). But when we measure the voltage at LOAD+, we don't see the Battery voltage at LOAD+ 

2) Then from Host we set FET control register to Turn ON CHG, DSG FET's. Now we see the Battery Voltage at  Load+.

Another thing we observed is that, if we don't set the pre-discharge bit manually like above and directly set the Discharge FET, we don't see the Pre-discharge FET bit set in the FET status. I was expecting that the Pre-discharge bit should set automatically because we enabled in the registers. Am i wrong?

Any thoughts on why the Pre-Discharge may not be turning ON?

  • 0
    TI__Mastermind 25556 points

    Hello Naga,

    For a quick test on the PDSG FET, Try sending FET_EN(), then ALL_FETS_OFF(), then ALL_FETS_ON(). While doing this, measure the gate-to-source voltages on the PDSG and DSG FETs. You should see something like this.

    Note: this is without having a load attached. Simply sending the above commands.


    Your settings look ok. 

    Naga Penmetsa said:
    4) Dsg Current Threshold => 0x3E8, discharge current threshold set to 1000mA

    Why are you setting the Dsg Current Threshold to 1A? This means that the BQ76952 will enter discharge mode when a discharge current greater than 1 A is applied. For the moment, it 'd be good to leave it as default.

    Regards,
    Jose Couso