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BQ76940: BQ76940 workflow

Chan PK
Prodigy 30 points
Part Number: BQ76940

Hi,

I am designing BSM based on BQ76940 however I am bit confuse with workflow such as when to turn off CHARGE mosfet with charger detection function.

1. power off BMS (with power switch, when power off no power will be feed to BQ76940 therefore both DSG and CHG mosfet are turn off.

2. with power is still switched off, no inrush or spike occurred when battery pack plug insert.

3. power on BQ76940, at this moment, both DSG and CHG rise up to 12v both CHARGE and DISCHARGE mosfets turn on, SysCtrl2 = 0x43.

4. now when charger is inserted, CHARGING immediately started, charge current also measured, but since the CHARGE mosfet was turned on early (in step 3 above), therefore the charge detection couldn't detect the charger insert action

5. when charging is completed, system can terminate charge process by turn off the CHARGE mosfet SysCtrl2 = 0x42.

my question is : 

1. do I need to turn off the CHARGE mosfet (sysctrl2 = 0x42) all the time or by default for charge detection ready ? when charger insert is detected, turn on CHARGE mosfet (sysctrl2 = 0x43), when charge process is completed, turn off CHARGE mosfet again (sysctrl2 = 0x42) ? 

PK Chan

  • 0
    TI__Expert 4640 points
    Hi Chan,

    The general practice is to manage the turn off and turn on of your system from the system side not from the BMS. Usually the BMS system is an always on system so that it can protect your battery pack and if you are doing some kind of gauging the gauge can log data over time. In addition, cutting power of the bq76940 will force a POR which will then clear the EEPROM settings for your protection settings, and your MCU will then have to rewrite those again and again. We don't recommend for your to have this practice of current power from your system by opening the protection FETs and turning off the bq76940. The protection FETs are there for cut power only when a protection trip happens not as a general off / on function.

    Thanks

    Miguel
  • 0 in reply to Miguel Angel Rios
    Prodigy 30 points

    Hi Miguel,

    thanks for your quick reply, let put the reset BQ issues aside, my main question was : 

    1. my system always ON both CHG and DSG mosfets (sysctrl2 0x43) but when CHG mosfet is turned on I wasn't able to detect CHARGER plug-in action and charging start immediately, assuming detect CHARGER insert is unnecessary how's after charging complete ?

    2. when charging is completed, immediately turn off CHG (sysctrl2 0x42) (of course I can use buzzer to inform user charging is completed), then he / she un-plug the CHARGER (at this moment sysctrl2 still 0x42) and because it is 0x42 therefore I can detect whether CHARGER is un-plugged or not right ? 

    3. keep monitor the un-plug action (voltage on CHG_DET), when un-plug is detected, I can set sysctrl2 to 0x43 ready for DISCHARGE ? 

    sorry for my bad english, but I am new to BMS design, hope you can point out my mistake or you (T.I.) has better workflow suggestions.

    PK

  • 0 in reply to Chan PK
    TI__Guru* 81035 points
    Hi PK,

    Can you monitor the current through the sense resistor to determine if the charger is connected rather than shutting off the CHG FET. We do not recommend shutting down the CHG FET as this is meant for protection and not as part of the normal BMS workflow.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Prodigy 30 points

    Matt Sunna said:


    Can you monitor the current through the sense resistor to determine if the charger is connected rather than shutting off the CHG FET. 

    Hi Matt,

    of course I wouldn't shutting off CHG FET for CHARGER plug-in detection, yes we have monitor current, what I mean was : 

    1. both CHG and DSG FET need to be turned on normally for discharge (sysctrl2 = 0x43)

    2. when CHARGER is connected / plug-in, since CHG FET is turned on (syscrtl2 = 0x43) therefore we couldn't tell CHARGER is inserted by detection circuit (suggested by T.I.), however we can monitor CHG current, when OV is occurred BQ will change sysctrl2 to 0x42 shutdown the CHG FET immediately, or  charge process is completed (cell voltage delta meet our preset value) our MCU will change sysctrl2 to 0x42 to shutdown the CHG FET.

    3. at this moment, CHG FET is off, DSG FET is on, sysctrl2 = 0x42, CHARGER is still connected, pack voltage not full because cell voltage delta already met our preset value

    4. this STATUS will hold forever unless user intervention such as un-plug CHARGER

    5. in order to detect user un-plug CHARGER action is based on charger detection circuit (suggested by T.I.), because now CHG FET is off (sysctrl2 still 0x42) I can tell whether CHARGER is un-plugged or not.

    6. if CHARGER is un-plugged (charger detection circuit voltage drop), I turn on CHG FET back (sysctrl2 0x43) for normal discharge operation 

    Sorry Matt, look like I repeat my explanation again and again, please help to review my workflow :)

    PK

  • 0 in reply to Chan PK
    TI__Guru* 81035 points
    Hi PK,

    Thanks for re-explaining the flow. I think I understand clearly now. I think this should work since it sounds like there is no way to turn of the charger without unplugging it. Normally the FETs are only used during fault conditions, but I can see why you would want to use it this way in your application.

    Best regards,
    Matt