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BQ76920: BQ76920 + BQ78350-R1+ BQ76200 BMS design

Christopher
Prodigy 80 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ78350-R1, BQ76200, BQ78350, TIDA-00792

Hello. I have several questions on a bms design I am working on using BQ76920 + BQ78350-R1 + BQ76200. I intend to use internal cell balancing using BQ76920. My maximum pack voltage will be 21V with each cell having a maximum of 4.2V.

1) In the bq769x0 app note https://www.ti.com/lit/an/slua749a/slua749a.pdf?ts=1687276824595 pg 21, zener diodes are used at the cell inputs of bq769x0 during cell assembly to limit excessive voltage at the pins. It is stated that the zener diodes should be removed during normal operation of the bms. My question is, can I use tvs diodes instead and leave them on the board after assembly? For example, my maximum cell voltage is 4.2v, the tvs diode I want to use is a bidirectional one with a 0.1uA leakage current at 5Vrwm. Can I place such a tvs diodes at the cell inputs i.e before the rc filter network that is usually placed before the bq76920 cell input pins, and then leave the tvs diodes on the board during normal operation? 

2) I intend to use high side mosfets driven by bq76200 together with bq78350-r1 and bq76920. How does load detection and charger detection work with this highside configuration?

3) If I use the precharge pin of bq76200 for predischarge, how can I implement load and charger detection, when using bq76920 + bq78350-r1 + bq76200? 

4) If I use split charging and discharging paths, how can I implement load and charger detections when using bq76920 + bq78350-r1 + bq76200? 

5) What is the state of the PCHG, CHG and DSG outputs of bq76200 if there is no load or charger connected? I am asking this because I want to use the PCHG pin for predischarge and I would like to understand whether the PCHG pin has more priority over the CHG and DSG pins i.e do the CHG and DSG outputs go low when the PCHG output is enabled so as to turn off the CHG and DSG mosfets?

6) From bq76200 datasheet, there is an on resistance of the internal fet between PACK and PACKDIV with a typical value of 2.5kohms. Should this resistance be included in the calibration of the resistor divider that is applied to the VAUX pin of bq78350-r1 from the PACKDIV of BQ76200?

7) From the bq76200 app note, https://www.ti.com/lit/an/slua794/slua794.pdf?ts=1688543704038&ref_url=https%253A%252F%252Fwww.ti.com%252Fsolution%252Fbattery-pack-ebike-escooter-light-electric-vehicle-lev%253FkeyMatch%253DBQ76920, pg 16, it is stated that the zener diode between the gate and source of the CHG mosfet is usually removed after assembly. What will be the impact of leaving the zener diode on the board during normal operation? my understanding is that if the zener diode is present, with the CHG output low, the flow of current is through the zener to the CHG pin then to the BAT pin but the BAT pin will already be charged to the battery voltage so there will be almost no current flowing through the zener. Kindly correct me.

8) In the case of reverse charger connection when using bq76200 with the protection circuit in the app note, https://www.ti.com/lit/an/slua796/slua796.pdf?ts=1689090372433&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ76200%253FkeyMatch%253DBQ76200%2526tisearch%253Dsearch-everything%2526usecase%253DGPN-ALT, pg 5, does the DSG output go low when the PACK input drops to zero volts or below ground (upto -0.3V)? I am asking this because , if the DSG pin is high during reverse charger connection and a diode is used at the PACK input to limit the negative voltage seen at the PACK input to the forward voltage drop of the diode, then a significant amount of current will flow from the diode into the PACK pin when the DSG pin is high which will cause a high power dissipation. Kindly explain to me what I can do to avoid such a situation where the DSG pin is high at the same time as a reverse charger connection? kindly correct me for any wrong assumptions I have made.

  • 0
    TI__Mastermind 35780 points

    Hello Patty, 

    Just to let you know, E2E will be down today and the weekend. 

    So I will give you a response on this on Monday. Apologies for the delay!

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 80 points

    Thanks Luis. 

  • 0
    TI__Mastermind 35780 points
    Christopher said:
    Can I place such a tvs diodes at the cell inputs i.e before the rc filter network that is usually placed before the bq76920 cell input pins, and then leave the tvs diodes on the board during normal operation? 

    This should be okay, as long as the voltage of the diodes do not interfere with normal operation. 

    Christopher said:
    I intend to use high side mosfets driven by bq76200 together with bq78350-r1 and bq76920. How does load detection and charger detection work with this highside configuration?

    The TIDA-01093 has some discussion and circuits showing how load/charger detection can be achieved using this configuration. These circuits however interface with a host MCU to make decisions. If only using the BQ78350-R1 these circuits may be more difficult. We do not have a reference for such case, nor a split paths case.

    Christopher said:
    5) What is the state of the PCHG, CHG and DSG outputs of bq76200 if there is no load or charger connected? I am asking this because I want to use the PCHG pin for predischarge and I would like to understand whether the PCHG pin has more priority over the CHG and DSG pins i.e do the CHG and DSG outputs go low when the PCHG output is enabled so as to turn off the CHG and DSG mosfets?

    This will depend on what the BQ76920 is outputting, which would depend on the BQ78351. 

    Christopher said:
    What will be the impact of leaving the zener diode on the board during normal operation? my understanding is that if the zener diode is present, with the CHG output low, the flow of current is through the zener to the CHG pin then to the BAT pin but the BAT pin will already be charged to the battery voltage so there will be almost no current flowing through the zener. Kindly correct me.

    You misunderstood this section. This section talks about adding a Zener/Resistor in series between the BAT pin and the CHG pin, similar to what is shown between the DSG pin and the LD pin. 

    Christopher said:
    does the DSG output go low when the PACK input drops to zero volts or below ground (upto -0.3V)? I am asking this because , if the DSG pin is high during reverse charger connection and a diode is used at the PACK input to limit the negative voltage seen at the PACK input to the forward voltage drop of the diode, then a significant amount of current will flow from the diode into the PACK pin when the DSG pin is high which will cause a high power dissipation. Kindly explain to me what I can do to avoid such a situation where the DSG pin is high at the same time as a reverse charger connection? kindly correct me for any wrong assumptions I have made.

    Figure 3 shows the most common reverse-charger circuit we use. This is also seen in the BQ769x2 family EVM schematic for some reference. If you do use a diode, this as you said runs multiple risks, including damaging the part. Again, this is explained in the same section of the application note. The DSG pin likely would not see much current out of it due to the high resistance of the gate-source resistor. Adding Schottky diodes at both the DSG and PACK pins may help at carrying the current.

    I would recommend using a different circuit for reverse charger protection in this high-side application. If you want to understand in more detail a different circuit, I would recommend you read (albeit for a different device family) Section 2 Reverse Charge Circuit and 3 Reverse Charge Component Selection of the  Multiple FETs with the BQ76952, BQ76942 Battery Monitors.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 80 points

    Hello Luis. Thanks for your reply.

    Luis H. S. said:
    The TIDA-01093 has some discussion and circuits showing how load/charger detection can be achieved using this configuration. These circuits however interface with a host MCU to make decisions. If only using the BQ78350-R1 these circuits may be more difficult. We do not have a reference for such case, nor a split paths case.

    I am using the TIDA-00792 as my reference design for an AFE + BQ78350 + BQ76200. How does a system of BQ76920 + BQ78350 + BQ76200 know when the battery is supposed to charge or discharge? For instance, once you wake up the system via TS1 pin in BQ769x0, kindly explain to me what happens if there is no load or charger. Are the power fets always in a off state if the battery is not charging or discharging? 

  • +1 in reply to Christopher
    TI__Mastermind 35780 points

    Hello Patty,

    This system is meant to protect the battery in the case of a fault condition. When everything is working correctly, the part will have both FETs ON at all time and would only turn them off when there's a fault. It does not turn the FETs on/off based on a load/charger. The system is not a battery controller, it does not control when a  battery will charge or discahrge.

    If there's no load and charger, both FETs will just turn-on, based on BQ78350 settings. Both power FETs will always be in an ON state (based on settings) if all is working correctly.

    Best Regards,

    Luis Hernandez Salomon