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Original question:

BQ77915: External Cell Balancing Schematic Review

BQ77915: Continuation

Muki
Prodigy 220 points

Part Number: BQ77915
Other Parts Discussed in Thread: BQ77905

Hey Luis, I have a question from the last design review

You may also have to use multiple external balancing resistors in parallel in order to handle higher currents. At 20-Ohms (Ignoring MOSFET resistance) your current external balancing resistor should be able to handle 3.6-V*0.18-A = 2 Watts of power. So that would be something else to take into account.

Regarding this statement, if 20-ohms resistor is used then the external balancing current is 3.6 V / 20 Ohms = 180 mA and the power being dissipated should be 0.18A*3.6V = 648 mW. Am i missing something here ? why would it be 2 W ?

Regards

  • +1
    TI__Mastermind 42780 points

    Hello Muki,

    You are correct! I apologize for the confusion here. I must have inputted the values incorrectly in my calculator. 

    Apologies for the inconvenience! It is definitely 648-mW of power dissipation. 

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 220 points

    Hey Luis,

    Thank you for the clarification

    per your suggestion, i consulted the "bq77905 using multiple FETs" and "cell balancing with bq769x2 battery monitors" reference documents

    can you take a look at the new schematic and review it ?

    New_Schematic

    I'm having doubts with the CHG and DSG pin path - whether they are sufficient for the higher current design

    pls let me know, if there are any recommended changes

    Thanks and Regards

  • 0 in reply to Muki
    TI__Expert 4085 points

    Dear Muki,

    Luis is out of the office today, but he should be back Wednesday. He will perform the review then.

    Best,

    Asher Nederveld

  • 0 in reply to Asher Nederveld
    Prodigy 220 points

    Hey Asher,

    appreciate the update

    Regards

  • +1 in reply to Muki
    TI__Mastermind 42780 points

    Hello Muki,

    Like in the previous E2E thread about the schematic, I notice that the external cell balancing circuit is done incorrectly. I do remember this was fixed in the schematics I saw previously, but it seems that it is back to being wrong on this schematic. Refer to the previous E2E thread for the comments on it and image of the proper implementation. 

    Besides that, I would just recommend to change R15 to 10-Mohm and R14 to 3-MOhm. This will minimize current consumption on the device. 

    The current capabilities depends on the FET and what heat dissipation you are expecting. Some FETs are rated to handle hundreds of amps at DC, however heat dissipation may be too large, so in that case you would want more FETs in parallel to distribute the current and distribute the heat generated across the FETs. There are multiple FET application notes that talk about FET behavior/allowed currents at different current pulses.

    Do let us know if there's anything else Smiley

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 220 points

    Hey Luis,

    I see my mistake with the balancing MOSFETs, I've changed the source and drain accordingly

    I've changed R14 and R15 as you suggested

    I have made some changes to the schematic, do let me know if they're executed correctly

    1. enabled HIBERNATE mode by connecting PRES to PACK+ with a 1K resistor and a switch

    2. changed OCD1 and OCD2 delay time by connecting OCDP to GND with a 750K resistor

    3. Balance resistors have been replaced with 4 Nos 15 ohm resistor(s) in parallel to form a  3.75 ohm resistor - to facilitate 1A passive balancing

    4. I calculate the Vgs for the balance MOSFETs as 1.72V. So, i selected a MOSFET with Vgs(th) of 0.4V to 1V and Rdson defined at 1.8V as 28 mohm

    5. should the values of R6, R8, R10, R12 be increased ?

    Thank you for help Slight smile

    Regards

    Schematic V2

  • +1 in reply to Muki
    TI__Mastermind 42780 points

    Hello Muki,

    I apologize, I made a mistake when naming the resistors I meant change R22 to 10-MOhm and R23 to 3.3-MOhm. You can leave R14 and R15 as they were before.

     On your points:

    1. enabled HIBERNATE mode by connecting PRES to PACK+ with a 1K resistor and a switch
    This should be fine. Keep in mind that when the switch is connected to PACK+, the device will be in NORMAL mode, if it is OFF (PRES floating), then the device will be in HIBERNATE mode.

    2. changed OCD1 and OCD2 delay time by connecting OCDP to GND with a 750K resistor.
    Sounds good! Delay here would be 1.42-seconds for OCD1 and 0.7-seconds for OCD2.

    3. Balance resistors have been replaced with 4 Nos 15 ohm resistor(s) in parallel to form a  3.75 ohm resistor - to facilitate 1A passive balancing
    Do you actually need a 1-A balancing current? Most applications do not require such a high balancing current. With 1-A of balancing current spread across the four 15-Ohm resistors, the power dissipation across each resistor would be roughly (0.25-A)^2 * (15-Ohm) = ~0.94-W. So ideally your resistors should be rated for higher than this.

    4. I calculate the Vgs for the balance MOSFETs as 1.72V. So, i selected a MOSFET with Vgs(th) of 0.4V to 1V and Rdson defined at 1.8V as 28 mohm.
    Is your FET rated to able to handle 1-A of current and a turn-on of 1-V? 

    5. should the values of R6, R8, R10, R12 be increased ?
    It can be if you'd like, but it is not necessary. 

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 220 points

    Hey Luis,

    i'll make all the changes that you've suggested

    Do you actually need a 1-A balancing current? 

    The 1-A balancing current is requested by the customer and since the pack has a nominal capacity of 360Ah, 1-A of balancing seems reasonable

    Is your FET rated to able to handle 1-A of current and a turn-on of 1-V? 

    The FET is rated for a source current of 20A and diode forward voltage of 1.2V (I'm hoping 1.2V won't be an issue)

    With 1-A of balancing current spread across the four 15-Ohm resistors, the power dissipation across each resistor would be roughly (0.25-A)^2 * (15-Ohm) = ~0.94-W. So ideally your resistors should be rated for higher than this.

    Understood, will take this into account

    Thanks & Regards