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BQ77915: Customization and use for high current discharge using multiple mosfets in parallel

Abdul Arif
Prodigy 10 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ77905,

1.       How does TI support on customization of this IC for required threshold levels? Is there pre-requisite of MOQ for customization?

2.       What are the precautions I should take while connecting multiple MOSFETs in parallel?

3.       I would like to make 20S configuration, since lower IC drives CHG & DSG MOSFETs, will there be imbalance created between lower 5 cells and others?

4.       How do I implement reverse polarity protection?

  • 0
    TI__Guru 54715 points
    Hi Abdul,
    1. For possible new configurations contact your TI sales representative to discuss the potential volume and schedule.
    2. Different MOSFET vendors provide different levels of documentation, if your vendor does not have good application notes refer to other industry or academic literature. Generally you want to isolate the gates with some small resistance to avoid oscillation. See the application note bq77905 Using Multiple FETs linked in the bq77915 product folder technical documents tab.
    3. Yes, and while the bq77915 has balancing available, you may want to match the load from the FETs on the upper devices. See section 3 of www.ti.com/.../slua774 . While it is not specifically for the bq77915 the load current matching is the same concept.
    4. Reverse polarity is essentially built into the basic circuit structure. When the discharge FET opens from the event (either SCD, OCD, or UV), PACK- will go high. Refer to figure 19 of the bq77915 data sheet. (Note in figure 19 that the top ROCD should go to the local VSS rather than battery- as shown.) The discharge FET voltage rating must be sufficient to handle the high voltage. The current paths into the CHG pin and LD pin must protect the limits of the charge FET and the bq77915. See figure 24 and related discussion in the data sheet. Size the resistors limiting the current into CHG and LD to keep the inputs within their specified limits. Also with a 20S pack the charge voltage could be high compared to a depleted battery, see figure 23 and 25 if needed.