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bq76920: Configuration for additional VCx input protection?

John M
Expert 2135 points
Other Parts Discussed in Thread: BQ76920, BQ76PL536, BQ76PL536A

Hello,

For our prototype boards using the bq76920 we would like to add additional protection beyond the RC filter. The operating environment is automotive with a huge variety of EFT/surge voltages and durations and we're considering zener or TVS diodes at the VCx inputs.

The series resistors (100 ohms in our case, for increased cell balancing current) will limit the current through the zener/TVS, which is good. Do you recommend placing the diodes across the inputs, acting to limit voltage differentially, or from each input to ground? We can see advantages and disadvanteges (especially concerning breakdown voltage selections for TVS') for both so we wanted to defer to any practical experience you might have in recommending the best approach for protecting these inputs.

JohnM

  • 0
    TI__Guru 54715 points

    Remember that the bq76920 is not an automotive qualified device.  The general architecture of the circuit is for simpler system than automotive may demand.

    The device has abs max limits of both input to ground (single ended) and input to input (differential) which must be considered.  The simple circuit uses differential capacitors to limit voltage changes and the input resistors limit current into the leakage paths of the IC. If it is pushed too hard, it will break.  For demanding applications, some additional protection may be needed.

    Zener diodes at the cell or near the cell will limit voltage into the circuit, but will not specifically limit voltages at the IC pins which could be induced across the input resistors by pull on the capacitors.  When the diodes are at the cells they may need to carry large currents.  If balancing is used in the bq76920, remember that the balance current is obtained by the inputs swinging.  The operation of the balance mechanism may give a small margin between the needed voltage swing for balancing and the abs max limit of the pins.  Zener diodes will have some leakage also.  Unfortunately there is not a simple "do it this way" type of circuit I can recommend.  Protection will involve some tradeoffs and testing in your specific environment.

    You may look at the application circuit for an automotive part like the bq76PL536.  While the device architecture is differnent to better support the more challenging environment, you may find some techniques which are applicable to your implementation.

  • 0 in reply to WM5295
    Expert 2135 points

    Thank you! After reviewing the bq76PL536A documentation we have a better appreciation for the advantages that an automotive-qualified chip can give. Especially with the cell input voltage limits.

    I was a bit surprised to see only 500mW zeners on the cell side of the cell input filters in the datasheet schematic. I guess the fact that the zeners are used differentially helps to limit the power levels the zeners must handle? It gives me hope that perhaps a similar zener setup might work for our application. Especially since we have very agressive transient protection at the BATT+/- and PACK+/- terminals on the board.

    We were also considering using zeners on the bq76920-side of the input filters. The filter's series resistor would limit current through the zener and how much power it had to dissipate. Not sure which placement might be better. After reading your reminder that balancing (which we will be doing) causes voltage swings, we're not very sure how to spec the zener's breakdown voltage (on either side of the input filter). Any insights you might have regarding this would be great to hear.

    JohnM