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Omit or move TVS protection, BQ76930

Kristoffer Stork
Prodigy 60 points

In the TIDA 0049 reference design (10S 50A toolpacks design) there is a TVS fitted between both pack + and - and load + and -.

Is there a need for the TVS? For our application, an extra wire will need to be installed to bring the top cell connection to the BMS with low impedance (other than the sense wire).

Interrupting current by stopping discharge will certainly create a voltage spike, depending on the switch speed of the mosfet - and the discharge current being interrupted. Can the TVS be moved to Drain-Source of the Mosfet instead of Pack + and -?

Looking for input on this issue, than you in advance.

  • 0
    TI__Guru 54715 points
    The TVS D16 and flyback diode D15 provide clamps to prevent the battery voltage from excessive levels and PACK- from rising significantly above the PACK+ voltage. Some designers will not like these structures since they are single point fault opportunities and they will implement transient protection in some other way. Your situation seems to be a mechanical restriction and again you may choose to implement transient protection in another way. The important consideration in the circuit design is to be sure your components operate within the limits of each component, how you do that may have options.
    When switching, the MOSFET will likely go into avalanche if the transient is sufficient. You might select a FET with sufficient avalanche rating. A TVS across the FET can share that current or take most of it depending on the component parameters. The TVS across the FET is a single point failure opportunity to bypass the FET.
    Another consideration in the battery design is the transients from the system current switching when the protection FETs are on. If the battery is removable, consider if it is possible that the user might mistakenly unlatch the battery during operation. Regardless of the transient cause, if filtering is sufficient to prevent excess voltage at the ICs the TVS would not be needed, or selected low current clamps might be placed in the circuit where needed rather than the high current clamp on the cell stack.