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Original question:

BQ76942: Transient protection in external balance circuit (SLUAA81)

BQ76942: Transient explanation across external balancing mosfet gate

Nick
Intellectual 672 points
Part Number: BQ76942

The app note SLUAA81 says:

A Zener diode is needed to protect the external FET gate from pack transients. For example, in the event of a
short across the pack in a 10-cell battery, Cell 10 would have approximately 40V across R n during the event and
the opposite transient at the release of the short circuit. The gate voltage should be connected through a resistor
to limit the current when the diode conducts.

I am trying to understand the reason for transients on Gate. please explain how come the voltage be 40V across Rn during short circuit event ?Lets say Point A is 40V but why point B voltage will drop ? 

Please help.

  • +1
    TI__Mastermind 42060 points

    Hello Nitin,

    Immediately following a short-circuit event, the voltage of the battery collapses to ~0-V, the VCn pin will still have a high voltage due to the capacitor Cn.

    Assuming a case you like assume, of 40-V at point A. Before short circuit, point A an point B are ~40-V. When short-circuit first occurs, the voltage at point A would drop to ~0-V, while the capacitor Cn would still hold some charge at ~40-V, so the voltage across Rn would be ~40-V. Rn is across the gate-source of the FET, so the Zener diode clamps the gate-source voltage of the FET.

    Although for a different part, I'd recommend you see Figure 7. External Balance FET Gate Protection and to read Section 4. Cell Balancing of the bq769x0 Family Top 10 Design Considerations application report. These describe this effect and how the Zener protects the FET.

    Best Regards,

    Luis Hernandez Salomon