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BQ76952: Zener diode across External balancing BJT

Shyama Agrawal
Intellectual 860 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ76942

Hello,

In the document SLUAA81  "Cell Balancing with BQ76952, BQ76942 Battery Monitors", it has been mentioned to use zener diode across each BJT used for balancing for protection against pack transient. We wanted to know what is the recommended voltage rating of the zener diode. For MOSFETs, protecting with 16V zener diode is clear but what about BJT? Please see the image below for reference.

 

  • +1
    TI__Guru 54715 points

    Hi Shyama,

    As shown the apnote says "A Zener diode is also used in this circuit to protect from pack transients similar to the FET circuits."  For the FET the zener will limit both the positive and negative gate voltage.  With the NPN shown the base-emitter voltage will limit so that the zener should never conduct.  An NPN will usually have a maximum emitter-base voltage, Emitter Base Breakdown Voltage, VEBO.  When the cell voltage rises above the VCn this limit could be at risk.  The zener will conduct in the forward direction to limit the VEB. A conventional diode could work for this purpose.