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I am drawing the schematic of a battery management system (BMS) based on the BQ76920. The application circuit in the datasheet shows the MOSFETs for balancing the voltage of each cell:

enter image description here

I compared it with a github project with the same circuit. It uses a Zener diode in paralel to the MOSFET.

What is the purpose of using a Zener diode in this? I have read that Zener diodes conduct energy in reverse polarity instead of rectifier diodes.

Could somebody explain me the operation of the circuit please? How do the MOSFETs perform the balancing process?

enter image description here

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What is the function of the Zener diode in this BMS schematic?

enter image description here

The Zener diode limits the peak gate-to-source voltage to around 15 volts typically and, protects the MOSFET's gate-source region from over-voltages. In your particular circuit, a 16 volt Zener diode is used (MMSZ5246B).

I have read that Zener diodes conduct energy in reverse polarity instead of rectifier diodes.

They conduct current when the "Zener" voltage is exceeded and act as a "shunt" voltage regulator.

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  • \$\begingroup\$ OP says "Zener in parallel to the MOSFET", not "in parallel to the MOSFET gate-source". \$\endgroup\$ Commented Mar 15, 2023 at 14:52
  • \$\begingroup\$ A MOSFET has three terminals and can only be made to be in parallel with another 3 terminal device so, I used my imagination LOL @RohatKılıç \$\endgroup\$
    – Andy aka
    Commented Mar 15, 2023 at 14:53
  • \$\begingroup\$ @RohatKılıç It also does not specifically say in parallel to the MOSFET drain-source either, it just asks what the Zener does in the circuit. \$\endgroup\$
    – Justme
    Commented Mar 15, 2023 at 14:54
  • \$\begingroup\$ @Justme true but when we say "parallel to the MOSFET" 99.9% of us will think "parallel to the MOSFET drain-source". And, again, it should have been quoted with its designator (D5). Because there are lots of Zener symbols "in parallel with MOSFETs" in the circuit. \$\endgroup\$ Commented Mar 15, 2023 at 14:54
  • \$\begingroup\$ Thank you Andy for your detailed explanation, now it is really clear for me! And excuse me Rohat for not beeing explicit \$\endgroup\$
    – imt_blake
    Commented Mar 15, 2023 at 15:01

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