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BQ76952: No. of Blocking FET for blocking charge through the Discharge Path

Naveed Shiliwala
Prodigy 30 points
Part Number: BQ76952


Hello All,

              I’m designing a BMS around the TI BQ76952 using the “parallel path / separate charge & discharge path” approach. My discharge requirement is ~200 A continuous, so I’m paralleling twelve identical MOSFETs on the discharge path to handle current.

The BQ76952 application note shows a blocking FET arrangement on the discharge path to prevent current flowing back through the discharge path, but the note only shows a single discharge FET (or a single back-to-back pair). My question:

• If my discharge switch is implemented as 12 MOSFETs in parallel, do I need 12 corresponding blocking FETs (i.e., one blocking FET per MOSFET), or can I use one (or a small number of) blocking FET(s) ?
Blocking through the discharge path.png

Regards

Naveed S. 

  • 0
    TI__Expert 3175 points

    Hello Naveed,

    Theoretically, only one blocking FET would be needed.

    However, with the multiple you are likely using a smaller resistance on the gate path. If you are planning to use a single blocking FET, you will need the larger gate resistor so it may defeat the purpose of having a single blocking FET. Just something to keep in mind.

    Regards,

    Rohin Nair

  • 0 in reply to Rohin Nair
    Prodigy 30 points

    Hello Rohin,

                     Thanks for the reply. I believe a single MOSFET cannot safely handle a continuous 200 A load. The MOSFET currently in my discharge path has an RDS(on) of 2 mΩ, which would result in around 88 W of dissipation. While the junction-to-case thermal resistance is low (RθJC​ = 0.4 °C/W), that only accounts for heat from the silicon to the case, the heat still needs to be transferred to ambient through the interface and heatsink. Even with an excellent heatsink, the total thermal resistance required to keep the junction below 125 °C is unrealistically low.

    In short, running 200 A continuously would require extreme cooling, or alternatively, paralleling multiple MOSFETs. I’d appreciate your thoughts on this approach.

    Regards

    Naveed S.

  • 0 in reply to Naveed Shiliwala
    TI__Expert 3175 points

    Hello Naveed,

    Paralleling multiple MOSFETs can help avoid this issue of overheating due to high discharge current. My statement above was just to say that theoretically only one blocking FET would be needed. Since FET overheating is a concern in your application, you will likely need to have parallel blocking FETs as well to avoid this issue. The number of blocking FETs you want to add is up to you.

    Regards,

    Rohin Nair