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BQ24640: MOSFET Selection

Brian Wang0928
Genius 11650 points
Part Number: BQ24640

Condition:

Device: BQ24640

VIN: 12V

VOUT: 2.7V

Icharge: 1A

 We used the FDD3642 to do the charge function with bq24640 for super cap application. However, the charge current is limited to few mA. after checked the Id vs. Vgs characteristics. it looks the Vgs=4V doesn't work with this device since it doesn't be fully turn- on.

We tried to use the bq24640EVM's MOSFET (SiR426DP) on customer system board, it's ok for charger function of 1A. However, we also tried to use similar part of SiR426 as FDD8447L. it doesn't work with charger. From the Id vs. Vgs characteristics it looks ok.I don't why? and what critical parameters of MOSFET could also improve this problem? how to choice the MOSFET correctly? Very thanks.

FDD8447L


 

 

  SiR426DP


 

FDD3642


 

  • 0
    TI__Mastermind 46671 points
    Have you tried FDD8447L on the bq24640 EVM and does it work normally? Thanks!
  • 0 in reply to Ning T
    TI__Mastermind 46671 points
    We haven’t heard back from you for a while, the thread is considered solved and closed. Thanks!
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    TI__Genius 11650 points

    1. When FDD8447 MOSFET is build on the EVM, it's ok for charger.

    2. When FDD8447 MOSFET is build on the customer board with boostrap resistor of 10 Ohm, charger current can be up to 1A .  

    When it is 5 Ohm for boostrap resistor, charge current can be only up to 0.7A.

    Could you comment the boostrap resistor value design? is 10Ohm ok on the design? What's the side effect?

    Regards,

    Brian

  • 0 in reply to Brian Wang0928
    TI__Mastermind 46671 points
    Brian,

    The bootstrap resistor will slow down the MOSFET switching speed. So bootstrap resistor can’t be too big. The BQ24640 EVM uses 4.7ohm bootstrap resistor.

    Could you please double-check the board layout especially the sensing resistor layout? Please refer to 10.2 Layout and Figure 16. Sensing Resistor PCB Layout on the d/s for the details.

    Thanks,

    Ning
    Please click "This Resolved my issue" button if this post answer your question.