This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

  • Resolved

BQ24618: Plugging the battery in before powering the IC

Part Number: BQ24618

Hi,

I'm still investigating, but it seems as though anytime I plug the battery in first I need to replace the Low-side NMOS. Is this something you have seen in the past? 

  • Can you please show scope capture on the issue when damage occurs?

    Regards,

    Jing Zou

    Application Engineer, Battery Charging Products

    Please click "This resolved my issue" button if this post answers your question.

  • In reply to Jing Zou:

    Before I do that, I was wondering if you could explain step by step what should happen when you plug a live battery before you apply input power. Maybe I can try to fix the problem before I blow another board.

    Thanks,
    JP
  • In reply to JPAlleyne:

    What is the cell count of your battery? What is the voltage rating of the low side mosfet?
    When battery is plugged in, the device won't power up until VCC is powered. In battery only mode, the converter should be off and the part will not switch.
    Do you have battery FET? Could you please upload the schematic?

    Regards,

    Jing Zou

    Application Engineer, Battery Charging Products

    Please click "This resolved my issue" button if this post answers your question.

  • In reply to Jing Zou:

    Cell count= 6

    FET Voltage Rating = 40V

    Here is the Schematic (we added the missing cap on SRP):

      

  • In reply to JPAlleyne:

    Since there is no BATFET in your schematic, when battery is plugged in, a large current would go though the inductor to the body diode of Q3 then to the ground. There will be a voltage spike on switch node. However, not sure why Q4 get damaged but PH pin did not.

    Regards,

    Jing Zou

    Application Engineer, Battery Charging Products

    Please click "This resolved my issue" button if this post answers your question.

  • In reply to Jing Zou:

    Well I haven't fully tested it but initial findings showed that there is the potential that the IC got damaged as well. You said that a large current would flow through the body diode of Q3 and then go to ground.

    1) What path would it take to go to ground through Q3's body diode?

    2) Is the voltage spike caused by the large current flowing through the inductor?

    3) Is it the voltage spike that causes damage to components on the PH node?

    4) What can I do to protect the circuitry right now, without major redesign?

  • In reply to JPAlleyne:

    Regarding 1, please see the path marked below.

    Regarding 2, yes.

    Regarding 3, I cannot conclude without look at scope captures.

    Regarding 4, when battery is plugged in, the drain of the lowside mosfet rises fast which might bring up the gate voltage and cause damage. One solution is to add more capacitance at Cgs to hold gate voltage down. Or try to find a replacement of the low side FET which Cgd is less than Cgs.

    Regards,

    Jing Zou

    Application Engineer, Battery Charging Products

    Please click "This resolved my issue" button if this post answers your question.

  • In reply to Jing Zou:

    So did you mean Q4? And not Q3?
  • In reply to Jing Zou:

    Regards,

    Jing Zou

    Application Engineer, Battery Charging Products

    Please click "This resolved my issue" button if this post answers your question.

  • In reply to Jing Zou:

    I'm still a bit confused as to why the current would flow through the capacitor to ground. Isn't it a large DC current?

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.