• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 554 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

BQ76200: Need of extra Cgd to remove gate oscillation

Shyama Agrawal
Intellectual 860 points
Part Number: BQ76200
Other Parts Discussed in Thread: CSD19536KCS

Hello,

In SLVA729A, page 7 it has been mentioned that extra Cgd might be needed to remove gate oscillation. The first question is that does this problem occurs when we are trying to turn off the DFET or turn on the DFET? The terminology of Page 7 is a bit confusing.

Secondly, A document by Alan Elbanhawy, says that normally Crss/Ciss of a MOSFET should be low if it is to be used in a situation of fast switching but in SLVA729A, Page 7 it clearly asks us to keep Crss/Ciss high. Can you please clarify what should be done keeping in mind the application of BQ76200. I have attached both the referred documents. Please refer to Page 4 of the document by Alan Elbanhawy.bq76200_FET.pdfWorst case cross conduction in MOSFETs.pdf

  • 0
    TI__Guru 54715 points

    Hi Shyama,

    The Alan Elbanhawy document appears focused on shoot through current in MOSFETs used in converters.  Switching is repetitive and high speed. It is important to switch quickly and avoid shoot through for high efficiency in the converter. 

    The BQ76200 uses FETs for a load switch.  In a best case situation the battery would always be on and the FET would never switch, but if a fault occurs it must operate successfully.  Switching a FET should basically be the same in both cases, a FET with low Ciss and using low gate resistance should switch fast.  Larger FETs tend to have large Ciss, so you may not get have much option on this.  In a battery the FET may need to carry high current, or the fault current may be high, so to avoid a large inductive spike in the system you may not want to turn off too quickly.  The BQ76200 has significant internal resistance, it is not a driver rated in amps like those used for converters, its specifications are rise and fall times.

    The author of SLVA729 encountered an oscillation with switching in the situation noted and fixed it with the Cgd capacitor which provides some feedback to slow the turn off.  Other techniques could also be effective.  Select something which works well in your design.

  • 0 in reply to WM5295
    Intellectual 860 points

    Hello,

    Thank you for your reply. One small query, was the issue faced by the author of SLVA729 while trying to turn the DFET OFF or ON or in both situations? The language in the document is a little confusing.

    Quoting the related paragraph - "When using a minimal number of FETs (1–2) under increased battery pack voltages, that is, 48 V, and a short circuit occurs, the CSD19536KCS FETs Cgd capacitance requires an increase of approximately 220 pF in order to provide feedback to keep the gate on as it switches"

    Why would you want the gate to be ON after a short circuit has happened? Does he mean it'll oscillate even when BQ76200 is trying to turn it off?

  • 0 in reply to Shyama Agrawal
    TI__Guru 54715 points

    Hi Shyama,

    Perhaps the wording could be different.  The picture included and the text seem to indicate the oscillation was on turn off.  Turn off with a short circuit is typically where there is a large inductive spike from the rapid turn off of the high current.  The spike can couple to things and cause oscillation.  Apparently the Cgd capacitance slowed the turn off and avoided the oscillation.  

    Each design will have different parasitic impedances which will affect the behavior in addition to the components.  Be sure to test your design and make refinements as needed.  You may not need a Cgd, or you may have a different solution.