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BQ24133: ACFET, RBFET and BATFET selection (Qg and Qgd specs)

Deniel Go
Intellectual 355 points
Part Number: BQ24133

Hello, 

From this TI question: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/945449/bq24171-acfet-and-rbfet/3493046?tisearch=e2e-sitesearch&keymatch=bq24133%252520ACFET#3493046

The recommendation for the ACFET and RBFET is to get low RDSON on and low Qg and Qgd. 

The BQ24133 EVB (15V Version) has the ACFET/RBFET with Qg = 2.1nC @4.5Vgs and Qgd = 0.4nC and the BATFET with  Qg = 2.6nC @4.5Vgs and Qgd = 0.5nC

Our Application will have a maximum current of 6-6.5A going through the ACFET/RBFET and 8-8.5A going through the BATFET.

We have a initial list of N Channel and P Channel MOSFET that handle the current going through them and have a low RDSON but the Qg and Qgd of the MOSFETs are not as low as the ones on the EVB

Qg on these initial list of FETs range from 4.3nC to 24nC. and Qgd ranges from 1.7nC to 5.1nC. What is considered a low Qg and Qgd? Can we go higher than the Qg and Qgd of the FETs on the EVB? How will that affect the Power Path Performance of the Charger IC? Or should we try to find FETs that have Qg and Qgd similar to the EVB FETs?

 

Thanks,

Deniel

  • 0
    TI__Genius 17102 points

    Hi Deniel, 

    Thank you for reaching out!

    I will look into this and get back to you by mid-week. 

    Best Regards, 

    Aya

  • 0 in reply to Aya Khedr
    TI__Genius 17102 points

    Hi Deniel, 

    The MOSFETs on the EVM were selected for the optimization of the device applications.

    We highly recommend choosing FETs that have very similar RDSON, Qg, and Qgd as the EVM. If you would like to select one from your list, then I would recommend the FETs on the lower range that would not vary much from the EVM. 

    Please let me know if you have additional questions/concerns.

    Best Regards, 

    Aya

  • 0 in reply to Aya Khedr
    Intellectual 355 points

    Hello Aya,

    Thanks for the response. I have some additional questions.

    1. For the BQ24133 EVM - 15V, what is the maximum allowable output current of VSYS if it is coming from the battery via the Q4 P Channel MOSFET?

    2. If we only have a choice between the two: Should we prioritize getting a lower RDS, ON or Qg/Qgd?

    3. What is the effect of having a larger Qg / Qgd? For example, the EVM P Channel FET has Qg of 2.6nC @ Vgs = 4.5V and Id = 3A. Is there an upper limit Qg value that we should not go for? Can we go for Qg = 15nC or even 20nC  for Vgs = 4.5V?

    From our list, MOSFET that have current carrying capacity (continuous drain current rating), generally have a higher Qg value. 

    Thanks,

    Deniel

  • 0 in reply to Deniel Go
    TI__Genius 17102 points

    Hi Deniel, 

    I'll get back to you on Tuesday with feedback. 

    Best Regards, 

    Aya

  • 0 in reply to Aya Khedr
    Intellectual 355 points

    Hello Aya,

    Are there any updates regarding my follow up question?

    Thanks,

    Deniel

  • +1 in reply to Deniel Go
    TI__Genius 17102 points

    Hi Deniel, 

    Thank you for your patience. 

    1- We do not control the current through the discharging path. The device relies on battery protectors to detect any over-discharge. This may also be influenced by the MOSFET current rating. 

    2- There is a tradeoff here and it just depends on your system requirements. RDSON correlates to efficiency while Qg/Qgd correlates to how fast the switch will open/close. 

    3- Unfortunately, because this is a legacy device, we have limited data on various Qg/Qgd values. Therefore, I would not be able to provide an upper limit threshold because we haven't tested in the past. 

    Best Regards, 

    Aya