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BQ25750: Observed BATFET behavior during hand-off, with and without RC soft-start

Allen Yang
Prodigy 25 points
Part Number: BQ25750

Tool/software:

Setup notes (for context only): Back-to-back N-MOSFETs as BATFET (source-to-source), BATDRV driving the gates, optional capacitor between BATDRV and BATSRC. VSYS has load 20W.

Observed behavior

No RC (no soft-start) between BATDRV and BATSRC

When enabling BATFET with load present on VSYS, the hand-off VBAT→VSYS fails: over-current protection is triggered and the path shuts down. A reset is required to recover.

With RC soft-start (capacitor from BATDRV to BATSRC)

The VBAT→VSYS hand-off under load succeeds. No OCP trip is observed.

With the same RC soft-start, when VAC is plugged in (adapter re-applied) while load remains on VSYS

The device transitions to forward operation (BATFET OFF, ACFET ON). During/after this transition the BATFET is damaged (burned).

  • 0
    TI__Mastermind 31305 points

    Hello Allen,

    Thanks for working on this.

    What's the adapter voltage and battery voltage for your system?

    When the BATFETs get damaged, are the ACFETs also damaged?

    Also, you've probably already seen this, but have you seen the BQ25750 power path switchover FAQ?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    VAC = 13.5 V, VBAT = 16–18 V. Only the BATFET fails. Even after replacing the MOSFET, it often fails again—still only the BATFET. The ACFET shows no abnormalities. I’ve reviewed the material you provided; it only addresses the inrush current issue. Adding an RC has resolved that, but it does not cover the BATFET burnout problem I’m encountering.

  • 0 in reply to Allen Yang
    TI__Mastermind 31305 points

    Hello Allen,

    Got it. Thanks for the new information. There might be shoot through from the ACFETs to the BATFETs. Would it be possible to graph BATDRV and ACDRV when VAC is plugged-in?

    Also, could you use a diode in parallel with the ACFETs RC to increase the turn-off strength?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Hello Ethan,

    Based on our latest tests, the contention/overlap between the ACFETs and BATFETs is confirmed. Given this, should I also add a soft-start to the ACFETs?

  • 0 in reply to Allen Yang
    TI__Mastermind 31305 points

    Hello Allen,

    Thanks for the lab tests.Are you measuring ACDRV and BATDRV with a normal measurement or differential measurement? Which channel is ACDRV and BATDRV?

    Adding a softstart to the ACFETs may reduce the overlap. You could also add a diode to bypass the BATFET's gate resistor and help pull the BATFETs to GND faster.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 25 points

    Hello Ethan,

    Since I don’t have a differential probe on hand, I used conventional single-ended measurements of both gates with respect to GND. CH1 (yellow) is the ACFET gate/ACDRV, and CH2 (blue) is the BATFET gate/BATDRV.

    I also tried placing a diode in parallel with the BATFET’s gate resistor to strengthen the turn-off, but the MOSFET still ended up damaged.

    Best regards,
    Allen

  • 0 in reply to Allen Yang
    TI__Mastermind 31305 points

    Hello Allen,

    Got it. The single ended measurements are not a problem. I just wanted to make sure I was reading the waveforms correctly.

    Can you measure the SRN/SRP and ACN/ACP resistors? These resistors can get damaged in a current surge. These resistors might still be damaged after replacing the FET.

    For your system, you might have to add soft-start to the ACFETs or reduce the soft-start of the BATFETs. Will either of these options work for your system?

    Best Regards,
    Ethan Galloway