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BQ25798: The chip got damaged if battery is unplugged while a power supply is connected to VBUS

Avi Hamavi
Prodigy 30 points
Part Number: BQ25798


Hello,

I have encountered a situation where during the normal operation, if an external power supply is connected to VBUS, and I disconnect my battery from the system, the charger chip is damaged.

It seems that there is a short circuit in the chip VBUS circuit (there is a sharp increase in the current and PSU voltage dropped, the charger gets hot).

When disconnecting the external PSU and reconnection the battery, I am able to read A/D battery voltage via TI CHARGER GUI, however once I reconnect the external PSU, the input short circuit remains and the chip is unusable.

(I think it also happened with the EVM PCB and I could not figure out what was going on due to the register complexity - i guessed I did some forbidden register operation)

Last week I was able to pinpoint the problem since I worked with my own PCB via TI CHARGER GUI for few days and chip behaved fine until i removed the battery while the PSU was connected.

The chip immediately got damaged as described above.

I could not find any restriction to do so (remove battery) on datasheet, please consult.

Many thanks,

Avi 

  • 0
    TI__Guru**** 169686 points

    Hi Avi,

    When you removed the battery (called battery hot unplug), the BAT and/or SYS voltage overshoots due to inductance and finite battery charger feedback loop response time.. You can monitor on an oscope. If your battery is 3S-4S, that overshoot could easily exceed the BAT and SYS pins 20V maximum, causing IC damage.  The EVM user's guide recommends adding a TVS diode to prevent damage.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Dear Jeff,

    I use a 2S configuration, so less prone to overshoot as would 3S/4S configuration.

    (the EVB states that for 4S without battery transistor a TVS should be added)

    However, I've damaged ~4 PCBA's and I suspect most of the due to overvoltage spikes.

    This our design

    We use a 20W PV with 21V open circuit voltage and~ 17.5V MPP.

    The chip got damaged after I toggled few times the enable charge & enable MPPT checkbox.

    I got a  VBUS_OVP_STATUS & VBAT_OVP_STATUS red, than the chip stopped charging.

    I guess the voltage got overshoot due to the voltage release when i disabled charging.

    I consider adding TVS diodes on the battery, sys and vbus pins near the chip.

    Assuming the Vbus max ~ 21V & 2S configuration please consult where and what values should be a good TVS selection.

    Many thanks in advance,

    Avi

  • 0 in reply to Avi Hamavi
    TI__Guru**** 169686 points

    Hi Avi,

    The VBAT_OVP is expected and, since 2S, is likely not the issue.  The VBUS_OVP is likely the issue.  I suggest adding either a TVS diode and/or the 2.2ohm plus series 2.2uF snubber on the input.

    Regards,

    Jeff