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BQ25798: Issues using multiple BQ25798 chargers in parallel.

skander MTAWA
Prodigy 10 points
Part Number: BQ25798

Tool/software:

Hello everyone,

I'm currently working on an Electrical power System project. I've successfully used a single BQ25798 charging IC, and it operates as expected. Howerver, when I try to use 2 ou 3 bq25798 in parallel, I start noticing some unusual behavior.

Has enyone had success with using multiple BQ25798 chargers in parallel ? Anyadvice on current balancing, Synchronization or resolving instability issues would be greatly appreciated. 

Thanks in advance for your help.

  • 0
    TI__Guru**** 166996 points

    HI Skander,

    When paralleling multiple, identical NVDC chargers like BQ25798, you can only connect the BAT pins in parallel not SYS. This means that only one charger sees the SYS load. Tying SYS pins together requires the use of a diode OR circuit.  Also, since both chargers have the same I2C address, you would need an I2C switch to be able to talk to each charger individually. Lastly, when in constant current mode, the BAT output is a current source so tying both together is no issue.  But when near termination, BAT output is a voltage source.  Tying two voltage sources together can cause oscillation so it is highly recommended to set one charger to terminate before the other with either higher ITERM or lower VREG or both.

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Prodigy 10 points

    exactly,

    I've only connected the BAT pin in parallel

    the SYS path is connected using a diode for protection.

    for the I2C address, each BQ is connected in a separate I2C bus.

  • 0 in reply to skander MTAWA
    TI__Guru**** 166996 points

    Hi Skander,

    What do the status and fault registers (0x1B-0x27) report on each charger when you see odd behavior?  if both are connected to the same input power source and result in both running in VINDPM mode, because the input power source can't provide the total current of the sum of each IINDPM setting, that can result in oscillations too.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    I'll take a screenshot and send it to you.

    As for the input each one has its own independent power source ( the first BQ has 12V PV input (MPPT) & 12V charger and the second one has 12V PV input (MPPT)  or USB ) I use 2S2P battery config .