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BQ24650: Low Side FET Off, Hi Side FET 50ns Pulse, No Charge Current

stomp
Genius 4675 points
Part Number: BQ24650

Hi,

I've implemented a solar charger using BQ24650 and I am unable to get the device to draw current from the solar input and charge the battery, this is despite all all measurements and indications seeming normal.

My setup and schematic is very similar to the typical application in the BQ24650 datasheet:

16.8V Li-Ion battery Pack (Currently discharged to 13.8V)

Vfb = 1.7V.

TS = 1.92V

MPPSET is configured for 18V.

Input voltage (VCC) using a lab supply (16-24V)

Stat_1 = On

Stat_2 = Off

Term_En pull to VREF

VREF = 3.3V, RGEN = 6V

I have scoped the high and low FET gate signals. The high side FET has a regular group of about 5x 50ns pulse on it, the low side FET never switches on, despite Stat_1 being on.

I've confirmed all the other input voltages and are convinced my device is not in sleep.

The only possibility I can see is from section 8.3.11 of the BQ24650 datasheet in "cycle-by-cycle charge undercurrent".

Does anyone have any advice as to what is possibly going wrong?

Thanks

Stomp!.

  • 0
    TI__Guru 57541 points
    Stomp,

    You mentioned that your setup and schematic is very similar to the typical application in the BQ24650 datasheet. Are there any differences between your schematic and the typical application in the BQ24650 datasheet?

    Thanks,

    Ning.
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Genius 4675 points
    Hi Ning,

    I have four differences with respect to the typical application circuit:

    1. I'm using TI CSD19537 MOSFETS
    2. I have a forward facing diode at the output of R_sr leading to the battery pack. The VFB circuit is on the battery pack side of the diode in any event.
    3. I have a small 2.2R resistor leading into the BTST pin from the cathode of D2.
    4. I do not have reverse polarity protection D1 into the VCC pin.

    Removing the diode makes no difference to the circuit's operation.

    Any ideas?
    Thanks.
  • 0 in reply to stomp
    TI__Guru 57541 points
    Hi,

    Please note that the MOSFETs selected need to be turned on fast enough. Please refer to e2e.ti.com/.../730482 and SiR426DP specs for the details.

    Thanks,

    Ning.
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Genius 4675 points

    Hi Ning,

    I have purchased a BQ24650EVM-639.

    I have replaced resistor R13 on the EVM to 510K, and I have replaced R15 on the EVM to 75K.  This gives me approximately 16.38V Charge on my battery.

    I have the CE jumper joined, I have the TERM_EN jumper JP3 set to EN, I have JP2 also set.

    I am powering the EVM through J1 at 18V and have my battery connected to V_OUT on J2 which is at 13.8V.

    I have exactly the same set of conditions as is on my hardware when measuring TP1. (Although the EVM has lots of ringing)!

    Could it simply be a condition of MPPSET not being correct? Will the BQ24650 pulse the MOSFET's if MPPSET is below the set-point threshold?

    Thanks

  • 0 in reply to Ning T
    Genius 4675 points
    Hi Ning,

    I've also compared my TI CSD19537 MOSFET's to the Si7288 parts as fitted to the EVM.

    From the information available it looks like the CSD19537 is a superior device, with a lower Vg(th), Lower Rds_On, Higher current and voltage as well as faster turn on times.

    Thanks for pointing out the web link though!.

    Cheers.
  • 0 in reply to stomp
    TI__Guru 57541 points
    Hi,

    What is the MPPT voltage and Voc (open circuit voltage) for your solar panel? The BQ24650 EVM assumes MPPT=18V.

    Referring to page 3 of the EVM User's guide, If the solar panel or other input source cannot provide the total power of the system and bq24650 charger, the input voltage drops. Once the voltage sensed on the MPPSET pin drops below 1.2 V, the charger maintains the input voltage by reducing the charge current.

    Thanks,

    Ning.
    Please click "This Resolved my issue" button if this post answer your question.
  • 0 in reply to Ning T
    Genius 4675 points

    Hi,

    I'm powering the EVM and my hardware from a lab supply up to 22V.

    My question really is why does the upper MOSFET pulse at a few hundred millisecond intervals?

    I could not find any reference to this in the datasheet. Is this the intended operation of the device?.

    Thanks.

  • 0 in reply to stomp
    TI__Guru 57541 points
    Hi,

    Please follow the BQ24650 EVM user's guide to do evaluation.

    Thanks!
    Please click "This Resolved my issue" button if this post answer your question.