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BQ24650 application issue

Sulyn ZHANG
Expert 8700 points
Other Parts Discussed in Thread: BQ24650

Hi Team,

My customer is using our BQ24650 in new project. But they met some problem and want to check with you, thanks a lot.

Customer use the typical application circuit in page 1 of BQ24650's datasheet. They set the MPPT to 17.8V, and connected to solar panel. When the light is not strong, the solar panel's open circuit voltage is about 20V. When connected to BQ24650 circuit, as these is no enough current, the panel's output voltage dropped, but in our datasheet, it seems charger current will be decreased and keep the charger voltage about 17.8V, however, in customer's situation, it charge Intervally, that means not continuously charge by changing the current. Can you give some expertise advice for this problem? Is these something wrong with customer's application or is these something wrong of the datasheet?

The solar panel voltage waveform as below:

Thanks a lot for your support!

Best regards,

Sulyn Zhang 

  • 0
    TI__Genius 11480 points
    Hi Sulyn,

    Do you have the datasheet for the panel available?

    If the solar panel VOC is 20V, it's MPP will be ~16V. However, it sounds as if the panel cannot even source that much current. If this is the case, the Solar Panel I-V curve is likely shifted down due to the low light levels.


    Does the circuit work as intended under better lighting conditions?
  • 0 in reply to David Wiest Jr
    TI__Expert 8700 points

    Hi David,

    Thanks for your support. Sorry that the Open circuit voltage is about 22V, not 20V. The datasheet of the panel is attached as below:

    TDB125×125×2÷3-36-P.pdf

    As for your question: Does the circuit work as intended under better lighting conditions?

    Answer: Under better lighting conditions, it works well but the efficiency is low, not as showed in our datasheet.

    Can you help give some advice on this? Thanks a lot!

     

    Best regards,

    Sulyn Zhang

  • 0 in reply to Sulyn ZHANG
    TI__Genius 11480 points
    Hi Sulyn,

    Thank you for sharing the datasheet.

    If you want to increase the charger circuit efficiency, the quickest place to look is at the FETs. You want to look for the FET's with the smallest Qg*Rds(on).

    A smaller Rds(on) results in smaller conduction loss in the FETs. A smaller Qg causes fewer switching losses, as well as less current to be pulled into VCC. You can also optimize DRC of the inductor and use Schottky Diodes to lower Vf of any diodes you are using.


    It seems that the panel crashes periodically. They can try to increase the MPPSET voltage, which would try to stop the panel from crashing a little bit sooner.
  • 0 in reply to David Wiest Jr
    TI__Expert 8700 points

    Hi David,

    Customer tried to increase the MPPSET value from 17.8V to 18.5V, but the issue still exist, no obvious improvement. So not sure have we got similar test before and what is our test result? It seems customer can't realize what our datasheet described when the solar panel's output current is small when light is weak, can you help check this?

    Customer's schematic is just like our typical schematic in datasheet, almost all parameters same(Except the MPPSET resistors value different).

    As customer is deciding use or not use our BQ24650 solution in this project, so your great support are appreciated!

    Best regards,

    Sulyn Zhang

  • 0 in reply to Sulyn ZHANG
    TI__Genius 11480 points
    Hi Sulyn,

    What happens if they keep increasing MPPSET? What charge current are they trying to use, and what is their system current?



    Can the customer provide switching waveforms, as well as look at currents (they just need to measure voltage across the sense resistors). If the converter is working ok, it sounds like the panel is just not up for the job under low-light.