• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 63 subscribers
  • Views 656 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

bq24650: SLUA565 application note - issue with charge current

Darren DeBattista
Prodigy 200 points
Part Number: BQ24650

HI all,

I have built a battery charger for LifePo4 battery based on the SLUA565 application note. The circuit is exactly the same accept that I have removed D12 and R6, L1 and C7+C8 are size for a charge current of 5A according to what is specified in the datasheet. My issue is that during charging, I noted that the charge current was being regulated to around 3.8A. After some debugging, I realized that the issue was coming from switching noise in the PV supply rail. This noise was being 'seen' by the MPPT input pin making the bq24650 to think that the solar voltage is below 18V and hence regulating at a lower current. Removing capacitor C17 solved the issue. This capacitor is shown in the SLUA565 application note but not in the datasheet.

My question is, what is C17 used for? Is it really needed for the correct functionality? (I know that ideally I would try to limit the noise on the PV supply rail but this solution is left for a future update). 

Thanks

  • 0
    TI__Intellectual 2320 points

    Hi, Darren

    What is D12 and R6? why did you remove it?  How about use a power supply(20V)? current still 3.8A? The power of solar panel might not enough.

    Thanks!

    Alen Chen

  • 0 in reply to Alen%2520Chen
    Prodigy 200 points
    Hi Alen,

    Thank you for your reply. The components designators are taken from the SLUA565 application note schematic. D12 was removed as we are taking supply only from the battery pack and leaving the solar just for charging (we are disabling the charger when solar input is smaller than 16V). R6 (together with L1 and C7+C8) was not removed , but its value was changed to set the charger for 5A charge current.

    I am currently testing with a power supply. When supply voltage is greater than 20V, the charge current is correct but with a supply voltage smaller than 20V, the negative spikes on the supply rail caused by the power mosfets switching are being induced (through capacitor C17) on the MPPSET pin cause the bq24650 to 'think' that the MPPSET voltage is below the 1.2V threshold causing it to decrease the output current to try to maintain the 1.2V on the MPPSET pin.

    My question is regarding the functionality of C17. What is the reason for placing C17 in the SLUA565 application note (nothing that it is not placed in the bq24650 datasheet application circuit). Is it critical for the functionality/stability of the circuit? Can it be removed without jeopardize the stability of the circuit?

    Thanks
  • 0 in reply to Darren DeBattista
    TI__Intellectual 2320 points
    Hi Darren

    C17 just for modification input voltage regulation loop. You can charge the value or remove it .

    Alen Chen
  • 0 in reply to Alen%2520Chen
    Prodigy 200 points
    Hi Alen Chen,

    Thank you for your answer. My last question, why should the input voltage regulation loop be changed? Is it because of using a LifePo4 battery?

    Thanks
  • 0 in reply to Darren DeBattista
    TI__Intellectual 2320 points

    Hi Darren,

    The change isn't necessarily,  generally we don't need the capacitor. Just for balence respose speed and stability of input voltage regulation loop.

    Alen Chen

  • 0 in reply to Alen%2520Chen
    Prodigy 200 points
    Hi Alen Chen,

    Thanks for your answer