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BQ25504 - MPPT

Daniel Lenz
Prodigy 210 points
Other Parts Discussed in Thread: BQ25504

Hi all,

i have some questions to the MPPT of the BQ25504:

On Vin_DC i have a solar cell with a open circuit voltage of 4V.
When the booster is aktiv, the voltage on the input pin is 0.35V.
I think, that the generated current of the solar cell is to small.
But the solar modul is a big modul with enough power.
I think the MPPT is incorrect and the load on the BQ25504 is to big.

Can you agree this?

Thanks

 

  • 0
    TI__Mastermind 31120 points

    Please read the data sheet for the correct operation.

    Typically solar cells have a MPP at 80% of the open circuit voltage so this would suggest programming MPPT to 80% which would give a MPP of 3.2V with an OC voltage of 4.0V.

    For the boost convert to work well the output OV would have to be 3.3V to 5V.

    If the input voltage is greater than OV the input protection will pull down the input to the OV threshold and the input can't take more than 400mW.

  • 0
    TI__Prodigy 170 points

    Daniel,

    If the VSTOR voltage is less than 1.8V, then the part is in cold start and the MPPT is held to that fixed voltage.  Cold start will have a much lower power efficiency.

    Since you are seeing 350mV, I think that cold start is what you are seeing.  Either wait until VSTOR is above 1.8V or try pulling it up to 1.8V

    to see the MPPT start tracking the input.  You will also have a much higher power effiency.

  • 0 in reply to John Carpenter2
    Prodigy 90 points

    Good morning !

    I am using bq25504 for ultra low power highly efficint energy harvesting system. I am using 2.3V,10F supercapacitor for storing the harvested energy from the solar panel. I just want to know that as we know the super capacitor with the higher voltages are not available due to technology limitation.I want to use the capacity of the supercapacitor until .7V. But the storage device under voltage protection range is from 2.2V to VBAT_OV. This means I can use only .1V capacity of the supercapcaitor??? please provide me the solution. I know that I can use two supercapacitor in series but then the individual supercapacitor will need to have  voltage balancing to prevent any cell from going into over-voltage and it will also increse the system complexity as well as the extra circuit means the power wastage and it is serious in the ultra low powered applications.

    Can we configure the Under voltage UV .7 or .9V?

    please suggest me the best solution for the problem. 

  • 0 in reply to Sunil Kumar
    TI__Prodigy 170 points

    Please see the spec for more details, but if VSTOR is less than 1.8V you will enter cold start and have lower efficency.

    Run directly from the super cap, and then UV setting is ignored and you will get the full range from your super cap.

  • 0 in reply to John Carpenter2
    TI__Mastermind 31120 points

    The method John is suggesting works well when the BAT pin (super cap) stays above ~1.45V, which is the votlage that will keep the IC out of the low efficiency cold start (1.8V to ~1.9V).  The difference between Vbat and Vstor is the diode drop below the UV threshohld which is ~0.4V.....thus Vbat = 1.45, Vstor=1.85 and not in cold start.

    The other option is to short the BAT and VSTOR together and use a couple FETs to 1) short the input source to VSTOR when Vstor is less than 2V (use BAT_OK set to go low at ~2V driving P-CH FET on) and drive the N-CH FET between the input source and Vin off. 2) When Vstor gets above 2V the P-CH turns off and the N-CH turns on connecting the input source to Vin and using the effiicient boost converter.