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BQ25570: MPPT algorithm

Timothy Chinowsky
Prodigy 40 points
Part Number: BQ25570

Tool/software:

I am building a low-power solar energy harvesting circuit using the BQ25570.  It seems to work, but I am wondering if I am seeing the expected behavior from the MPPT.  Every 16s it samples the open circuit voltage, as expected, and then it adjusts the charging current, but only  a little bit - say enough to change the loaded input voltage by 10 mV.  It seems to converge, slowly, to a charging current of ~1.5mA, which is appropriate given the low input energy.  Is this how the part is supposed to behave?  There is little information in the datasheet about how MPPT is implemented.  Thanks!

  • 0
    TI__Guru**** 166996 points

    Hi Timothy,

    The MPPT algorithm uses a very simply fractional open circuit sensing method.  The converter stops switching every 16s, measures the panel VOC and then stores the VOC x the MPP % that you previously selected (typically 80% for solar panels).  Then the boost converter only turns on if the input voltage is above VOCxMPP%, essentially regulating the charger's input voltage to VOCxMPP%.  The output current is simply the average of the converter output bursts, i.e. output current is not regulated.  Example waveform is shown below:

      

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Ah, that makes perfect sense.  I must have a circuit problem somewhere.  Thanks!