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BQ25570: Efficiency of BQ IC in buck and boost mode

godwin nishanth
Prodigy 20 points
Part Number: BQ25570

Hi, Will there be any difference in  charger efficiency in boost and buck mode. We are using Li-ion rechargeable battery (Nominal - 3.7V, Full charged battery voltage - 4.1V) (Solar panel Voc-4.57V, Vmp-3.85V rating). What's the charger efficiency we can expect with 80% set MPPT(Vin connected to VSTOR). 

  • 0
    TI__Guru*** 143910 points

    Hi Godwin,

    The datasheet efficiency curves should help with that.  Without knowing the panel's ISC (IIN), it is a bit hard to predict exactly but, based on the curve below with VIN=3.0V, well above 92% is a good estimate.

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Thanks Jeff, The panel Isc=807mA, Im=754mA 

    At high lux (80k-90k) measured from HTC lux meter, we observed 3.85V-Vin(that means MPPT has been properly set), 110mA-Iin, Charger output voltage - 4.2V, Output current-85mA(measured form rms multimeter). Battery voltage before connecting to charger output was 3.7V(9000mAh). So Pin=423.5mW & Pout=357mW. Converter efficiency of boost mode practically we are seeing is of 84%. Is our understanding correct or are there any other factors to be considered like lux value(for low lux less than 5k or high lux more than 70k), Ah of batt  in calculation. In data sheet we could only see efficiency w.r.t input voltage & efficiency w.r.t input current. 

  • 0 in reply to godwin nishanth
    TI__Guru*** 143910 points

    Hi Godwin,

    Those numbers seem reasonable.  You are operating at the max input power with 110mA average input current.  The efficiency starts to dip at higher currents due to the high Rdson internal FETs.  If you don't need the internal under voltage lockout protection provided by the ~1 ohm FET between VSTOR and VBAT, you can short VSTOR to VBAT and get even higher efficiency.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 20 points

    Thanks Jeff, Will try and get back with results(connecting VSTOR to VBAT for higher lux efficiency). With low lux say(5k ,10k or less than 30k lux) where Vin ranging b/w 1.5V to 3V efficiency was found 40-50%. Can you pls suggest any configuration to be set for low Vin application. Referred to the MPPT resistor 20Mohm table but didnt help us.

  • 0 in reply to godwin nishanth
    TI__Guru*** 143910 points

    Hi Godwin,

    That's odd.  At low lux, I would expect higher efficiency than 50%.  Is VSTOR>2.0 when measuring so that the charger is not in cold start?  The MPP may be slightly different (likely lower) than the fixed 80%. Is your measurement equipment for current capable of filtering the pulsing input and output currents of PFM/hysteretic converter?  The appnotes below might help with measurement issues:

    https://www.ti.com/lit/pdf/slua691

    https://www.ti.com/lit/pdf/slyt558

    Regards,

    Jeff