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BQ25505: Energy harvester with small indoor solar cell for USB C charging (BQ25504, BQ25505, BQ25570)

Chris Dreessen
Prodigy 10 points
Part Number: BQ25505
Other Parts Discussed in Thread: BQ25570, , BQ25504

Hi,

my end goal is building a USB C charger (for an eInk device) that is powered by a solar cell (ca 300 cm^2) in indoor lighting conditions.

The ouput power of the solar cell is about 10 mW under 1000 lux. I can manufacture the solar cell to deliver around V_MPP = 0.4 V and I_MPP = 25 mA (chinging with light intensity), but I can also connect several smaller solar cells in series and, therefore, increase the voltage and lower the current. The more cells I connect in series, the more power will be lost. So it is possible to reach 1.2 or 1.6 V (with 8 or 6 mA), but making a solar cell with 5 V will not be efficient.

For the USB charging, I need 5 V and 50 - 100 mA output for the device to charge properly. To reach these values I thought about charging up a supercapacitor with the solar cell and, once it is charged, the supercapacitor should be able to charge the eInk device for a short time, until the supercap falls below a certain voltage and needs to be charged again.

In the search for an IC, I found the series BQ255xx (BQ25504, BQ25505, BQ25570) and my question would be if you could recommend any of them (or also any other part of TI that I might have missed).

My thoughts so far: I liked the BQ25570 because it seems to have both parts (charging the supercap and charging the device) included, but my doubts are about the buck converter. As far as I understand it, a buck converter is a step-down converter. So this would mean that the voltage of the supercap would need to be higher than 5 V and most of the supercaps I saw are at 2.7 or 3 V. So this wouldn't work, correct?

That would leave me with the BQ25504 or BQ25505 and an additional boost converter. I don't see any advantage of the additional gate driver of the BQ25505 and I could see that the adjustable VBAT_UV of the BQ25504 could come in handy, so I tend towards the BQ25504. Do you agree with me? Can you recommend me a supercap and an appropriate boost converter that would make sense in my application?

Do I need anything else to make the system work?

The difference between a normal boost converter and the energy harvesting circuits is the MPP tracking?

Thank you very much for your help!

  • 0
    TI__Genius 17725 points

     HI,

    Chris Dreessen said:
    So this would mean that the voltage of the supercap would need to be higher than 5 V and most of the supercaps I saw are at 2.7 or 3 V. So this wouldn't work, correct?

    That's right. The step down rail is typically used to power an MCU or the device system load but not necessarily used for charging and outputting 5V. This would likely require an external boost converter since your capacitor voltage can vary.

    The BQ25504 may be the best solution for your application.

    Chris Dreessen said:
    The difference between a normal boost converter and the energy harvesting circuits is the MPP tracking?

    This is one big difference, however the energy harvesting circuits also include low voltage shutdown and regulation voltage for safer battery charge and discharge.

    Best Regards,

    Juan Ospina.