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BQ25756: BQ25756 charging characteristics at low power.

cameron Ryan-Pears
Prodigy 10 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ25176, BQ25820, BQ25798

Tool/software:

Hi. I am looking into using the BQ25756 but I would like to know more about the charging characteristics when it is using MPPT when there is not very much power being delivered from the solar panel, less than 2W.

Below is a graph comparing a linear charger (BQ25176) to a buck MPPT charger (LTC4015) that I have tested. From this testing it seams to be that when it doesn't have enough power to do the buck switching and tracking the MPPT falls back to doing linear charging, then when it got a bit more sun it could do the proper MPPT and the power increased a lot, at least that is how I interpreted it.

For my application the solar panels will often have to be placed in places where there is a lot of shade and might only get an hour or less of direct sun light. Because of this it is important that the charger I design works well in both low light and direct sunlight.

I do see that in the datasheet for the BQ25756 figure 7-1 shows charging eff. and it looks like when it gets down to 0.2A or so the eff will be very low. This is why I am hoping that it will fall back to linear charging or if there is some way I can make it work better in low power/low light MPPT charging situations.

If you have any information that could help me with this design that would be appreciated.

Cheers,

Cam

  • 0
    TI__Expert 6755 points

    Hi,

    I don't fully understand the design requirements. The BQ25756 is a high power charger. We can do low power levels, but that isn't really what the device is meant for. Are you looking for a switching charger with MPPT? If so, does the MPPT need to do a full panel sweep and have perturb and observe functionality? We have some more basic MPPT functionality in chargers at lower power levels. This MPPT essentially sets a lower limit to the voltage based on a percentage of the open circuit voltage to keep the solar panel in the maximum power region. Would this be more aligned with your application?

    If that does not work for the application, the efficiency of the BQ25756 can be different in each application since the components are external.

    The efficiency measurements are taken off of the EVM BOM. I also think that the measurements were taken without PFM enabled, which should be more efficient at lower currents. The efficiency can change depending on the external components. We have this calculator that can help estimate efficiency:

    This will require you to select a MOSFET and input the characteristics into the file.

    Also, the LTC4015 is a buck only battery charger. We have the BQ25820 which is also a buck-only device. This is pin-to-pin with the BQ25756.

    Let me know if I need to clarify anything.

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    Prodigy 10 points

    Thanks for the help. Yes I know the BQ25756 is intended for high power chargers but I was wanting the MPPT full panel sweep and observe, so this is why I was looking at that part.

    As you mentioned some of the other chargers have that more basic MPPT like the BQ25798 with it working by setting the voltage as a percentage of the Voc from the panel. Maybe I should try a charger with one of those MPPT algorithms and see how well it works for our design requirements.

    Thanks for mentioning the PFM, I missed that when looking though the datasheet, I have had good experience from other TI buck-boost converts for low power applications that use the PFM so maybe if the BQ25756 can use PFM I could get some good results for charging at low power.

    Cheers,

    Cam

  • 0 in reply to cameron Ryan-Pears
    TI__Expert 6755 points

    Hi Cam,

    The BQ25798 would be the charger I would have recommended. If you were planning on using host controlled method, this app note might help:

    I think there are a couple of caveats with this app note, however. I think that the VOC_DLY or the VOC_RATE needs to be a bit longer. I would have to double check with the primary expert on this device to be sure. Additionally, I would measure the input current rather than the charge current, especially if there is a system load.

    Additionally, if the battery will fully discharge and there will be low light conditions like you mentioned, I would recommend looking at this app note:

    Just a side note on PFM: the BQ25756 does not have an OOA PFM, so there might be some audible noise while using PFM on that device.

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    Prodigy 10 points

    Thanks for those resources. I think I'll try out the BQ25798 and see how it goes.

    Cheers,

    Cam