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BQ25970: Charge Pump Efficiency issue

Jun-Hao LAI
Prodigy 10 points

Part Number: BQ25970
Other Parts Discussed in Thread: BQ25960H

Why can a charge pump achieve higher efficiency than a buck or 3-level buck converter? Is it simply because it doesn’t have an inductor?

Thank you.

 bq25970_1.pdf 

  • 0
    TI__Genius 11665 points

    Hi Jun-Hao,

    Thanks for reaching out.

    Yes, an advantage of the charge pump is that it uses capacitors instead of an inductor to move energy from the input to the output. This means it avoids inductor-related losses seen in a buck converter. The main losses in a charge pump are charge-sharing losses, conduction losses through the switching FETs and ESRs, and switching-related losses, including gate drive losses and output capacitance losses. The charge pump also typically operates at a lower switching frequency than a buck, such as a few-hundred kHz compared to >1MHz, which helps with switching-related losses, although the charge pump does have several more switching FETs than the buck. Additionally, due to the nature of the 2:1 switch cap topology, the input current is half of the output current, which reduces losses across the input charging cable.

    Let me know if you have any questions about the BQ25970. I'd also recommend the BQ25960H.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 10 points

    Thank you.

    So, under the same conditions—such as identical switching frequency and switching FET sizes—is the efficiency difference between a 2:1 charge pump and a 3-level buck mainly due to the presence or absence of the inductor? Since both use four switching FETs.

    If so, does that mean a 3-level converter could be used for charging, and when fast charging with a charge pump is required, the inductor could be shorted and the switching signals of the FETs changed so that it operates as a charge pump?

    In addition, in Figure 5 of this document, is the reason why the 2:1 switched-cap topology does not show such a severe efficiency drop at high current also related to the presence or absence of the inductor?

    Thank you.

    Best regards,

    Jun-Hao Lai

    charger comparison.pdf

  • 0 in reply to Jun-Hao LAI
    TI__Genius 11665 points

    Hi Jun-Hao,

    Thanks for following up. The presence of an inductor in a buck is the main difference in efficiency between a buck and a charge pump, but not the only difference.

    In a typical application, the charge pump is used for fast charging while the buck is used for precharge and charge termination. We don't have any 2-in-1 buck and charge pump battery chargers. Such a device may be theoretically possible, but switching between topologies would likely introduce several challenges.

    And yes, at higher charge currents, conduction losses dominate in a buck while the charge pump efficiency remains relatively steady since it doesn't have inductor losses.

    Best regards,

    Alec