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BQ25756: BQ25756: Inductor & Capacitor Values for 33.6 V / 20 A Charger

Umut Öznar
Prodigy 140 points
Part Number: BQ25756

Tool/software:

Hello,

I am designing a charger using the BQ25756. My input voltage range is 15–40 V, and the output is 33.6 V, 20 A. I will be charging an 8S Li-ion battery. The switching frequency is 250 kHz.
When using the BQ25756 calculation tool, it recommends an inductor value of 75.49 µH. However, this value is not readily available and also falls outside the recommended inductor range of 2.2–15 µH for the BQ25756. Could you suggest a suitable inductor value or a specific inductor part for this application?

For input and output capacitances, I based my design on the BQ25756 EVM, using ~600 µF for input and ~800 µF for output. Could you please advise if these capacitance values are appropriate for this application?

Best Regards.

  • 0
    TI__Expert 6315 points

    Hello,

    That minimum inductance is based on the saturation current of the inductor. The selected saturation current is only 0.22 A above the average inductor current. This minimum value is the inductance to keep the inductor peak current below the saturation current based on the worst-case inductor ripple at the selected switching frequency.

    This value can be reduced if the saturation current of the selected inductor is higher, the switching frequency is higher, or a combination of the two.

    The assumption here is the worst-case boost current. (15V to 33.6V) Will you be limiting the input current? This could also change the selection of the two values as the maximum average inductor current would be ~20A rather than 49.78.

    With regard to the input and output capacitances, I do not believe that the EVM has that much capacitance. We have the space for that much, but not all of that is populated by default. I think reducing the capacitance to 150-200 uF should be fine.

    Let me know if there are any follow up questions.

    Best regards,
    Michael