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BQ78350-R1A: Efficient Precharge Circuit Design for BMS in E-Bike Application

Ahmed MOSRATI
Prodigy 195 points
Part Number: BQ78350-R1A
Other Parts Discussed in Thread: BQ76940, BQ76200, BQ25756

Tool/software:

Hello,

I'm working on a BMS design (BQ76940 & BQ78350-R1A & BQ76200) for an e-bike application and need advice on an efficient precharge circuit. The simple design with a MOSFET and resistor in series for precharge is causing high power dissipation and requires a high power rating for the resistor.

My battery configuration is 10S4P, and the recommended precharge current is 303mA per cell, which means 1.2A for the 4 cells in parallel.
Additionally, the indicator LED on our charger needs a consumption current of more than 600mA to switch from green to red.

Moreover, the current with the simple precharge circuit decreases during charging (from battery = 25V to 30V) and takes more time to charge in precharge mode.

I'm looking for a design recommendation that uses a current generator like a DC-DC converter with Rsense feedback. The goal is to minimize space with SMD components on the PCB and reduce power dissipation in the precharge circuit.

Any suggestions or design ideas would be greatly appreciated!

Best regard,

Ahmed

  • 0
    TI__Mastermind 31185 points

    Hello Ahmed,

    Thanks for being patient with the wait.

    For a 10S4P battery charging, I recommend using the BQ25756. The BQ25756 can charge 2S to 16S batteries and can charge at up 20A of current. The BQ25756 has built in trickle charge, precharge, fast charge, and taper charge modes. The BQ25756 will also automatically terminate once the battery is fully charged.

    Let me know if this part will work for your application and let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 195 points

    Hello Ethan,

    Thank you for your response and recommendation. The BQ25756 does seem like a robust solution, but it is a bit expensive for my application. I'm looking for a simpler design with fewer components and lower power dissipation.

    This is the simple architecture :

    My opinion is to design something like the architecture below:

    Do you have any alternative suggestions that might meet these criteria? Ideally, something that can still handle the precharge requirements efficiently.

    Best regards,
    Ahmed

  • +1 in reply to Ahmed MOSRATI
    TI__Mastermind 31185 points

    Hello Ahmed,

    Thanks for the new information. The BQ25756 is the only battery charger we have that can support charging at 42V.

    For suggestions on a DC-DC buck converter with current control, can you open a new E2E thread for the DC-DC experts?

    Best Regards,
    Ethan Galloway