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BQ3060 Cell Balancing through pulsing FETS?

A. Rebel
Genius 11740 points
Other Parts Discussed in Thread: BQ3060

Hey Battery Fuel Gauge Team,

A client of mine is utilizing the cell balancing feature of the BQ3060 set up with the external balancing circuit of P-channel MOSFETs, with 39 ohm 1W resistors. (~90mA of balancing per cell.)

When monitoring the Vgs of the FET, they are seeing pulses ON about 100ms, and off for about 150ms, with an amplitude of ~-2.3V. They are seeing current through the balance resistors corresponding to the ON period. (As expected) However, it is not ON long enough to balance the packs being used.

Is this pulsing normal?

If not, what's actually happening?

If it is normal, how can the ON time be increased?

Thank you,

-Rebel

  • 0
    TI__Guru 73830 points
    The pulsing is expected and the duty cycle looks correct and the duty cycle is fixed at about 44%. You will need to adjust the load resistor to increase balancing. The gate should be at the source voltage when the FET is off and about 2.5V below the gate when it is on.

    Have you seen the Cell balancing app brief on the bq3060 website (SLUA509)? It covers what is expected and how to adjust the parameters.
  • 0 in reply to ThomasCosby
    Prodigy 30 points

    Hi Tomas,

    Thanks for the info and for confirming that the PWM behavior we are seeing is correct.  We have also reviewed the app brief, beefed up the resistors and replaced the MOSFET with a device that is rated for 1.8V gate drive operation (SSM3J332R) .  We have these follow on questions:

    1. Can you please confirm that the expected on voltage to be ~ 2.5V below the source voltage

    2. Is there a way for the bq3060EVSW software to show or log the balancing action per cell as opposed to the pack?

    3. What settings would you recommend for a 4S1P 12V 3.0AHr LiFePO4 battery with a 0.1A charger and 22 ohm balancing resistors? 


    Thanks,

    Gilbert

  • 0 in reply to Gilbert Amine
    Prodigy 30 points
    Also, what is the maximum VGS value that the FET will see?
    Thanks
  • 0 in reply to Gilbert Amine
    TI__Guru 73830 points
    The maximum VGS will depend on the cell voltages and the series resistors used in the cell balancing circuit. The 1k resistors form voltage dividers with the internal resistors shown on page 7 of the datasheet. The Vgs should be less than 3V when the balancing FETs are active.