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Original question:

BQ76940: Cell Balancing not working

BQ76940: Cell Balancing Mosfet N-Ch or P-Ch

Yusuf KAYA
Intellectual 630 points
Part Number: BQ76940
Other Parts Discussed in Thread: TIDA-00792, BQ78350-R1

Hi,

7.1 paragraph of SLUUAN7B technical document says that it is necessary to use P-Ch Mosfet for balancing feature. But Schematic of TIDA-00792  Reference Design shows us N-Ch Mosfet for balancing. According to the schematic I used N-Ch Mosfet and the balancing is not working.

Would you please share some information about that?

Thanks.

  • 0
    TI__Guru 54715 points

    Hi Yusuf,

    See the application note http://www.ti.com/lit/pdf/slua749 section 4 for a description of the N and P channel balancing.  Figure 18 has an additional characteristic of P-ch which is sometimes undesired. An example balance waveform for channel 1 is shown in figure 5 of the scheduler apnote http://www.ti.com/lit/pdf/slua775 .  Using these explanations you should be able to troubleshoot your circuit.  Waveforms look different depending on the type of probe used, so check the circuit with the tools you have available.

    An observation or question from users is that the voltage does not change when balancing operates. Two aspects of this are first that the part duty cycles during balancing (Figure 5) to measure the cell voltage, so the voltage is not measured on individual cells during current load.  Second balancing must run a long time to show a change in the cell voltage.  When implementing a battery with your own MCU you can choose how long to balance, with the BQ78350-R1 such as TIDA-00792 the balancing time is limited and it may take many cycles to bring an out-of-balance pack into balance.

    If this does not help to find the issue with your circuit, please describe the behavior.