• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 65 subscribers
  • Views 355 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ77PL900 EVM charge FET

Andrea Ciuffoli
Prodigy 60 points
Other Parts Discussed in Thread: BQ77PL900

Hi,

I am performing some test on the BQ77PL900 EVM. Most of the functionality are working fine but when I connect the charger to the EVM I notice that the charge FET is always off, blocking the charge current, even if the CHG (BIT7 of the STATUS register) is set to 1, indicating that the FET should be ON.

If I bypass the charge FET by shorting DRAIN to SOURCE, then the EVM works fine, charging the battery pack, detecting OV and entering in balancing mode, but since the FET is bypassed I need to manually disconnect the charger in order to really balance the cells.

Thanks for your help.

  • 0
    Prodigy 60 points

    I found the problem, D5 on the EVM was shorted and the Charge MOSFET could not turn on, now is working fine but without the protection diodes, difficult to replace due to the small footprint. Are those protections really needed?

    Thanks.

  • 0 in reply to Andrea Ciuffoli
    TI__Guru 54715 points

    Glad you found it.  Was the board used or modified previously or did it arrive not working?

    In quiet electrical environments the board will function OK without the diode. The bq77PL900 provides a regulated voltage for the CHG output with respect to the PACK pin. The diode becomes important in transient events.  Imagine a pack charged to OV or in host mode with the CHG output off. If the PACK pin voltage is ~40V, the CHG output is also ~40V.  Suppose a sudden heavy load drops the board PACK+ = charge FET source voltage to ~ 10V.  The PACK and CHG pins are still at ~40V due to the hold up circuit of D8 and C24. Without the FET gate capacitance and C21, the instantaneous Vgs would be 30V and could damage the FET.  Even with the time constants the maximum voltage limits of the FET might be exceeded and potentially damaged. Be sure to analyze and test your system and circuit thoroughly, this should show what protections you need to keep or if additional ones may be needed for your situation.

  • 0 in reply to WM5295
    Prodigy 60 points

    Thanks so much for your answer!

    The board arrived new from TI with the CHARGE function not working, due to the shorted diode.

    Thanks,

    Andrea