• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 868 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.

BQ77905: Reverse charger protection

Alen%2520Chen
Intellectual 2320 points
Part Number: BQ77905

Hi,

Does BQ77905 support reverse charger protection? if not how to protect battery pack if the reversed charger is connect?

Alen

  • 0
    TI__Guru 54715 points
    Hi Alen,
    Yes, see the bq77905 data sheet figure 22. With reverse charger the PACK- will have a high voltage. Select appropriate FETs for the voltage also.
  • 0 in reply to WM5295
    TI__Intellectual 2320 points

    Hi WM,

    Got it. thanks.

    Two more questions,

    Why does the circuit need a zener to clamp Vgs of CHGFET?  I think a diode(1N4148) is enough, didn't see any risk that VGS exceed V(FETON)  even if the voltage on Packe- goes to negative(Once the Vgs > Vth, the pack- will be back to 0V). How about your opinion? Are there is any thing I missed?

    Do we need to add a zener to protect LD pin(or the ESD cell is strong enough) if a reversed charger is plugged in? I think the voltage on LD would exceed the ABS rating(20V) in this condition.

    BTW, do you have any waveform about reverse charger protection? Will the SCD be triggered first when a reverse charger is plug-in?

    Alen 

  • 0 in reply to Alen%2520Chen
    TI__Guru 54715 points
    Hi Alen,
    If a battery using the bq77905 recovers from a temperature fault in a charger the gate voltage will come on suddenly. The instantaneous Vgs on the FET could exceed 20V, or be limited as the FET comes on. The recommendation from the FET expert I received was to protect the gate, you might check with your FET supplier to see if it is a concern for your FET in your application. The forward diode behavior of the zener protects the gate in situations where CHG is driven low and PACK- exceeds 20V.
    The LD pin has both an ESD cell and an internal clamp. See the data sheet section 7.5 electrical characteristic table "Load Removal Detection" section for details. Size the external resistor so that the clamp does not exceed the abs max current limit of 500 uA in section 7.1 You might also look at the pin equivalent diagrams application note.
    I don't know of a reverse charger protection waveform. The behavior will depend on the charger characteristic. Normally it is expected to be a short circuit or overcurrent event followed by an increase in the voltage lasting indefinitely rather than the short pulse of the inductive response. If the charger limits current to within the battery capability the battery may discharge to UV before the voltage rises.
  • 0 in reply to WM5295
    TI__Intellectual 2320 points

    Hi WM

    Got it, thanks for you valuable reply!

    Alen