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BQ2954 based Li-Ion charger continues to deliver current after battery is fully charged

BriaN-D
Prodigy 30 points
Other Parts Discussed in Thread: BQ2954

A few of a batch of chargers, based on the BQ2954, and using a circuit very similar to the one in the data sheet, are

intermittently failing to terminate the charge on a 2-cell Li-ion battery.

End of charging for this circuit (which works great most of the time) is indicated by the red LED going off and the green

LED coming on. The LED normally switches from red to green when the charging current level has dropped to Imax/15=Ifull. Current continues flowing until the level drops to Imax/20=Imin at which point the current to the battery actually terminates. The LED still remains green until the battery is removed from the charger, at which point the LED goes off.
The battery is considered 'charged' anytime after the LED turns green.

The problem I have is that some of the chargers occasionally do not switch to green and continue to stay red, delivering current to the battery even though the current level has fallen below the normal Ifull and Imin current levels. 

At this point, the battery can be actually disconnected from the charger, and the RED charging LED doesn't go off! The

output voltage of the charger is still ~8.4 to 8.5 Volts, even though no battery is present (Under normal operation, output voltage of this 2-cell charger would rise to about 15.4V when battery is removed).

Even though the defective chargers work perfectly some of the time, changing the BQ2954 IC usually seems to fix the problem. (This is not a good production solution as chargers are usually tested only once before shipping to customers; they may fail on the 2nd or 3rd they are used).

I have also found, that on two of the 'defective' chargers, increasing the regulator timebase capacitor at TPWM pin 9 from 470pF to about 800pF seemed to make the problem go away (This lowered the frequency from about ~190KHz to ~110KHz: the data sheet example circuit uses a 470pF for C9.
Although I have not been able to get these two units to fail yet, after testing for a number of cycles, I'm not confident enough to suppose this would take care of the production problems -- I don't want them failing in the field.

The problem almost appears as if the internal 'charge control state machine' is sometimes jumping into an illegal state when a battery is plugged into the charger. 

Any suggestions or help would be much appreciated.                     ....Brian

  • 0
    TI__Prodigy 720 points

    Try to increase Ifull or Imin, usually termination current is set to 10%, if too small, the error is increased and not accurate.

  • 0 in reply to Mao Ye35
    Prodigy 30 points

    On your suggestion, I tried changing the termination current from Imax/15 to Imax/10 on one of the chargers I had that were failing, but it performed exactly the same: it never switches to the green LED and the current to the battery never terminates. --Thanks for responding...

    On the other hand, I tried an experiment and took a 'good' charger (our orignal evaluation unit) that had never failed and changed the regulator timebase capacitor at TPWM pin 9 from 470pF to ~390pF and it exhibited exactly the same failure as the two units I had been evaluating: never terminating battery current AND red LED staying on when the battery is pulled from the charger.

    ...Brian

  • 0 in reply to BriaN-D
    TI__Prodigy 720 points

    From the result you have, it seems IC is on frequency limit edge.

    Please check cap variation range, if cap is too small, IC can not work properly.

    From equation 7, max frequncy is 200kHz, this gives min C=500pF.

    For a 470pF, low end value may beyond IC range.

    You have to increase C to reduce frequency.

  • 0 in reply to Mao Ye35
    Prodigy 30 points

    You're right! From equation 7, it looks like we shouldn't be using 470pF at all (so the example circuit using a 470pF and f=200KHz on data sheet page 5 is a bit misleading).

    The good 'evaluation' unit (160KHz) that I modified by reducing the cap value to produce the problem, actually measured 202KHz at the MOD pin after modification (i.e. above the MAX spec)..
     
    This charger board was originally 'designed' and supplied by our Li-Ion battery pack assembly house, so we hadn't really looked at the design until we started having problems.

    In fact, the frequency of the two problem boards (both which use a 470pF / 5% cap) is lower than 200KHz (as measured at the MOD pin): one is 185KHz and the other is 176KHz.
    The one that is 186KHz most consistently shows the problem; the other one less so. Dropping the frequency of both to ~115KHz made the problem seem to go away.

    If this is the case, it looks like increasing the capacitor to drop the frequency closer to 100KHz is the best bet for our production chargers. I'm not sure if there is a downside to the lower frequency; hopefully the output buck inductor will still be adequate (33uH).