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BQ51051B: Charge current reduced during fast charge phase

Andrea Lombardo
Prodigy 185 points
Part Number: BQ51051B

We have a wearable application using your BQ51051B. The version is the correct one as we have a 4.35V battery.

We experience a random problem that afftects more than 50% of the units. During the charge process, presumibly in the costant current phase, we often see a sudden drop of the current powering the recharging pod.

Starting from that moment, monitoring the battery voltage, we see a significant reduction of the voltage slope.

The chip will go on signalling CHARGE IN PROGRESS (CHG pin).

The transmitter (a off the shelves, Qi certified transmitter, based on a ST chip ) will go on signaling CHARGE IN PROGRESS, but the battery will never reach the voltage needed to switch from constant current to the constant voltage phase. The charge never ends until the 10 hours time out triggers.

At first we thought of a temperature problem, but I tend to exclude it as we are monitoring the temperature carefully and it always remains well inside the safety margins.

Another thing that let me think that it is not a temperature problem is that if you simply remove the device from the pod and put it back again after just few seconds, the charging procedure will immediately restart with the usual charging current.

After this "off the pod - back on the pod" procedure, it is likely that the charge will correctly ends. Other times, another stop could occur and you need to repeat the procedure.

What could be the reason for this behaviour? Any clue? Any area we should investigate?

What is really annoying is that the first 10 prototypes were (and still are)  just perfect. Now a significant amount of units of the 100 units of pre - production shows this behaviour.

Theoretically no change of any kind. Just a slight decrease of coil distance (less than 1 mm) that we thought could have been an improvement.

Unfortunately, because of mechanical restriction, is very difficult to test new PCBs with old enclosure.

Thanks

Andrea

  • 0
    TI__Guru* 75656 points

    Hello Andrea

    Could you send the schematic for review?

    Any info on the coil would help.

  • 0 in reply to Bill Johns
    Prodigy 185 points

    Hello Bill,

    the coil is Wurth code: 760308105214

    To improve efficiency of the coil and reduce the effect of the battery presence, we also have: 354002 WURTH EMI Shielding

    Thanks to this shielding (and reducing the thickness of the plastic enclosure of 0.5mm we solved all temperature problems we had at the beginning.

    The schematic and the PCB are attached.

    Let me know if you need anything else.

    Thanks

    Andrea

  • 0 in reply to Bill Johns
    Prodigy 185 points

    Hi Bill,

    did you get any chance to look at our schematic? We have tested everything here and we are really running out of options. What struggles us is that some units (unfortunately a minority of the total) work just perfect. 

    We are investigating in two directions now:

    1) Is the battery protection circuti part of the problem? We are running some tests excluding it.

    2) Is the distance between the two coild part of the problem? We are running some tests removing the palstic enclosure and reducing to "zero"  the distance betwwen the TX and RX coil.

    We would really appreciate any input from you

    Thanks

    Kind regards

    Andrea

  • 0 in reply to Andrea Lombardo
    TI__Guru* 75656 points

    Hi Andrea

    I have several things we can look at:

    1.) Is the battery protection circuti part of the problem?--Do not think it is the battery protector.

    2.)  Is the distance between the two coild part of the problem?--The distance will change the coupling between coils.  If the system is on the edge getting coil closer will help. 

    3.) 760308105214--The coil L is low 6.3uH is very low, size is small 20mm both will make the gain low.  May have trouble starting.  This could lead to a  number of problems.

    4.) Schematic -- C11, C12 CLMP1 / 2 ---The two caps should be the same value 470nF

    5.) Schematic -- C3, C4 COMM1 / 2 -- The standard value is 22nF, higher value of 47nF may work but need to be evaluated in your system.

    6.) Schematic -- C16, C14, C15 -- Cs (resonate caps), what Ls' value was used for the tuning?

    More info later 

  • 0 in reply to Bill Johns
    TI__Guru* 75656 points

    Hi  Andrea

    Do you have a TI EVM you can use for testing?

  • 0 in reply to Bill Johns
    Prodigy 185 points

    Hi Bill,

    with our instrument the induction is 6.8uH in air and basically the same when everything is close.

    We are looking into other items you advised.

    Thanks

    Andrea

  • 0 in reply to Andrea Lombardo
    Prodigy 185 points

    Hi Bill,

    regarding C11 this was a schematic error. Actually all the products we are testing have the 470nF in C18 position. So, this is not the cause of the issue. Sorry for sending you a wrong documentation.

    On the contrary, is confirmed that we C3 and C4 are 47. We are trying to reduce C3 and C4 to 22nF.

    In the meantime, do you have any comment on L and L1 values (basically the same)? Would you reccomend different values of Cs and Cp capacitors?

    Thanks

    Andrea

  • 0 in reply to Andrea Lombardo
    Prodigy 185 points

    HI Bill, 

    unfortunately also the devices with the 22nF stopped during charging. 

    By the way, we got the 47nF value form bq51051 data sheet at page 23. 

    But I have realized that in the test schematic the value is reduced to 22nF. However, this doesn't look to be the only problem.

    Looking forward to receiving your thoughts.

    Thanks

    Andrea

  • 0 in reply to Andrea Lombardo
    TI__Guru* 75656 points

    Hi Andrea

    Working with you off line on this question and will close the post for now, can re-open later if needed.