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BQ51050B: Device appears to communicate with Tx but won't start charging

Juan Pablo DT
Prodigy 130 points
Part Number: BQ51050B
Other Parts Discussed in Thread: BQ500210

Hello,

We designed a wireless charger (Rx) based on the reference design for the bq51050b as per schematic shown below

with coil TDK WR-483250-15M2-G , there is a fuel gauge in between the battery and the charger (bq27421).

On the Tx side we use a Logiix (QI)10W Fast charger as well as a UNITX-8610 (with no certfications) based on the bq500210. (We also have a POWERQI T-100 (5V, 1.5A) charger which we test occasionally with but also isn't communicating).

Testing using the setup as shown in the schematic was not successful, so we proceeded to modify the board to match the values in the reference design fully. C20 was updated to 68 nF, C23 to 1800 pF and R10 to 42.2k. This yielded better results as shown below but still failed to charge the battery, COMM caps were also increased from 22 nF to 47 nF with no noticeable differences (higher voltage peaks)

Pictured below is the result of placing the coil on the UNITX charger with no battery connected;CH1 is the RECT voltage and CH2 is the voltage on BATT. Some communication appears to succeed but the battery voltage fails to stabilize. charging light blinks whenever communication appears to succeed.

Connecting a battery in the system (and removing/replacingt the coil) shows the following behaviour; CH1 is the RECT voltage and CH2 is the voltage on BATT. Using a discharged battery yields very similar results.

Using the Logiix fast charger does not seem to improve the issue as shown below, voltage is higher but behaviour is the same as with the other Tx.

Using a different coil 1461791001 we get different results, communication is attempted, fails and a fault is generated (Tx stops trying). Trying to match Cs and Cd does not seem to improve the communication either. Also attempted using the coil recommended in one of your designs http://katalog.we-online.de/pbs/datasheet/760308103206.pdf which somewhat improves the communication when no battery is present (it continuously communicate) but the battery voltage does not settle and when a battery is present the same behaviour is there; also once you put any distance between coils, it completely fails.

We've removed R12 and swapped R13 with a 10k with (again) no improvements. This procedure has been attempted in 4 different boards so at this point we are running out of debugging ideas and we are hoping to get some guidance as to what to attempt next. We did notice a capacitor is missing on FOD but again the addition of it on a board does not appear to have an effect on successfully charging the battery.

For batteries we have attempted using protected Li-ion batteries from EBL, Canwelum and Orbtronic as well as an un-protected Ultrafire battery.

Thanks in advance,

Juan Pablo Diaz

  • 0
    Prodigy 130 points

    Any guidance/help on this would be greatly appreciated. We've gone through multiple posts on this issue and attempted the recommendations but are still unsuccessful.

    I should add, we've also removed the /CHG resistor (from LED) with the same behaviour observed.

    Thanks,

    Juan Pablo

  • 0 in reply to Juan Pablo DT
    TI__Guru 70796 points

    Hello Juan
    With no battery connected the unit will send end power transfer (EPT) to TX and it will turn off. We see on the RECT voltage a packet then power transfer stops, this is probably the EPT. We also see the battery voltage reaching 4.2V.

    The middle scope capture with battery shows a possible communications problems. We see the RX reply to TX ping but the ping is not extended.

    The connection for Temp Sensor (NTC) at TS does not look right. The 20k pull up resistor is internal to the device and TS would go directly to NTC & resistor network.

    Some things to check:
    1.) Double check the coil tuning, measure RX coil in case and on the TX (Ls') and use that value for tuning.
    2.) Comm capacitors can be increased to 68nF
    3.) Check coil to coil distance should be in the 3mm to 5mm range.
    4.) Rework the TS pin resistor network.

    Update from customer:

    The TS pin resistor network appears to be the culprit, we had attempted to disable it by removing the thermistor and swapping the pull-down resistor to 10k without success but removing the 20k pull-up fixed the issue. The root of the problem was a misunderstanding on our part of the schematic on the spreadsheet provided.