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BQ51050B Wireless Charge Problem

Tony Wong12
Prodigy 20 points
Other Parts Discussed in Thread: BQ500211A, BQ51050B, BQ500212A, BQ51013B

Hi, Ti,

 

We are manufacturer of wireless charger and receiver. We are using BQ500211A for the charger transmitter and the BQ51050B for the charging receiver. We have question as below.

 

When VBAT fully charged to 4.18V, the receiver turned off the transmitter, and when VBAT dropped to around 4.00V~4.15V, the receiver turned on the transmitter and charging the battery again.

 

Our buyer wants to keep the transmitter continue on to transmit the power even the VBAT fully charged. Please advise how to modify the receiver circuit not to turn off the transmitter?

 

Herewith our BQ51050B receiver circuit diagram for your reference.

 6761.RX Schematics.pdf

We are looking forward to receive your answer soon. Thank you!

 

Mr. Tony Wong

Sunlux Industrial Co., Ltd.

  • TI__Genius 14890 points

    Tony,

    The bq51050B sends a charge termination message to the bq500212A when the termination threshold is met.  The bq500212A then terminates power transfer.  The bq500212A continues to "ping" the transmitter to see if the bq51050B is still there (and the battery voltage is still higher than the re-charge threshold).  If the bq51050B is still there, it checks the battery voltage and sends a charge termination message again.  There is not a way to prevent this from happening.  As you note, once the recharge threshold is met, the bq51050B turns back on at the next ping and charges until termination again.

    In general, it is not good for a battery to be charged once full.  

    What is the reason for wanting the power to remain on?  How long does the system take to discharge to the recharge threshold?

    If the device is still active and the user wants to power from the wireless power source instead of the battery, there is a way to do that. Instead of using the bq51050B wireless charger, you could use a bq51013B wireless power supply plus a stand-alone charger.  When the charge is complete, the charger turns off, but the bq51013B will remain active.  The system could then be powered from the bq51013B instead of the battery.

    SCHEMATIC

    Can you describe the purpose of R17 in your schematic?

    The Cs capacitors (in line with AC1, C14, C16 and C18) are the same as the standard EVM.  If you have selected a different coil, you will need to calculate the correct value for Cs as described in the datasheet.

    FOD does not show a resistive connection to RECT.  This is required for proper Foreign Object Detection (FOD) operation.

    I would suggest that D4 and R12 be tied to RECT instead of BAT.  Otherwise, any leakage on the /CHG pin would be a drain on the battery.

    TS/CTRL shows R15 an R16.  To disable the TS function, only R15 is needed (10k resistor to ground).

    Based on the ILIM resistance  your charge current is about 300mA.  

    The schematic does not show a resistor from TERM to ground.  What do you have there?

    Regards,

    Dick

  • in reply to Dick Stacey
    TI__Genius 14890 points

    Tony,

    I did some more checking on this.  After discussions with the designer and bench verification, the TERM pin holds the key.

    The TERM pin has a comparator that checks the ILIM voltage in relation to the TERM voltage.  When IOUT drops, the ILIM voltage follows.  As the ILIM voltage drops, the TERM comparator will trip when it reaches the termination current.

    If the TERM pin is floated, the comparator is disabled.  At that point, there will be no termination.

    If the TERM pin is grounded, the effect is quite similar.  The ILIM pin can not go below 0V.  However, due to comparator offset and accuracy it would not necessarily be as consistent as floating.

    There is a fast-charge time-out of ~10 hours.  The bq51050B will send an EPT (Battery Failure).  The TX will stop power transfer at that time.  TI Transmitters will not restart power transfer.  That is not a WPC requirement.

    Regards,

    Dick

  • in reply to Dick Stacey
    Prodigy 20 points

    Dear Dick :

     

    Thank you for your information, we have receive the reply from our customer as below in blue.

     

    Thank you for the important information, this response explains well what we have experienced.

    Please ask Dick Stacey if he can explain how to calculates the RTERM so it will terminates when charge current reach 5 mA while the ILIM is 300 mA.

     
    We are calculate the RTERM by an equation of BQ51050B Data sheet is
    5mA / 300mA x 100% x 240ohm = 400ohm, if it is right, we will info our customer to take a evaluation.
     
    If you have any questions, please do not hesitate to contact me.
    Thank's !
    Best Regards
     
  • in reply to Tony Wong12
    TI__Genius 14890 points

    Tony,

    Glad to see the progress.

    RTERM = %ILIM * KTERM

    %ILIM = 5/300 = 1.7, KTERM = 240

    RTERM = 1.7 * 240 = 400 Ohms - your calculation is correct.

    I do have a concern with this termination though.  The bq51050B was designed with higher current charging and termination in mind (1A charge, 100mA termination).  As such, we do not have the details on the low current termination accuracy.  We are characterizing the termination at low currents and will add details to the datasheet.  The issue is not mean, but the standard deviation.  If you evaluate 5-10 units, you may get good results.  At 100-200, you may see some units that do not terminate as expected.  This could be early termination, or termination.  The bq51050B has a maximum charge time that will stop the charging.

    Regards,

    Dick