• Resolved

BQ51050B: How to improve range on designs using the BQ51050b

Part Number: BQ51050B

I've been using the BQ51050b RX in a design for several months now, and I'm just not getting the range that I need. To get any boards to work, I have had to take apart our transmitter so that the RX coil is actually touching the TX coil in order to get power transfer to begin, and a fraction of the boards fabricated never even get above the UVLO to start charging. I know WPC 1.1 only supports up to 7 mm, but we're not even getting close to that. If I need more range, should I move toward the WPC 1.2 technology?

  • Hi Brad
    1.) What RX coil are you using?
    2.) What is the tuning value of coil, C1 or Cs?
    3.) What TX are you using?

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    1.) Rx is Wurth 760308101214 (26 uH)
    2.) C series = 22 nF, C parallel = 100 pF
    3.) I don't know the details of the TX coil, it is an off-the-shelf Samsung pad similar to this:
    www.samsung.com/.../

    Thank you for your help,
    Brad

  • In reply to Brad Nelson99:

    Coil is OK but it is small (19mm) and will need good alignment, larger will help your Z-distance.

    Double check the C1 (Cs) value and calculation. With an L value of about 26uH you should have tuned to 100nF. See data sheet section "Series and Parallel Resonant Capacitors".

    Note that the Cd value is not a big deal for your test, this is only used with the moving coil TX.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    I tried Cs = 100nF on two different boards (while leaving Cd at 100 pF) : one that works when the coils are basically in contact, and one that never gets above UVLO. It had no significant effect on either board. We are using a coil this small because the final device needs to be as small as possible, and the coil is currently one of two major size limitations. Is it possible that the range cannot be increased when using such a small coil?

  • In reply to Brad Nelson99:

    Can you post a scope capture of voltage at RECT pin during a no-start condition.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Brad Nelson99:

    This looks like a communications issue.
    We can see that the voltage at V-RECT is high enough to start the RX, UVLO is about 2.7V.
    Also can see some of the comm pulses, but TX turns off indicating it did not decode them.

    Try increasing comm 1 and comm 2 capacitors to 68nF.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    Changing the comm capacitors didn't have any effect on the behavior of the circuit. It stays in the no-start condition.
  • In reply to Brad Nelson99:

    Could you send schematic of the board for review.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    This is the schematic without the changes from this thread. The current board I am testing has the following changes:

    C10 = 100 nF

    C3, C4 = 68 nF