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WPC Charger Range Issue

Zac Blume
Prodigy 240 points
Other Parts Discussed in Thread: BQ500212, BQ51050B, BQ51013B

Hello,

I've created a design using a BQ500212 (transmitter) and BQ51050B (receiver) to charge a Li-Po battery. I'm very limited on space, so I've gone with a 12mm coil for the Rx (Abracon AWCCA-12R12H11-C01-B), and I've tested multiple Wurth Tx coils (760308101103/4/5), as well as a TDK WT151512-22F2-ID Tx coil. I have ordered the matching TDK WR121210-27M8-ID Rx coil to do more testing, but it has very similar parameters to the Abracon Rx coil.

My problem is that my charging distance is no greater than 2.5mm. Any greater than this and charging won't initialize. I can get it to start charging and then move the coils to 3-3.5mm and charging will continue, but I need it to be able to start charging at 5mm. I've looked at my capacitor selections per WPC and the datasheets, and everything seems to equate correctly. Schematics below. Any help is greatly appreciated.

  • 0 in reply to Zac Blume
    TI__Genius 14890 points

    Zac,

    As you've seen, moving to smaller coils does create a significant impact on the charging distance.  When you were able to get charge activity, what current were you able to achieve.  Smaller coils can also limit the power transfer.

    The 2.5mm "starting" distance indicates that at larger distances, there is not enough coupling between the systems to achieve the UVLO required on the RX.  This needs to be achieved during the ping operation before power transfer can be initiated.  Once initiation occurs, the bq51050B needs to get a high enough RECT voltage to enable the OUT pin to initiate charging.  Looking at the RECT pin can help determine exactly what is happening.  Section 8.3.2 in the datasheet goes into some detail on this.  A good plot of VRECT and VOUT can be seen in the bq51013B datasheet with Figure 17.

    As debug of your overall system, start with standard TX and RX coils.  Once that is up and getting the behavior expected, then moving to smaller coils may assist with debug.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 240 points

    Hi Dick,


    Thanks for the reply. I have actually been working with a very helpful TI employee on this issue, and it seems I was using a Tx coil with double the necessary inductance. Since this is a half-bridge design, it requires a Tx coil of ~3.3µH. Removing some windings from my Tx coil has yielded excellent results.