• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 62 subscribers
  • Views 1538 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ51013B capacitive load modulation

Luis Olvera41
Prodigy 160 points
Other Parts Discussed in Thread: BQ51013B, BQ500212A

Dear TI wireless power support staff,

We are designing a product that features the BQ51013B IC to charge a battery. Currently we are testing one prototype with a known good TX. Our prototype uses the Mingstar 312-00015 coil and after tuning it we find it difficult to connect to the TX station, however after finding a "sweet spot" it just connects, works fine and fully charges the battery without interruption. I was wondering if COMMx capacitors may have an influence in this issue since another prototype we tested had 22nF COMMx caps and in this prototype we switched to 47nF COMMx caps.

The first prototype with 22nF COMMx caps connects easily but will not charge properly. (This one had the coil detuned)

The tuned-coil prototype with 47nF COMMx caps will not connect easily but charges properly without interruption. This one features 57nF as series coil caps and 702pF as parallel coil caps.

Thanks for your advice,

Luis.

  • 0
    TI__Genius 14890 points

    Luis,

    Can you tell me what your Ls' measurement was and the Cs / Cd values used?

    I have used this coil, but did not specifically characterize alignment.

    The COMM capacitors certainly have an impact.  They change the modulation depth of the communication.  As the coupling increases, the modulation depth does as well.  Increasing the X, Y or Z distance decreases the coupling which then decreases the modulation depth.  Increased COMM caps compensates for that and allows for better communication with poorer coupling.  The trade-off is that it does impact the efficiency.

    Please take a look at this the TIDA-00334 Reference design (www.ti.com/tool/tida-00334).  It uses the bq500212A transmitter but with a smaller TX coil.  As you know, the ratio of the TX coil to the RX coil has a great influence on coupling.  For smaller solutions, it's sometimes wise to go with a smaller transmitter coil.

    One note on this approach is that the Qi certification requires a standard coil and the TIDA-00334 coil does not comply.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 160 points

    Hi Dick,

    I'm using the 312-00015 Ls & Ls' values from the 51013B datasheet, that is: Ls 36.5uH & Ls' 45uH, right now we are just testing the receiver coil without its enclosure.

    Regarding Cs we are using 57nF (47nF || 10 nF) and Cd 702pF (680pF || 22pF).

    Best regards,

    Luis.

  • 0 in reply to Luis Olvera41
    TI__Genius 14890 points

    Luis,

    Thanks for the information.  I used 54nF, so your value is good.

    I'm not sure what  you meant by "without its enclosure".

    The reason I asked about your Ls' measurement was that the standard measurement is done with 50mmx50mm ferrite material and the "standard" thickness spacer. Once the coil is in you end equipment, the Ls is expected to change.  So, you may want to keep that in mind when you do the final design.

    What is the distance between the surface of the transmitter and the coil?  You may consider adjusting the z-distance to see what happens to the sweet spot.  How big is the sweet spot?

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 160 points

    Thanks Dick,

    What I was referring to its that we are not testing the coil inside the end equipment yet.

    The distance between the surface of the transmitter and the coil as we are testing right now is 2mm-2.5mm. The sweet spot is a little bit left-south center from the TX coil's center. The sweet spot is about 3-4mm in diameter.

    BTW I changed COMMx capacitors to 22nF and it seems to be working fine now. I will see what happens adjusting the z-distance as you suggested.

    Best regards,

    Luis.

  • 0 in reply to Luis Olvera41
    Prodigy 160 points

    Hello Dick,

    I have a follow up question...

    In our application, the z distance between primary and secondary coils is 5-6mm, with this in mind what are the pros and cons of capacitive vs resistive load modulation?

    I ask this because I've seen that as z-distance is increased coupling is very sensitive to COMMx cap values.

    Best regards,

    Luis Olvera.