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Rx coil tuning, Ls and Ls'

KJ Hamblin
Prodigy 85 points
Other Parts Discussed in Thread: BQ51013B, BQ500212A

What are common symptoms of a poorly tuned Rx coil?

I am using a bq51013b receiver design based on the EVM and believe that my issue is most likely an improperly tuned Rx coil but I want to confirm.  My OUT pin remains at 0 volts aside from some noise during pings and my RECT pin has a periodic wave form.   Communication is never established.  I am using a bq500212a based transmitter design the follows the EVM very closely.  A problem could lie on that end, but the pings are at 175khz and 0-5V so my confidence for the Tx side is higher.  Plots of AC1, AC2, and RECT of the bq51013b as well as the bq500212a ping are below. 

Also, what procedure does TI follow to calculate the Ls and Ls' of different coils?  

Both the bq51013b datasheet and WPC specification leave a bit to be desired in terms of specifics.  I know calculating inductance in neither difficult nor challenging, but knowing that I am following the exact procedure as those who designed the part I am using will give me more confidence in my results.

Thank you very much for your time.

AC1

AC2

REC

Tx Ping Length (no secondary present)

Tx Ping Freq (no secondary present)

  • 0
    TI__Genius 14890 points

    KJ,

    My first suggestion is to start with a known good Tx.  That would be with a TI EVM or retail purchase of a Qi Tx.  

    The "Series and Parallel Resonant Capacitor Section" gives some discussion on how to select the capacitors.  Simply put, the Ls is the inductance of the coil (with the shield in place) in free air.  The Ls' is measured with the coil on an emulated Tx system.  This is done by having the correct spacing (dZ is shown as 3.4 mm) and with the correct shield in place (50 mm x 50 mm x 1 mm thick).  In addition to the 3.4 mm will be the whatever will be between the Rx coil and the Tx system in the final solution.  Based on those measurements, the Cs and Cd values can be calculated.

    What is the nominal inductance of the Rx coil you are using?

    I have some plots attached below that I tried to mimic the plots you took that may be used as a general indication of proper performance.

    AC1 ping from the Tx (measured AC1 to ground).  No Rx is present.

    1682.TX_AC1-NoRx.TIF

    AC1 ping from the Tx (zoomed in).  No Rx is present.

    5140.TX_AC1-NoRx_Zoom.TIF

    AC1 on the Rx when the Rx is first placed on the Tx.  This shows the ping then is followed by the startup with full response from the Rx.

    5545.AC1_Startup.TIF

    RECT and OUT on proper startup.  This shows the ping followed by proper RECT followed by OUT.

    0677.RECT_OUT-2.tif

    Let me know if this helps in your debug.

    Regards,

    Dick

  • 0
    TI__Genius 14890 points

    KJ,

    I want to add one more note.  There is an application note that goes into great detail on coils "Designing a Qi-compliant receiver coil for wireless power systems".  This is an excellent discussion on how to design the coil for your system.

    http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slyt479&docCategoryId=1&familyId=64

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 85 points

    Dick,

    Thanks for the reply and the plots.  A retail Qi charger and receiver are in the mail.  

    The nominal inductance of my coil is 10uH.  I'm using the Vishay IWAS3827ECEB100J50 coil at the moment. The plots are definitely helpful and appreciated.  

    With respect to the Ls', if you are using an emulated Tx how does the 1Vrms, 100khz mentioned in the datasheet fit into the equation?  The 1Vrms called out in the datasheet and WPC spec is what was tripping me up.  Is it sufficient to measure Ls and Ls' with an LCR meter set to 100khz?

  • 0 in reply to Dick Stacey
    Prodigy 85 points

    I just say the last note about the application note, but after posting my last reply.  I do not believe I have read that app note so I will read through it.  Thanks for pointing it out to me.

  • 0 in reply to KJ Hamblin
    TI__Genius 14890 points

    KJ,

    Yes, using an LCR meter set to 1Vrms and 100kHz is the correct way to measure the coil inductance for Ls and Ls'.  My note "an emulated Tx" meant only the spacer and ferrite material called out in the discussion.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 85 points

    Dick,

    The retail Qi receiver and transmitter showed up today.  Turns out my receiver design works just fine.  So now I have to debug the transmitter.  I haven't put any time into that task yet, but have you seen any common issues that might point me in a good direction?

    Thanks again for the help and fast replies.  It is very helpful and really appreciate it.

  • 0 in reply to KJ Hamblin
    Prodigy 85 points

    Well after a short time I discovered that the 12.1K pull up to 3.3V on the COMM transmitter circuit did not make it to layout.  So after a little rework the transmitter now responds to a receiver.

    However the transmitter indicates power transfer for about 1 second then indicates fault for both my receiver design and the purchased Qi receiver.  I will continue looking into this, but wanted to post an update.

    Both receiver designs still work flawlessly with the purchased Qi transmitter.

  • 0 in reply to KJ Hamblin
    TI__Guru* 75906 points

    The 12.1k pull and 10k pull down will form a voltage divider to bias Comm+ signal at +1.5V.

    Problem described here could still be a communications fail.  The TX will power the RX during digital ping and during this time RX will send information in the form of packets.  Load modulation at a 2kHz rate and will appear as voltage and current steps on the TX coil voltage.  Comm ckt will pick off the coil voltage and divide it down for processing inside the bq500212A. 

    The TX digital ping will last about 80mS during this time the RX should reply, if it does the TX will extend the digital ping time.  Look at V-Rect test point on the RX, see bq51013B EVM users guide fig 5 Start-Up.

    Requirement 1 is that the voltage at Vrect rise above UVLO, 2.7V.  Next the RX will power up and send signal strength, ID and Configuration.  If you have a communication problem the TX will removed power before all three are sent.  Next the RX will send Control Error Packets (CEP) and the TX will change its operating point to generate about 7.5V at Vrect before output voltage is turned on.

    TX must maintain communication with the RX or TX will restart.  If a valid CEP needs to be received every 1.2 sec to continue.

  • 0 in reply to Bill Johns
    Prodigy 85 points

    Bill,

    Thanks for the reply.  

    Below is my plot of the VRECT while on the Tx before the fault condition sets in.  During the 4 second period where communication is established (I did not measure the timing for my last post) I did measure the Tx coil frequency.  It did vary from the 176kHz ping to about 109kHz before stopping.  I am assume that is due to the CEPs attempting to make the RECT voltage reach 7.5V.

    I also included a longer time scale plot of VRECT.  The fault is not consistent at this point.  It will eventually get set, but not after every failed power transfer start up.  When the fault is set is it most likely EPT-08 from the Rx since RECT is not converging to 7.5V?  

    RECT Voltage - Failed to reach 7.5V

    RECT Voltage - Longer Time Scale, No Fault

  • 0 in reply to KJ Hamblin
    TI__Guru* 75906 points

    Something is wrong with the tuning of TX.  You should have more gain at freq below 175kHz, but you have less.

    In the bq51013B UG figure 5 you can see that VRECT increases as the freq is decreased from digital ping freq of 175kHz. 

    TX is tuned to have a resonance at 100kHz, this is set by L of the TX coil and C of the resonant capacitors.  For the 5V bq500212A EVM the coil L is about 6.3uH and C is 400nF ( EVM C27, C28, C29 and C30).

    Check that area and the power section.

  • 0 in reply to Bill Johns
    Prodigy 85 points

    Bill,

    That absolutely was the problem.  I did not check the inductance of the primary coil because I thought we had purchased the same coil as the EVM, but we did not.  We actually purchased Wurth Electronic's 24uH coil which looks very similar to the 6.3uH coil.  A little rework to the capacitors and the Tx works with both my Rx and the purchased Rx.

    Everything I encountered was due to an error on our end, but I thank you and Dick for the assistance.  It was greatly appreciated.  Hopefully this thread will be useful to someone in the future.

    I do have another question, but I think it is out of scope for this thread.  I'll start a new one.