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BQ51003 and BQ500212A resonance capacitance calculation issue

Srujana Patel
Prodigy 250 points
Other Parts Discussed in Thread: BQ500212AEVM-550, BQ51003EVM-764, BQ51003, TINA-TI, TIDA-00762, BQ500212A, TIDA-00318

Hello,

I am designing a receiver and a transmitter module for wireless charger using bq51003evm-764 and bq500212aevm-550 boards. I want to test the boards with different pairs of transmitter and receiver coils.

I have few doubts regarding the design and test processes. They are:

1. Are the transmitter resonant capacitors (C27,C28,C29,C30) to be changed everytime I  change the transmitter and receiver coils? To what frequency are these coils already calculated?

2. I know that I need to change the receiver side resonant capacitors (C1,C2,C3,C4,C5). But, to measure these capacitors, I need to use the test fixture mentioned on page 27 of the BQ51003 spec. document.

Here, they mentioned that I must apply 1V RMS voltage at 100 KHz frequency to measure Ls and Ls'. Where do I apply this 1V RMS voltage at 100KHz frequency? On the receiver EVM (across J1 and J2) or transmitter EVM (across J1 and J2)? And where do I measure the inductance (Ls and Ls')? Is it across TP2 and TP4 of receiver evm?

3. How to set the best-operating frequency to the setup after load-line analysis? Are these resonant capacitors just maintain and tune the module to a frequency range of 110-205KHz?

Expecting a quick reply soon.

Thanks & Regards,

Srujana Patel.

  • 0
    TI__Guru 68686 points

    Hi Srujana

    See below answer also take a look at bq51003 data sheet "Series and Parallel Resonant Capacitor Selection" 9.2.1.2.2

    1. Are the transmitter resonant capacitors (C27,C28,C29,C30) to be changed everytime I  change the transmitter and receiver coils? To what frequency are these coils already calculated?----Bill J-Tuned with RX coil and only retuned if the coil or mounting changes.  This would be the mounting of RX coil or metal around it.  Freq is 100kHz for C1 (Cs) and 1MHz for C2(Cd).

    2. I know that I need to change the receiver side resonant capacitors (C1,C2,C3,C4,C5). But, to measure these capacitors, I need to use the test fixture mentioned on page 27 of the BQ51003 spec. document.----Bill J-Yes, the coil L will change when placed on the TX and this fixture will simulate the TX.  IF you are only using ONE TX and Qi certification is not required,  then use that TX for the test. 

    Here, they mentioned that I must apply 1V RMS voltage at 100 KHz frequency to measure Ls and Ls'. Where do I apply this 1V RMS voltage at 100KHz frequency? On the receiver EVM (across J1 and J2) or transmitter EVM (across J1 and J2)? And where do I measure the inductance (Ls and Ls')? Is it across TP2 and TP4 of receiver evm?---Bill J.- the LCR meter used for measuring inductance will be set to 100kHz and 1V RMS.  The measurement is on the RX coil with no connection to circuit.

    3. How to set the best-operating frequency to the setup after load-line analysis? Are these resonant capacitors just maintain and tune the module to a frequency range of 110-205KHz?---Bill J - this is a special test fixture that will be used to operate the coil in and open loop configuration.

  • 0 in reply to Bill Johns
    Prodigy 250 points
    Hello Mr. Johns,

    I want to change both the receiver and transmitter coils to smaller ones and test the power transfer.
    So, would I need to change capacitors at receiver and transmitter? That is, equivalent calculated capacitance for C27,C28,C29,C30 in TX module and C1,C2,C3,C4,C5 in RX module?

    Please tell me if my understanding is right, I need to set 1V rms voltage and 100 kHz frequency to LCR meter and check the inductance at TP2 and Tp4 of bq51003evm with no transmitter for Ls.
    And then, I need to set 1V rms voltage and 100 kHz frequency to LCR meter and check the inductance at TP2 and Tp4 of bq51003evm with a spacer (of 3.4mm), primary coil and transmitter module under bq51003evm for Ls'. Here, TX module shall be plugged in with micro USB input.

    Am I right?

    So, is load-line analysis is just to make sure the coil is operable in a frequency range of 110-205kHz?

    And am using TINA-TI for the load-line analysis and unable to plot the curves for voltage and current. I was reffering to the article published by you 'TI-Desiging a Qi compliant receiver coil'. It woud be great if you could suggest how to get proper curves for different resistive loads (10ohms to 500kohms).

    Thank you in advance,

    Best Regards,
    Srujana Patel
  • 0 in reply to Srujana Patel
    TI__Guru 68686 points

    If your are changing both RX and TX coil a different set of rules apply, this would be a custom, non-Qi solutions.  The RX and TX would only be used together.

    For TX coil tuning the L & C would be tuned for 100kHz, (bq500212A) L of 6.3uH and C of 400nF.  Smaller TX coil with L value of 6.3uH are available, see TIDA-00762.

    For RX coil tuning yes TP2 and TP4 are the correct point but you only want the coil connected to LCR meter not EVM circuit.

    Yes for Ls it is coil in the RX case but no TX.  

    For Ls' this will be on your TX design with TX coil in the case ( plate and spacer used for Qi ).  The TX is not powered you only want the influence of the TX coil on RX coil to measure RX coil L.

    Load line is optional if you use TIDA-00762 and TIDA-00318 as refer the performance will be good.

  • 0 in reply to Bill Johns
    Prodigy 250 points

    Hello Mr. Johns,

    Thanks for the reply. I am using BQ500212A and BQ51003 ICs for this design. Because I need an output power of 2.5W from RX. 

    I have a transmitter coil (26mm OD and 7.36uH L) and a receiver coil (24mm OD and 47.5uH). So, using these coils would make the charger custom and hence I need to calculate and change the resonant capacitance on both TX and RX end. Right? If so, then what are the formulae used to calculate resonant capacitance for TX module?

    And, if I wish to make the design Qi compliant, then I need to buy 760308101103 (6.5uH) and can use it with my receiver coil (24mm OD and 47.5uH) by changing the resonant capacitors on RX end such that the module is tuned to fs = 100kHz and fd = 1MHz. Right?

    Or, I have another TX coil (29mm OD and 4.33uH), so can I use this coil with 400nF capacitance so that my TX would fall in a resonant frequency range of 100kHz to 1MHz?

    Also, please let me know if these two ICs are compatible enough to provide me 2.5W power. I am using BQ500212A EVM-550 and BQ51003EVM - 764 for my tests and evaluations.

    Thank you in advance,

    Best Regards,

    Srujana Patel.

  • 0 in reply to Srujana Patel
    TI__Guru 68686 points

    On the TX side (bq500212A) I highly recommend using 760308101103 or similar coil, I have tested them and you will get good performance.  Any TX coil must have low DCR due to high circulating current in the LC tank circuit.  Note the Qi standard requires that the TX coil be larger about 50mm, standard has several types but not 30mm range.

    Question--I have a transmitter coil (26mm OD and 7.36uH L) and a receiver coil (24mm OD and 47.5uH). So, using these coils would make the charger custom and hence I need to calculate and change the resonant capacitance on both TX and RX end. Right? ---Yes

    Question--If so, then what are the formulae used to calculate resonant capacitance for TX module?--Coil L and resonant caps form a series LC tank tuned to 100kHz.

     

    Question-- if I wish to make the design Qi compliant, then I need to buy 760308101103 (6.5uH) --No not Qi coil, but good one that will work well.

    Question--can use it with my receiver coil (24mm OD and 47.5uH) by changing the resonant capacitors on RX end such that the module is tuned to fs = 100kHz and fd = 1MHz. Right?---Yes

    Question--I have another TX coil (29mm OD and 4.33uH), so can I use this coil with 400nF capacitance so that my TX would fall in a resonant frequency range of 100kHz to 1MHz?--This L value is low do not recommend it, most good TX coils will have L near 6.3uH and low DCR--DO NOT try to use and RX coil as TX coil you will have thermal problems.

    Question--Also, please let me know if these two ICs are compatible enough to provide me 2.5W power.---Yes.

  • 0 in reply to Bill Johns
    Prodigy 250 points

    Thank you so much for the answers.