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10 mm coils for wireless receiver

Other Parts Discussed in Thread: BQ500212A, BQ51013B, BQ51050B, TIDA-00318, BQ500212AEVM-550, TIDA-00339, BQ51003, BQ51051B, BQ25100, BQ51052B

I have received a sample coil (10mm in diameter) from Wurth (760308101208). It is Evaluated and approved by Texas Instruments for wireless charging. Do you have any reference design?

  • Kjell,

    We do evaluate coils for wireless power, but we do not approve coils.  Since coils are an external component that we cannot control, and the end system is not identical to our environment, we do not approve components.  We do reference coils and other components that we use in our EVMs and reference designs.

    I have looked at very small coils.  The issue I see with small coils is the coupling factors between the TX coil and the small RX coils is not good.  To make a smaller RX coil effective, the corresponding TX would need to be reduced.  This makes the system non-Qi.  But, many solutions do not need that certification.  See www.ti.com/tool/TIDA-00334 for an example of a non-Qi transmitter solution based on the bq500212A.  

    Not the precise answer that you were looking for.  But I will say we are always looking at smaller (wearable type) coils and will release reference designs where possible.

    Regards,

    Dick

  • Hello

    Thank you for the reply.

    On Wurh page http://katalog.we-online.de/en/pbs/WE-WPCC_Receiver?sid=75a03ad4e3#vs_t1:c2 they state

    • Evaluated and approved by Texas Instruments and Integrated Device Technology

    What does this mean ?

    We need a solution with the 10 mm coil to charge a battery that is 50-100 mha at a distans about 3 mm.

    Do you think this is possible?

    /Best Regards Kjell Bystedt

  • Hello Kjell

    We are working with Wurth to clarify the evaluation and approved by Texas Instruments statement. 

    Qi Receiver coil evaluation has several parameters that will change performance and Pass / Fail:

    1. Output Power---RX design can operate over a very wide output power range, 100mA output current for some and 1A for others. 

    2. Physical design---case and mounting configuration will impact performance.

    3. Load and output voltage---Coil can be used with different RX, bq51013B 5V out, bq51050B battery charger and others.

    4. Qi or Non-Qi---If certification is required the RX must work with transmitters in the certification pool that use the standard TX.  If the transmitter will be designed to work with the RX in a non-Qi application then the design is more flexible.

     

    The Qi standard allows for a great deal of flexibility in the receiver and receiver coil design.  If the design is non-Qi you have more options.

    Note---The Qi transmitter is the opposite it has a large number of restrictions and specification.  The coil, driver configuration, input voltage and other are defined in the standard.

  • Hello

    Any news about "evaluation and approved by Texas Instruments statement"

    /Best Regards Kjell Bystedt CEO Sport and Health Sensors AB

  • Hi All,

    I know I'm jumping into this pretty late, but Kjell, the 10mm coil by Wurth should work with the 30mm transmitter circuit described by TI. Since the ratio of diameters is 0.3 and the distance only 3mm, you should be able to charge your battery with your coil.

    I don't have the luxury of the coils with me, but It would be nice if you can test it and share your results.

    I think that with this coil you can't get QI certification, but that's fine if you have a custom built dock. All the communication should still occur as expected, and your dock would be able to know if your battery is charging, charged or if a fault is generated of any sort.

    Dick, We have a question for you, we're planning on using the same coil with BQ51050, as per the guidlines given in BTIDA-00329. We have noticed that in the test results the temperature rose to 50 degrees, however it's not clear which part had the increase in temperature. Can you please clairfy this, as we are unable to find any relation to temperature and wattage curves on the datasheet.

    regards,

    apoorva

  • Hi Kjell,

    How's it going with what you are doing? I'm just using the exactly same coil as yours and trying to find a wireless receiver and charging board for it. My choice is TI's TIDA-00318 board. I'm just wondering are you using the same board? If so, where did you get it? I really want to find a place that I can easily buy it instead of asking the manufacture to do the PCB and assembly. And also do you have any other alternative choices there? Thank you very much!

  • Hello all,
    I've been doing some tests using this small coils of 10mm diameter , because I saw in an application reference design(www.ti.com/.../TIDA-00329) that it was possible to obtain a reasonable efficiency. The problem is that I cannot even achieve to hook with the transmitter(bq500212AEVM-550) as it is specified in the already mentioned reference design(TIDA-00329) using the 10mm coil. I've used a circuit that works, because I've tested it using a 30mm coil and the already mentioned Tx.

    One of the reasons why I think it doesn't work is because the mutual inductance is not so good as Kjell already mentioned, so we need to change the size of the transmitter coil in order to improve the mutual. And of course I've change the matching capacitors.

    But the question is: I'm certainly doing something wrong because on the document of TIIDA-00329 they have achieved a WPT and additionally with an improved efficiency simply using the BQ500212AEVM-550 as it is and a receiver with the famous 10mm coil. Or did they change the Tx coil aswell for a 10mm coil to improve the Tx and the mutual inductance?

    thank you guys, hope to hear about your opinions soon
  • Hello Edgar,

    I'm newbie to wireless charging, but from my understanding, the TIDA-00329 is QI compliant, so does it mean that it can be used with any QI compliant transmitter as long as the power is acceptable there? Have you ever tried any other QI compliant wireless transmitters?

  • TIDA-00339 uses either the bq51003 or the bq51050B/bq51051B.  All three of these devices are Qi-compliant.

    However, the solution is not Qi-compliant.  It is Qi-compatible which means it will work on many Qi Transmitters, but not necessarily all.  The main reason for this is that the tuning capacitors (called Cs and Cd in the documents) are not installed.  For a Qi-compliant solution, the RX must be tuned for a resonant frequency of 100kHz.  This assures the compatibility between all TX and RX systems.  In this case, the compatibility is dependent on the TX used.

    Additionally, the coil used has a great impact on compliance and compatibility.  If a solution is compatible or compliant, but uses a coil that does not work in the system, the solution will become non-compliant.

    At this time, a 10-mm coil is quite difficult to be Qi-compatible or compliant.  The magnetic field coupling is simply not strong enough to allow proper energy transfer.

    As a final note, to be Qi-certified requires extensive testing by an approved test lab.

    Regards,

    Dick

  • Hi Dick and Liqiang Du,
    I think by now, we all understand that a WPT Receiver with a 10mm coil is not compliant because it doesn't follow the standard specifications. So, even if the IC is Qi compliant the fact that the coil is not makes the whole system not to be Qi compliant.
    @Dick: I'm not sure if I understand what you just said concerning the Cs and Cd capacitors. What I understand is that in TIDA-00339, apart from that, this design is not the same reference design I mentioned( TIDA-00329), but in any case, how can the transmitter and the receptor succeed on transferring power if they are not well tuned for WPT and to communicate(they don't have Cs and Cd caps)? .
    @All: Cs caps make possible power exchange, so if you short circuit them, the system is not resonant at the same frequency, making WPT almost impossible due to the bad efficiency. Secondly, Cd caps make possible the WPT control communication, so if they are not present or the circuit is open the systems cannot communicate, so Tx IC wont allow start a wireless power transfer. Does the system work as I just described or Am I missing something?
  • Edgar,

    My reference was, indeed, to the TIDA-00329.

    Cs capacitors set the resonant frequency.  (100kHz target)

    Cd capacitors set an unrelated resonant frequency (discussed below).  (1MHz target)

    Cs and Cd do not impact communication.

    The Cs capacitors are set according to the  inductance of the coil in the end system.  It is used to set the resonant frequency.  If the Cs capacitors are not tuned properly, or they are not used, the system will have a different resonant frequency.  The idea of setting the resonant frequency the same on the RX and TX is to ensure that "higher frequency is lower power" and "lower frequency is higher power".  

    For example, use an RX system tuned for 140kHz.  The ping frequency is always 175kHz.  So, the RX will ping without issue and communicate just fine.  When more power is requested by the RX, the TX will decrease the frequency.  When the TX reaches 140kHz, the power level is still increasing.  But as this is the resonant frequency of the RX, if the TX continues to decrease the current, the RX will actually receive less power.  As such, it will request more power - which the TX tries to do by decreasing the frequency even more. It will continue until the TX reaches 110kHz - which is far below what the RX needs.

    An system without Cs capacitors will have a "self-resonant" frequency based on the coil and its surrounding components.  That will create unique resonant frequencies for unique designs.

    The Cd capacitance is slightly different in that it is tuned for 1MHz and is not part of the power transfer equation.  It is for a unique TX that is part of the WPC pool and requires the extra component.

    The Cs and Cd capacitors are NOT used for communication.  The COMM1 and COMM2 capacitors set are used for this.  Figure 26 in the bq51003 datasheet shows the deviation created in the coil current when the COMM1 and COMM2 capacitors are enabled (when their internal FETs are enabled).

    A very good reference can be found at ti.com/wirelesspower under the Technical Documents section.  An application note titled "Designing a Q-Compliant receiver coil for wireless power systems".

    Regards,

    Dick

  • Dick,

    Thanks for the answer, specially to clarify about Cd capacitor. I also read on the Qi standard that Cd is used to enable a resonant detection method. Now that we are speaking about Cd, I think I found an error on the equation (5) that you can find in the BQ51050 datasheet on page 21, because in my case when I try to calculate the C2 or Cd value I found a negative value, something like -4.81e-21, so I went back to the standard and used the formula that is in the Qi standard and found a more normal value of around 2nF.

    I propose the next correction:

    Cd = 1/((2*pi*Fd)^2*Ls - 1/Cs) , which basically is similar to the equation (5) of the datasheet but the only difference is that the therm 1/Cs is not multiplied by the factor (2*pi*Fd)^2.

    I insist once more about the reference design TIDA-00329, for which I couldn't succeed on transferring power.  How did you managed to transfer energy using a Tx coil of 4.5cm of diameter(the coil in the bq500212AEVM-550) and a reception coil of 1cm of diameter? or did you change the Tx coil as well for a 1cm diameter coil?

    Thanks in advance for your valuable feedback,

  • Edgar,

    Please verify the version of the bq51050B datasheet you are referencing.  The current datasheet was revised in January 2014 and has corrected equation 5 (as you note above).

    TIDA-00329 does not use a 10mm coil.  The coils used for evaluation were approximately 18mm and 24mm.

    WPC specifies standard coil requirements for the Transmitter, but not the receiver.  As such, there is much flexibility on the RX side.  Going down to 10mm is quite difficult with the current WPC TX coils.  Non-WPC compliant coils can be used, but require modifications to the TX circuitry.  The TIDA-00334 transmitter design uses a 30mm TX coil.  Still, 10mm is quite difficult, but with increased inductance it is possible.

    Regards,

    Dick

  • Dick,

    Thanks a lot for you help, I think with all these information I'm ready to make some tests on the Tx side. I hope that sooner rather than later I'll find something that could work and I will let you know if I succeed to recharge a device using a 10mm coil on the Rx.

    thank you all for the helpful information, Best regards.

  • Dear Dick,

    Thank you for your detailed explanation, it really helps us have more understanding for the receiver design. Can you please tell me more about the RECT pin connection? In the datasheet of BQ51050B, it only says that the capacitor value for this pin is related to the power level, is there any equation or specific information on how this relationship is defined? Because in the reference design of TIDA-00329, it is using 4.7uF and 22nF with charging current of 75mA, then in the evaluation board design it is using two 10uF and one 100nF with charging current much larger. For my design the charging current might be in the range of 50 to 100mA, so will one 4.7uF and one 22nF be good enough for that? Thank you very much for your help!

    Best Regards,

    Liqiang

  • Liqiang

    There is no formula for the RECT pin capacitors.  The points to keep in mind are that the RECT is used for smoothing the AC to DC conversion and it's also used in surge capacity during a load step.  If your current is 100mA maximum then the concern for a large current step is not an issue.  Although there are many factors that will impact the final factors, I believe the 47uF and 22nF you've chosen would work well.   Evaluation of your final solution looking a smooth signal at RX with normal charging conditions will indicate you've selected the correct values.  

    Regards,

    Dick

  • Dear Dick,

    Thanks for your reply. Other than the RECT pin capacitor value, its resistance connection to FOD pin is also not something I understand. In the EVM design, it is connecting to FOD pin with a 42.2K resistor, however in the TDIA-00329 it is connecting to FOD with a 100k resistor. Does the value of this resistor related to the capacitor values? As mentioned above, my charging current would be 50--100mA or even smaller and I was planning to use 4.7uF and 22nF capacitors, so should I stick to the TDIA-00329 design and use the 100k resistor since it is using the same capacitors? If you can point me to some documentation related to RECT, that will be really great! Thanks very much for your help!

    Regards,

    Liqiang

  • Liqiang,

    The bq51050B has a Foreign Object Detection measurement that's done in conjunction with the wireless transmitter. The resistance between the RECT pin to the FOD pin and from the FOD pin to ground form a calibration circuit. The actual values of the calibration are generated through an interactive measurement / correction methodology. Details can be found on the bq51050B web folder under the "Tools & Software" tab. From there, the "Software" link will take you to: "Foreign Object Detection (FOD) Calibration/Tuning Tool". It is a zip file that contains a user interface and a Reference Manual. Start the GUI to get the link to the Reference Manual. That manual will give the details of FOD calibration.

    The smaller capacitance values you suggest on RECT should work - lab data will confirm that.

    One thing to consider on the bq51050B is the low current charging. The accuracy of the bq51050B for low power charging is not as accurate as the two chip solution of the bq51003 + bq25100. A "typical" bq51050B will work fine at lower currents, but mass production may show some devices that do not meet your expectations. TIDA-00318 is a small board (5mm x 15mm) using those devices.

    Regards,

    Dick
  • Dear Dick,

    I've read the FOD calibration manual, thanks for pointing me to that. However, I do have some equipment limitations here so that I can't do the calibration properly. My solution is that I'll just go ahead and use the value of 200ohm for Rfod and 42.2k for Ros.

    My concern is that what will be the side effects of the system without FOD calibration? Because from my understanding, the purpose of this FOD calibration is to make sure that the receiver can estimate the received and lost power as accurate as possible to make sure if foreign objects exists, the temperature will be under controlled. So does it mean that it will not influence the charging function even without the FOD calibration? Because I don't need to get my device QI certified, as long as the charging parts works fine, even if the FOD is not working accurately, it'll be fine for my system. Looking forward for your opinions,thanks for your help!

    Regards,

    Liqiang

  • Liqiang,

    If you populate FOD resistors as you suggest (42.2k for Ros and 200 for RFOD) you will have no side-effects.  You may still have FOD faults.  What transmitter are you using?  Is this a prototype (single build) or is it in preparation for a large-scale build?

    The receiver sends its power level to the TX.  The TX compares it to the power level it sent.  If the TX is sending more than the RX is reporting, the TX monitors that amount of "excess" power.  If that level is too high, a Qi transmitter is required to terminate power transfer.  If you are operating at lower powers, the delta between sent and received may not get high enough (even with "poor" FOD tuning).  In that case, you will not have an issue.  If a foreign object does get in the field, then it could cause problems.  If your trim is not right, you might get an FOD fault too early.  But, worse, is you might not get an FOD fault when the object is heating.

    In this case, it depends on how you want your system to react.

    Regards,

    Dick

  • Dick,

    Thanks for your reply.The transmitter I'm using is some random QI universal transmitter like this one: www.adafruit.com/.../2162.
    We only build the receiver part inside our device and it is not for large-scale build. At lease for now it is only at design and test stage and is not for consumer use. We'll try to do more tests after it's finally made and to make sure chances for the worst case happening be lower. For now, we just want to make sure that the wireless charging can work fine for the device. Thanks again for the help!

    Regards,
    Liqiang
  • Dear Dick,

    I have another question here about the small charging current problem of BQ51050B. I'm just wondering that if it is really inaccurate, how bad can it be. Is there a tolerance for that? For example, if I want a charging current of 20mA, will it be as inaccurate as going up to 30mA or 40mA or even worse? Because for our application,space is really limited,it should be as small as possible. But if it's really inaccurate at that low current and there is the potential to damage the battery, I'll switch to the two chip solution for sure. Thanks for you help!

    Regards,

    Liqiang

  • Liqiang,

    There are two parameters to be aware of at these low currents.  The charge current is based on a ratio to KILIM.  That parameter is 314 at 500mA but at low currents, the standard deviation of the characterization material got very large.  The additional parameter, is the termination current (KTERM).  This parameter also has a large standard deviation at low power.  

    Combine both of these and the results may be that the current is lower than expected and the termination can occur earlier than anticipated - which indicates the energy transferred to the battery is less meaning lower run-time.  

    I would not expect a 20mA setting to be to result in charge currents higher than 40mA.

    I strongly recommend the two-chip solution (bq51003 + bq25100) at charge currents lower than 100mA.

    Regards,

    Dick

  • Hi Dick,

    Looking at the TIDA-00329 reference design, It is configured for a 75mA charging current per the note on the schematic. Looking at the data sheet (for bq51050B, bq51051B, bq51052B), the range for IBULK is from 500mA to 1500mA (on page 8 of data sheet SLUSB42E). How can it be that data sheet min is 500mA but the TIDA-00329 reference design is set for 75mA. I must be missing something.

    Does this really mean that these devices cannot be used for currents less than 500mA during fast charge mode?

    I'm just starting to look at wireless charging, surly I've miss-understood something.

    Thanks for your help!

    Mark

  • Mark,

    Thanks for the question.  The bq51050B, bq51051B and bq51052B can have charge currents set below 500mA.  The accuracy of the charge termination is the concern for lower charge currents.  The datasheet spec for KTERM shows IBULK (Charge current) at 500mA.  The standard deviation of the KTERM increases at very low currents.  For example, if KTERM sets the termination current at 10% of IBULK (50mA for 500mA IBULK), the KTERM will be within spec and termination at 50mA is consistent across the material.

    If the IBULK is set to 100mA and the same 10% termination is used, that sets the charge termination current to 10mA.  At 10mA, the KTERM standard deviation is larger (taking material and temperature into account) and the actual termination current may vary by 50% (possibly some outlier cases at 5mA or 15mA). 

    Controlling the charge current at 100mA is not considered to be a concern, but the termination may vary.  The numbers I used in the 100mA example are not specification limits.

    Let me know if this helps.

    In cases where very precise charge termination is required, we recommend the bq51003 wireless receiver be paired with the bq25100 Li-Ion battery charger.  Its termination accuracy goes down to 1mA.  This example can be seen in the TIDA-00318 reference design.

    Regards,

    Dick