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BQ51050B FOD topic

Other Parts Discussed in Thread: BQ51050B, TIDA-00669, BQ51050BEVM-764, BQ51013B, BQ50002, BQ500511

Hi,

I am doing a wireless battery charging design using BQ51050B.

I am referring to TIDA-00669 (500mA Wireless Charger BoosterPack with Gauge Reference Design) as the application circuit.

Because I don't want to do the FOD tuning (need extra equipment), can I just use the recommended resistor value for FOD (R15=200, R24=20k), and just change the R14 for the current limiting value?

But from bq51050BEVM-764, the resistor between FOD and RECT becomes 42.2k instead of 20k.

Which one should I follow, if I don't do the FOD tuning?

When I use an off-the-shelf Qi V1.1 TX product for charging (cover to cover contact), does the performance degrade without FOD tuning of my BQ51050B board?

Thank you.

Regards

Rongfeng

  • Rongfeng,

    There is no simple answer to your question.

    FOD is used by the RX and TX to determine if any foreign objects are present in the power transfer area (within the magnetic field generated by the TX).

    The RX calculates its power consumption and sends the information to the TX.  This power consumption is based on the power it uses and a factor created during calibration.  The calibration allows the system to compensate for any friendly metals in the RX system. This could be any part of the electronics in the RX (possibly a camera, screws, battery or any other object that may impact the magnetic field).

    The TX measures its power delivery and compares it to the power consumption reported by the RX.  If the TX is sending more than the RX (by a defined amount) then the TX determines there is a foreign object in the field that is absorbing the energy (and potentially heating up).

    If the RX is not trimmed properly (Ros and RFOD) then the amount of power it is consuming may not be accurate. If the RX reports less power than it is consuming, the TX may give a false FOD Error.  If the RX reports more than it is consuming, then the TX may not recognize when an object is in the field which could result in heating of the object.

    You can use either value and see the results that you get across the full load current.  Monitor the results for for FOD, including adding objects to create a true FOD fault.  This is not a recommended procedure as it will not cover all areas and would not be expected to pass Qi-certification or the testing on other transmitters.

    Regards, 

    Dick

  • Hi Dick,

    Thank you for your reply.
    During the TX and RX side communication, RX will inform TX the charging current that RX want, is it?
    And this RX charging current is set by the R(ILim) resistor, so RX will base on the resistor setting to inform TX how much current and power RX needs, am I right?
    That means no matter the current limit setting is, if the FOD tuning is not good, then the actual charging current will be lower than the target charging current, is it?
    If so, that means the current limit setting can be a little bit higher than the target value, to compensate the power loss to the foreign object.
    Does this way suitable? (if I will do the actual trial-and-error testing for the actual scenario)

    Regards
    Rongfeng
  • Rongfeng,

    The RX informs the TX it need more power, less power or power is stable.

    The RX measures the RECT voltage and current level to determine if it needs more power.  See figure 4 in the bq51013B datasheet for the RECT response to power level.  Increasing and decreasing the RECT voltage is done based on power level required.

    The RECT voltage is not impacted by the FOD setting.  The algorithm of the bq51013B for reporting power uses the RECT voltage, the current at RECT and the FOD settings to determine what power level to pass back to the TX.

    Please look at the Tools & Software tab on the bq51013B web folder.  The Foreign Object Detection tool has a User Guide associated with it that will help with a better understanding of FOD.  

    Regards,

    Dick

  • Hi Dick,

    Thanks for your reply before. I have 1 more question.
    If we use a magnet to fix the top and bottom cover, which magnet type,size or strength is recommend to be used in order not to affect the wireless charging circuit? If we put magnet at the edge area or the PCB (circuit and coil at center area), is it ok?

    Regards
    Rongfeng
  • Rongfeng,

    Since wireless power works with a magnetic field, adding another magnet to the area can cause interference.  You will have to investigate your system to determine the best place for a magnet.  Shielding the wireless power coil area (both TX and RX) is helpful.

    Regards,

    Dick

  • Hi Dick,

    Thank you for your explanation.

    For the pin AC1 and AC2, is there any polarity?

    Since from the reference circuit, there are 2x68n and 1x47n caps at the AC1 path but not AC2 path.

    But I can not find the pin 1 indication of the coil header J5 (I think it is just a normal header), even though J5 land pattern has pin 1 indication.

    And also for the inductor coil (like the reference part from Wurth), there is no polarity indication.

    So I am confusing about this.

    Regards

    Rongfeng

  • Rongfeng,

    There is no polarity for the coil.  The connection to AC1 sets the resonant frequency so the Cs capacitors must be placed on the IC AC1 line.  It is the IC that sets the polarity for this.

    You can connect either coil connection to AC1 or AC2.

    Regards,

    Dick

  • Hi,

    I have another question about the wireless charging.
    I want to design a product with both Charging TX and RX function. The Charging TX function will be used to charge another product with RX charging capability. The Charging RX function will be used to being charged by another commercial Qi charger.
    So in short, I want to design a wireless power bank with both Qi TX and RX function.

    I plan to use a thicker Li battery (e.g. 9mm thickness), and stick TX coil at one side of the battery and RX coil at the other side to minimise the interference between 2 coils.
    The charging RX chip is bq51050b, while charging TX chips are bq50002+bq500511.

    Do you think it is feasible?

    Regards
    Rongfeng
  • Rongfeng,

    Yes, I believe your concept is feasible.  A couple of items you should pay special attention to.

    The TX circuit needs 5V for operation.  You will need to boost the battery to get enough voltage.

    The TX, at times, can have high currents so the battery needs to be able to provide that in particular at startup and at full load.

    Keeping the RX and TX separated as you propose will simplify your design and development cycle.

    Regards,

    Dick