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BQ500100: RF1, CF calculation and RO, CO calculation

Lorenz
Prodigy 45 points

Part Number: BQ500100
Other Parts Discussed in Thread: BQ501210

Hello

we have a setup with a WPC transceiver with enabled FOD, pretty similar to the BQ501210 dev kit. Now we have a WPC receiver from Rohm. Without the FOD enabled we can transfer up to 10 W without any problem. But with the enabled FOD we are not able to transfer more than 5-6W after this we receive a FOD warning. We have the feeling that the Current measurement has too much noise / ripple on the I_SENSE line. What are good values for the RF and CF? In the datasheet there is 1 Ohm / 0.1 uF, in the dev kit it is 10 Ohm / 1 uF, what are the differences between these two possibilities? What are the advantages / disadvantages?

In the datasheet it is stated that when the output ripple is > 30ms the RO / CO values shall be changed. In which range is it meaningful to modify this? What are the advantages / disadvantage when we change the RO value from 1 kOhm to for example 10 kOhm?

Thank you ver much for your support

Best Regards

Lorenz

  • 0
    TI__Genius 17100 points
    Hi Lorenz!

    I am looking into this and will respond soon.


    Respectfully,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Genius 17100 points
    Lorenz,

    The 1Ω / 0.1uF combination in the datasheet offers minimal effect on the input signal, therefore higher accuracy as opposed to the 10Ω / 1uF combination in the dev kit. Advantage of the 10Ω / 1uF pair is a lower bandwidth, which means frequencies > 16kHz will be filtered out at the measurement point.

    For the output filter, an increase in resistance to 10kΩ will decrease the voltage drop across the capacitor and reduce the bandwidth – which might help attenuate output ripples.

    Sometimes high capacitive loads can cause devices to enter unstable states; if your I_SENSE line is connected directly to an output filter you may try reducing the capacitance to 1nF to test this. A quick method to check whether there is ripple on the I_SENSE line is to probe with an oscilloscope… can you provide a screen capture/photo/measurement of this?


    I will await your response,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    Prodigy 45 points

    Manuel

    I measured the I_SENSE voltage ripple with an osciloscope and an output filter of 1k96 / 1nF. The first 2 picture is the whole signal and the 2nd last picture is only the AC content of the signal. I connected on the WPC receiver side a load of 10V / 0.2A, if I connect the same load but with a different output filter 1k96 / 100nF I have a much lower ripple (see last picture). The I_SENSE line is after the output filter directly connected to the BQ501210.

    What should I try next? What is better, to change the output filter parameters or change the input filter parameter or both or is it possible to reduce the noise on the power path? On all measurements the input filter is the same with 2x10 Ohm / 1 uF and a Rsense of 20 mOhm.

    Thank you very much for your support

    Lorenz

  • 0 in reply to Lorenz
    TI__Genius 17100 points

    Hi Lorenz,

    Thank you for the oscilloscope screenshots and information. The output filter which reduces ripple most can be used for now, but switching noise is expected on the I_SENSE line. The issue here could stem from the BQ501210.

    This transmitter provides power based on inputs and the receiver communicates how much of this power is being received.  If the discrepancy between transmitted and received power is too great, FOD will alert and halt power transfer. Which receiver is being used here?

    The FOD resistor (on pin 23) is 100kΩ on the EVM, but can be changed to adjust the FOD threshold according to the table below. The issue being seen could arise simply because the FOD threshold is too low; to test this please try using the system with a 1% 178kΩ or 205kΩ resistor on the FOD pin.


    Also ensure that the coil used matches the coil in the TI design. Let me know if this is helpful!


    Best regards,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 45 points
    Hi Manuel,

    thank you for the answer. Today I found out that this ripple is ok for the FOD, but the WPC receiver need to be set up with more data. We are using a Rohm BD57015GWL receiver and want to transfer up to 10W. Now with our device this seems to be possible. But thank you very much for your support and ideas. We are keeping the 1 kOhm / 100 nF output filter and the 2x 10 Ohm / 1 uF input filter.

    Best Regards
    Lorenz
  • 0 in reply to Lorenz
    TI__Genius 17100 points
    No problem, Lorenz! We are happy to help.


    Success on your projects,
    Manuel Chavez