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BQ51013B Wireless Charging design is not working, please check C1 and C2 capacitance values

Other Parts Discussed in Thread: BQ51013B

Our customer says the BQ51013B design is not working.  He says when this design is put on a charging pad transmitter, nothing happens.  He thinks the C1 and C2 cap values might be wrong for the coil that they are using.  They are using a TDK WR483245-15F5-G coil.

  • Gary,

    First, the answer to your direct question.

    By C1, I expect you mean the AC1 series capacitors (C22, C23 and C24) and C2 would be the AC1 to AC2 capacitors (C25 and C26).  Those values are calculated based on the free-air inductance of the coil (Ls) and the inductance measured based on the ferrite material (Ls').  The TDK datasheet shows a nominal inductance of 13uH.  What was the value the customer measured with the ferrite material (typically done with the intended transmitter)?

    Equation 6 in the datasheet will give the required values for C1 and C2.  Depending on the Ls' measurement conditions, the expected value will be in the range of 15.5uH.  This must be measured and not estimated.  At 15.5uH, the nominal C1 value would be 148nF.  Using standard values for creating C1 (68+68+10) gives 146nF.  Using that in the calculation for C2 gives 1.6nF (1.2nF+0.39nF).  The numbers used in the schematic are not expected to be far enough off for total failure of the system.  The is more to be investigated.

    Next is an overall review of the schematic.

    Where are the EN1 and EN2 pins terminated?  What is their status?

    Please refer to the schematic in the bq51013B EVM user guide.  Note that the connections for the resistors at RECT, ILIM and FOD are incorrect in the included schematic.  There should be a resistor (nominal of 20k) from RECT to FOD.  There should be a resistor between ILIM and FOD and one from FOD to ground.  The resistance from ILIM to FOD to ground is what sets the current limit.  The resistance from RECT to FOD and the resistance from FOD to ground is what sets the calibration for Foreign Object Detection (FOD).  Without these, the device will not start up.

    The TS pin is the Temperature Sense pin.  The TS function must be disabled by using a 10k resistor from TS to ground, or an NTC circuit must be added to allow temperature control of the power transfer.

    Regards,

    Dick

  • Hi Gary,
    Any resolution with this issue?
  • Hi Sorry for the late reply.  Met with customer late last week and the suggested changes worked!  Customer is up and running.  Thanks again for all the help.

    Gary C.