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BQ51013B: BQ51013B Coil and Capacitor Specification

Peter Blank
Prodigy 20 points
Part Number: BQ51013B

Hello,

in our application we use the bq51013b for wireless power to charge a battery and drive a load, max. 300 mA in total. We designed an own PCB RX coil with specs R = 0.8 Ohm and LS = 12 uH. Based on the formulas in the datasheet, the capacitors are around 200nF and 2.1pF. The coil diameter is around 40mm with an inner hole of 15mm. We have a slightly higher distance that specified, around 8 mm.

With this setup, the connection to a standard 15W charger is not stable, but if we double either the inductance or both capacitances the connection becomes stable, either up to 10mm.

How is this behavior explained?

Do we make something wrong during computations?

Thanks four your help

  • 0
    TI__Guru 68856 points

    Hello

    The instability could be caused by low power transfer or communications failure. 

    Low Power Transfer would be the system running out of gain and not delivering the power needed to load due to the increased coil to coil distance.  Check the voltage at RECT pin to determine if it is below target value.  Check the control packets they should be at 250ms interval if you see 30ms interval this could be the problem.

    Communications failure would be the transmitter not decoding a valid packet in 1.5 sec.  This could be caused by modulation created by comm1 and comm2 capacitors.  The comm1 / 2 capacitors will shift the resonant point to modulate coil voltage.  It is harder to determine if communications is the issue, only transmitter knows if the not successful decoding comm.  But increasing comm capacitors is an option, try increasing comm to 40nF or 60nF.