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BQ51050 output current

Hector Solar
Prodigy 150 points
Other Parts Discussed in Thread: BQ500212A

Hello there,

We have design our own coil for the BQ51050 together with the BQ500212A transmitter. As first coil design we fabricated a 14uH inductance with Q=32. The coupling factor is poor with a value of 0.3.

The wireless charging works even for an output current of 1A using a battery emulator and a distance of 5mm. However, the ping voltage is too close to the limit: any misaligment or distance increase avoided the ping so we decided to fabricate a coil with higher inductance.

In this second case, we fabricated a L's=53uH (Ls=51uH) inductance again with Q=32 and K=0.3 and modified series and paralell capacitor accordingly (Cs=47pF and Cp=400pF). What we found here is:

1. We had to increase Cp because the transmitter did not recognize the receiver for 5mm so we had to further reduce the distance.

2. The output current (keeping RLIM to 300 Ohms) did not reach to 1A but 800mA although the transmitter frequency was at 125KHz.

Do you have any tip of what is happening? Maybe is a question of the parasitic resistance of the coil and low coupling?

Best regards,

Hector. 

  • 0
    TI__Guru 65686 points
    Hello Hector
    The low coil coupling factor is probably the problem. What physical size is your coil?
    Coupling factor will go down as the ratio between TX coil and RX coil size goes down. Best coupling will be with a 1 to 1 ratio. Standard TX coil diameter is 50mm. If your TX coil is small this will present a change getting to a 1A output at a 5mm distance.
    Also a small coil will require higher inductance which means smaller wire and higher DCR.
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Hello Bill,

    The coil is 10mm diameter, 10mm length rod coil with ferrite core for both 14uH and 51uH designs. The only differency is that we used one layer winding in one case and two layer winding in the other.

    The point is that it seems to work worse with the 51uH coil in spite of higher inductance.

    Is there any reason why the output current is reduced to 800mA? In addition, why the ping voltage is still too low? Do we have a limit for the coupling factor or the DCR?

    (By the way, we used 47nF for Cs, not 47pF)

    Best regards,

    Hector.

  • 0
    Prodigy 160 points
    Hello Hector,

    Just want to know that how did you fabricate the coil, asked help from manufactures or did it by yourself? Thanks!

    Regards,
    Liqiang
  • 0 in reply to Liqiang Du
    Prodigy 150 points

    Dear Liqiang,

    We did the coild by ourselves and characterized it by means of an LCR meter.


    Best regards,

    Hector

  • 0 in reply to Hector Solar
    Prodigy 160 points

    Dear Hector,

    Thank you for your reply. Another quick question is that what kind of wires are you using to make your own coil? Just normal copper wire? What about the dismeters? I also want to make my own coil to best match our design, just don't know where to start about the coil material. I'm really appreciated for your help. Thank you very much!

    Best regards,

    Liqiang

  • 0 in reply to Liqiang Du
    Prodigy 150 points

    Dear Liqiang,

    Thank you for your answer. These are the specs of our coil:

    Geometry

    The geometry of the coil core is cylindrical, 10mm diameter.

    Wire

    The copper wire diameter is AWG 29.

    Core

    The core material is 4B1ferrite, offered by Ferroxcube. This material has a permeability of 250 while its value is kept constant up to frequencies of 10 MHz, well above the maximum Qi frequency of 205 KHz.

    Length

    The length of the ferrite is 10mm.

    Best regards,

    Hector.