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BQ51050B: Wireless charging heating

Connor Landgraf
Prodigy 20 points
Part Number: BQ51050B


I'm using the BQ51050B module along with this coil and an OTS Qi charger: 

I've done my best to tune the impedance using the C1 and C2 capacitors. However, I'm still getting lots and lots of heating (like 47C)

Any tips to solving significant heating issues with the BQ51050B?

  • 0
    TI__Guru* 76756 points
    What is:
    1.) The coil tuning for C1 and C2 that you are using?
    2.) What is operating freq of the TX?
    3.) Is the 47C on the coil or case?

    Some things to look at:
    1.) Metal is located near the coil in the AC field could cause heating.
    2.) Large coil to coil distance (5mm) will increase AC field size and heating of adjacent metal / battery.
    3.) C1 (series resonant cap) typically X7R will heat up. Lower loss COG/NPO can be used to reduce heating.
  • 0 in reply to Bill Johns
    Prodigy 20 points
    Thanks for the quick response.
    1) The coil has a inductance of ~11uH. The C1 and C2 are calculated accordingly. www.vishay.com/.../iwas3827.pdf
    2) The operating freq of the TX is between 105 and 205 kHz.
    3) 47C is on the case beneath the coil

    1) There's a small metal part about 10 mm from the coil, however even when that is removed from the case, heating persists.
    2) We've removed the case and had the coils directly on top of each other and heating was still very significant
    3) What is typical heating for C1? 50C?
  • 0 in reply to Connor Landgraf
    TI__Guru* 76756 points
    Hello
    For tuning the RX coil C1 (Cs) value coil inductance should be measured in the case on the TX, Ls'. Value for inductance in the data sheet, 10.7uH is for free space and value will change due to metal around the coil. See section 9.2.1.2.1.
    Note the C2 (Cd) value is used for moving coil detection, will not have impact on this thermal issue.

    The operating freq range is between 110kHz and 205kHz, lower the freq the higher the output power. For a very high load or unit that has tuning issues the freq will be closer to 110kHz. During this time the TX coil current is high and magnetic field is large, source of heating can be the TX coil or AC field heating metal. If the freq at full load it 140kHz to 150kHz tuning is probably OK.
    The efficiency is another indicator of possible issues, how does this look?