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BQ50002A: Improve efficiency for low power requirements

Wout Geeurickx
Prodigy 235 points
Part Number: BQ50002A
Other Parts Discussed in Thread: BQ500511A, BQ51003, , BQ25101

Hi,

We are using the bq50002A together with the bq500511A as the TX unit in a wireless charging system. On the RX side, we use a bq51003 in combination with a bq25101 to charge a small battery of 60mAh. We noticed that the battery can become quite hot, possibly destroying the battery, if we charge at 1C. We limited the charging current to 0.5C, 30mA, to decrease the power received and minimize heating of the battery. When I did some efficiency measurements, I noticed that the overall system efficiency (USB to battery) is very poor, only around 11%. I noticed in the datasheet of the bq50002A a small graph indicating that the efficiency is indeed very poor at low power.

Are there ways to improve the efficiency significantly for low power, or would you suggest another part that can perform more optimally for such low power requirements?

Thanks!

Wout

  • 0
    TI__Guru 65846 points
    Hi Wout
    Thermal rise of the battery may be due to AC field heating metal of the battery. As a test connect lead to the RX coil so that only the coil can be placed on the TX charging pad while battery and rest of circuit is outside the AC field. If battery temp is reduced then shielding the battery with magnetic material may be a solution.

    The TX will take about 1W to generate a magnetic field with no load on the RX. Most of the power is consumed inside the TX. This will result in low efficiency at low power, as output power increases the efficiency will improve.
  • 0 in reply to Bill Johns
    Prodigy 235 points
    Hi Bill,

    Thank you for your reply. Due to size constraints, we had to put the RX coil directly on the battery. We already did some tests with shielding the battery from the magnetic field as we indeed expected that there might be problems with the metal there. But we noticed that it was actually the RX coil that was heating up the most and since it is mounted on top of the battery, the heat was conducted on the battery. That is why I am mainly looking toward electrical losses (instead of magnetic losses) for the moment. We put some thermal isolation between the coil and the battery, and it is much better now. I will try to do some testing to see if we can further limit the heating up by shielding the battery more.

    I am also interested to know if there is a better suited solution for low power WPT? Are there TX that require less than 1W to generate the magnetic field with no load?

    Best regards,
    Wout
  • 0 in reply to Wout Geeurickx
    TI__Guru 65846 points
    Hi Wout
    "I am also interested to know if there is a better suited solution for low power WPT?" -- Bill J - No low power option on the standard. A lot of work is being done in higher power but not lower power and higher efficiency.