STWBC86JR wireless charge efficiency

[Wendy]

Hi,

When we were using the STWBC86JR as the wireless charging TX, we encountered a problem. The design is defined as wirelessly charging the battery and supporting the product's operation. The tested charging efficiency was relatively low, only 43%. When the device was turned off, only supplied power to the battery, and the efficiency was only 32%(the battery was set to current limit). This means that a large part of the energy is converted into heat energy. We have adopted a cooling solution, but still hope to improve the charging efficiency. Can I ask if you have any suggestions?

The currently used TX coil is 6.2uH, with a diameter of 20mm. The working frequency measured at stability is 102kHz. We found that reducing the spacing between the TX and RX coils can increase the efficiency by 50%. However, since the shell (about 2mm) cannot be thinner, reducing the spacing is not feasible. Currently, there is no other solution. Could you help us find some ideas to improve efficiency?

Thanks！

Wendy

[Didier HERROUIN]

Hello,

By using a small coil (20mm diameter), the coupling factor will be limited whatever the distance or the Rx coil is.
You can achieve around 50 to 55% for instance if you use a 15mm coil placed at a 2mm distance.
Which coil do you use at Rx side ?
Then, the efficiency will depend on the operating point: what are the voltages and currents for which you get theses values ? Usually, the maximum efficiency can be observed for higher currents.

Finally, you can also monitor if the STWBC86 is set in half-bridge or full-bridge mode, it impacts the efficiency.

Best regards,

Didier Herrouin