STWBC86JR wireless charge efficiency

Wendy · ‎2025-05-28

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 · ‎2025-06-05

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

Wendy · ‎2025-07-17

Hi Didier,

Glad to recieved your message!

We found the root of the low effciency,but there's another issue.

The RX and TX cannot establish the communication in the coil space of 2mm we originally designed,it must inrease to 3mm/3.5mm that they can handshake successfully.At this time ,the efficiency is about 52%,After handshaked successfully ,and put it back to 2mm,it's efficiency is 58%.Or put the RX coil offset a little bit,it's efficiency is 58%,too.

Is this issue caused by the coil?The TX coil internal diameter is 10mm and external diameter is 20mm(maybe 19mm），and the RX coil diameter is 5mm/20mm(internal/external diameter),which cause the bad coupling I think.

what's your suggetion about the coil shapes and their diameter?I will appreciate it if can solve the issue of spacing of two coils.

And the TX coil are have a high-frequency sound when detects and charges,I don't know the root reason.It seems caused by TX coil when IC generate AC current.

Is it common in Qi standard?Or do you have some suggestion about this?

Best regards!

Wendy

Didier HERROUIN · ‎2025-07-23

Hello,

As you use different coils compared to our eval kit, you must ensure with your coils provider the coupling/compatibility of both coils working together. I can only recommend you keeping resonant frequencies equivalent to our wearable reference design which works at 2mm based on a 20mm Tx coil (Wurth 760308101104= 6.3µH associated with 347nF in series, so resonant freq ~ 108kHz) and a 15mm Rx coil (Wurth: 760308101219=11.8µH associated with 247nF in series, so resonant freq ~ 93kHz)

For your next requests, do not hesitate to open a case on st.com (https://www.st.com/content/st_com/en/support/support-home.html#), you will get a direct support.