STEVAL-WBC86TX and STEVAL-WLC38RX coil change

[David101]

Hello everyone,

I got the above mentioned Eval boards with the following adaptions to suit my application:

- Tx coil connected to STEVAL-WBC86TX: WE 760308101104 

- Rx coil  connected to STEVAL-WLC38RX: Custom PCB antenna with ~1.5 µH self-inductance, ~ 9 x 9 mm^2 size

- Rx Ctank: 1.57 µF --> leads to a resonance frequency of ~102 kHz for the receiver circuit

When I place the receiver coil in 2 mm distance to the transmit coil, I can see the ping by a repeating short light-up of D7 on the STEVAL-WLC38RX board. However, there is no power transfer and STSW-WPSTUDIO connected to the transmitter indicates a signal strength timeout as EPT reason. Here is my TX configuration:

Also, here is a screenshot of the AC1 & AC2 signal on the receiver coil that I can see when the transmitter sends a ping:

I assume that the coupling between my antennas is too bad for a sucessful communication and power transfer between the boards. Is there any measure I can take on the Tx side to improve the ping so that the receiver gets enough power for answering, such as increasing the Tx coil voltage? Do you have any other suggestions for making it work?

Thanks!

Best regards,

David

[willzhou]

Hi Sir

You have change RX coil from 8uh to 1.57 µF, so your RX Ctank should change to, or you can use 9V or 12V for STWBC86.

[David101]

Hi willzhou,

thanks for your answer!

I already changed RX Ctank to 1.57 µH in order to tune the frequency to 102 kHz. Increasing the voltage of the Tx board to 9V leads to a successful link between Tx and Rx, however after some seconds of power transfer, the STWB86 reaches temperatures >100 °C thus cutting off the power. Also, the Vout of the Rx board remains at 0V. Do you have further recommendations? Do you think it is even possible to tranfer power over such a small receiver antenna?

[Didier HERROUIN · Accepted solution]

Dear customer,

It is very difficult to get a power transfer with such a small coil, several parameters should be considered:

There must be a minimum mechanical overlapping between the winding of the Rx and Tx coil. Usually, we recommend a overlapping of 80% to get a good coupling between the coils

The inductance you get (1.5µH) is too small. Indeed, you can compensate the resonant frequency by changing the capacitor value as you did, but the coupling factor (~the power transfer gain) between Tx and Rx coil is impacted by the ratio of both inductances. With such a small inductance, the gain will be very low.

A pcb antenna usually has lower performances compared to a wired coil (higher impedance), even if in your case it is maybe less sensitive if you target low currents.

You should monitor Vrect at STWLC38 side to see if it rises (first step before enabling Vout) or if it is stuck to 0V (if received voltage is not enough).

Regards,

Didier