NFC WLC with ST25R3916 and ST25DV-i2c

[OSilv.1]

My project is to charge and receive data from a device that has on board energy storage (70F supercap). 300mW of received power is plenty for my application. I thought NFC WLC with iso15693 would be perfect for the job.

From ST's NFC WLC webpage:

https://www.st.com/content/st_com/en/support/learning/st25-education/nfc-for-wireless-charging.html#stm32trust-overview

I see NFC can charge up to 1 watt with an output power on the ST25R3916 of 1.8W. The webinar on the website demonstrates received power of 0.5W.

However, my X-Nucleo-NFC04A1 is only receiving 0.013W on its EH pin with the X-Nucleo-NFC06A1 as the reader. This was measured via a 50 ohm resistor between V_EH and ground. The rest of the board is supplied via 3.3 input, so I assume all of the received power is dissipated in the resistor.

My first thought is that the reader isn't outputting 1.8W. I have tried configuring the RF regulator at 0x2C and TX driver at 0x28 to output max power, which should cause the rf regulator to limit current, correct? I am not seeing the i_lim flag turn on however. Is it possible to ensure that the ST25r is outputting 1.8W? And, if it already is, is the efficiency really only 0.013W/1.8W = 0.7%?

Any help is much appreciated!

[Marcel Rossignol]

Increasing the antenna inductance does not lead automatically to higher mutual coupling / coupling factor. It´s more about antenna form factor and Q.

Usually the reader antenna which I use, have an inductance around 1µH (less windings --> less DC resistance --> higher Q).

Virtual ground floating for the reader antenna is OK. I do not use the option to tie the center to ground.

Best regards,

Marcel

[OSilv.1]

This is interesting! I would assume that more magnetic field produced is always better. Perhaps this is just the case for inductive charging as opposed to resonant inductive charging.

Is it fair to say that increasing inductance while keeping Q constant will improve wireless charging?

From what you write, increasing Q linearly offers better results than increasing inductance linearly. Is this correct?

By antenna form factor, do you mean the size of the coils and their orientation and distance from each other? Or do you refer to the shape of the coil? EX: circular vs rectangular.

I will try a new coil with a larger Q. Thank you for your advice!

Best,

Oliver

[OSilv.1]

Please disregard my first two questions...they do not make much sense as Q factor is a function of inductance :P.

I do wonder if all Q factor antennas provide the same power transfer given all other conditions are constant? For example, will a higher inductance and higher resistive antenna (Q factor 100) perform the same as a lower inductance and lower resistive antenna (Q factor 100)?

Using a higher Q antenna I am seeing faster charging!

With my 4.6uH (but 20 Q) antenna, charging took about 2.5 hours.

Using a tuned NFC06A1 antenna on the receiver, charge time is now 1 hour. I believe this translates to roughly 80mW.

I am wondering if I will be able to communicate via ISO15693 with such high Q? I know that it was said that the battery (capacitor) on the RX will bring down the Q considerably. Is this true if the battery/capacitor is fully charged?

Best,

Oliver

[Schuyler]

Hi,@JL. Lebon_O​ 

I noticed that the purpose of EH is to power small devices (diodes, up to small MCUS) that consume a lot of power.Therefore, I would like to ask how to power stm32L476rg without power through EH in X-NUCLEO-nfc07A1, because L4 is a low-power MCU.

Best regards,