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!

[Ulysses HERNIOSUS]

Hi OSilv.1,

the X-NUCLEO-NFC06A1 is not able to output 1.8W with its given antenna and matching. The output power is mostly defined by the matching/antenna impedance. Please see AN5276 for more details.

BR, Ulysses

[OSilv.1]

Hi Ulysses,

The X-NUCLEO_NFC06A1 "Getting Started" data sheet claims up to 1.7W of output power on page 2. Is this correct?

Thank you,

Oliver

[OSilv.1]

If max power is not 1.7W, what is the maximum output power for the X-NUCLEO-NFC06A1?

Thank you!

[JL. Lebon]

Hello OSilv,

Please note that EH (Energy Harvesting) feature of the ST25DV is not NFC WLC (WireLess Charging).

The two things are different.

Intent of EH is to power small devices that consume very much power (diodes, up to a small MCU), not to charge a battery. The tag takes its power from the RF field, and if there is more power that needed, the rest is output on the V_EH pin. But it will not deliver more than a few mW (~10mW in best conditions).

NFC WLC on the other end is a protocol defined in the NFC standard, the specifies a communication protocol (on top of NFC) and a power transfer method. When used in WLC, the ST25DV is there to manage mainly the NFC communication protocol. The power transfer is not done through the ST25DV EH pin, but is made directly on the antenna using a bridge rectifier. This helps to "harvest" more power.

Best regards.

[OSilv.1]

Thank you JL. Lebon, that cleared things up for me. I am wondering if adding a rectifier and PMIC on the receiver antenna will change the matching circuit on the reader? Also, have you seen if the added circuitry interferes with communications? I would love to see a reference schematic or application note on the subject. If ST offered a course for designing NFC WLC (for example, the process used to design the devices that were used in the webinar) I would happily pay to enroll.

I tried adding a rectifier made of four diodes (https://www.onsemi.com/pdf/datasheet/1n4001-d.pdf) to the receiver, but saw only microwatts on the output. I assume this is due to the large forward voltage required. I've ordered a rectifier IC with low forward voltage: https://www.mouser.com/ProductDetail/750-CDBHD140L-G. Hopefully this will solve the issue.

Any guidance would be very appreciated. Thanks again for your and Ulysses' quick responses!

[OSilv.1]

Hello @JL. Lebon_O​ and @Ulysses HERNIOSUS_O​,

I've replaced two serial and two parallel capacitors on the X-nucleo-nfc06A1 to bring the matching impedance down to 3.5 - 1j ohms. I've set the driver output resistance to the lowest setting. I've placed an antenna (4.6 uH) and rectifier bridge (4X MMBD330T1G onsemi | Discrete Semiconductor Products | DigiKey) at an optimal distance above the reader. This receiver circuit is also resonant at 13.56MHz. This provides 2.4V at the rectifier output, however adding a one-ohm resistor (a place holder for a battery) between rectifier positive and negative drops the rectified voltage to nearly zero.

I've attempted other resistances such as 10, 100, and 1000 ohms, the best power supplied to them has been only 2mW (less than the 10mW I've measured from the V_EH pin of the st25dv using a reader of a higher matching impedance. I feel I am missing something fundamental. Any help would be greatly appreciated.

Thank you,

Oliver

[Marcel Rossignol · Accepted solution]

Hello Oliver,

there are several points which you should take into account for NFC WLC:

• WLC needs a reader antenna matching which take already the mutual coupling with the tag antenna into account. This maximize the power transfer.
  • A high power VNA measurement would be needed to determine the mutual coupling impedance between reader and tag antenna (10dBm VNA output power is to less to activated the bridge rectifier on the tag side to "see" the tag load!). This impedance is depending on the antenna distance, the VNA output power, the tag tuning and tag load.
  • High antenna Q-factor is very welcome on the reader side (removing the serial resistors in the matching network)
  • EMC filter cut off- frequency is on the Nucleo board around 11.7MHz. This is to low for WLC! Suggestion: cut off - frequency of around 20.3MHz (110nH/560pF)
  • Take care on the EMC coil choice! Very low DCR (high Q) and high rated current is needed (e.g.: Coilcraft 0805LS-111)
  • Reader and tag antenna resonance frequency should have NOT the same value.
    • Reader: 13.56MHz
    • Tag: 15 - 18MHz (this is also tag load depending)
  • The AAT of the reader could help to find a better matching during power transfer. This is an interesting option if no high power VNA measurement is available and the "trial and error" method is needed :)

I hope this brings you a little bit further.

Best regards,

Marcel

[OSilv.1]

Hi Marcel,

Thank you very much for your insights! This is what I need to progress my design. Unfortunately, I do not have an expensive VNA, just a NanoVNA.

I have one more question:

To do both wireless charging and communications (iso15693) at the same time, the quality factor should not be too high. I have read to keep the Q factor below 35 for this reason. Is 35 reasonable, or could I go higher?

Thank you again!

Best,

Oliver

[Marcel Rossignol]

Hi Oliver,

well, the NanoVNA is better than nothing :)

But you should keep in mind that your impedance measurement do not reflect the true picture!

However it is important to measure the antenna parameter for the matching and the matching impedance with the tag antenna on the reader antenna.

Just a short definition of Q factor: I would separate between reader Q, tag Q and system Q (Q if the tag is close to the reader antenna)

Your statement regarding the reader Q is true for the usual NFC matchings. But for WLC is different, because you always operate reader and tag in very close distance with a relatively high load. This reduces the system Q tremendous (<5!!). Basically the reader Q can be as high as possible in an NFC WLC system, the tag load decreases the system Q in a range where communication is not an issue.

Best regards,

Marcel