[ST25DV]Strange Energy-Harvesting behavior

[mais]

I am developing an application that works with energy harvesting (EH).

In doing so, I examined the EH function more closely and found strange behavior.

What I have done:

hardware:

 - IC used: ST25DV04K-IER6S3

 - design of three custom tags (similar to some ST reference antenna boards)

 - antennas designed according to AN2866 (sizes [mm]: 25x40, 50x40, 75x40)

 - turned on EH by setting EH_mode to 'forced after boot'

measurement setup:

 - resistor between EH-pin and Vss

 - hooked up oscilloscope to examine output

 - android NFC device parallel over antenna with 3d-printed spacer

measurements:

 - the 3 tags I mentioned above

 - different resistors between 50 ohms and 10k ohms

 - 2 NFC devices (OnePlus 7T and Nexus 7 (2013))

 - variable distance between Tag and NFC device (5mm to 35mm in 5mm steps)

For each configuration, I used the oscilloscope to measure the rms voltage across the resistor.

In many cases this works well because the voltage is almost constant (unless the nfc device sends an inventory request every few 100 ms).

But with certain combinations (about 1/4 of the cases) the nfc tag does not come at all on or keeps going off over and over again.

This happens mostly for resistances between 500 ohms and 2k ohms.

The same strange behavior is also found if powered over Vcc.

Any ideas what I've done wrong or why this happens?

Thanks in advance!

[Ulysses HERNIOSUS]

Hi,

I would assume that you are running into issues of the phone/tablet understanding the load modulation of the tag. If the phone is not successfully recognizing the tag it will turn off the field.

Also phones typically use some low power card detection where they sense the loading of a card by emitting very short field pules (few us). If the phone detects a change then it would poll, possibly not detect anything and go back into low power card detection. With the card being in a static position it will not wake again. Maybe you are running into such a behavior as well.

Also the Nexus 7 is very old, did you check whether it does NFC-V in OOK or AM? OOK will give small field-outs. I expect newer phones to perform according to NFC Forum NFC-V at AM modulation.

Regards, Ulysses

[mais]

Thanks you for your response.

In the meantime I have tested other smartphones. My examined devices are now:

 - OnePlus 7T

 - Nexus 5

 - Nexus 5X

 - Nexus 7 (2013)

I was able to find out some of the NFC chips for the various devices (Broadcom BCM20793 / Broadcom BCM20793M / NXP PN548). My research showed that every chip supports ISO 15693.

This also confirms my investigations, as it is possible to write and read the tags with all devices.

However, there is still the problem that with certain loads the chip keeps going on and off or does not work at all.

Here is a concrete example:

 - device: OnePlus 7T

 - tag with antenna: 40x50 mm

 - distance tag-phone: 15 mm

The connected resistance is gradually increased from 50 ohms to 10 kohms. In the range of 450 ohms to 3 kohms, the tag no longer reacts and the EH no longer works. Even if the tag is removed and brought back into the field or the smartphone is switched off and on again, the behavior is the same.

Where could the fault be?

Thank you very much.

[JL. Lebon]

​Hello,

In the failing example, you said that the connected resistance is gradually increased from 50ohms to 10kohms, and that you only hava range of 450ohms to 3kohms where the tag is not responding.

 Can you check the EH voltage output value for resistor between 50 ohms to 450 ohm, then for the range 450 ohm to 3 kohm, then above 3 klhoms ?

I suspect that for resistors below 450 ohm the EH may not be delivered (~0V), and that in the 450-3k range, the available energy on the antenna is just enough to both power the tag and deliver the EH. What can happen in that case, is that tag is starting delivering EH, but as the amount of nrj is limit, it causes a reboot of the tag. Then after reboot, the tag is again starting to deliver EH, and reboot again, and so on. If you plug a scope on EH output, this phenomenon is visible: you can see EH start rising, then falling, then rising again... May be you can check this too ?

Best regards.

[mais]

Thanks for the fast response.

I notice that I have not described the behavior precisely enough.

I would actually expect the behavior you describe. But since that's not the case, I originally asked this question.

The three areas are:

• The strange thing is that even at 50 ohms to 450 ohms there is a voltage at the EH output. (In the case mentioned: starts at 0.5V and ends at 2.2V)
• In the range between 450 ohms and 3 kohms I can determine exactly the behavior you described.
• After 3 kohms, the tag and the EH voltage behaves normally as you would expect. It starts at 3V and increases to 3.15V at 10 kohms.

If I summarize the three areas, there is a voltage at low and high loads, but not in between. And that's exactly what I don't understand.

And, as mentioned, similar behavior also occurs with other configurations. The only difference is that the limits of the critical area (in which there is no voltage or the tag keeps going back and forth) vary slightly.