low energy harvesting voltage on a custom st25dv tag

[Speda.1]

Hi all ,

we have designed a custom board based on the ST25DV SMARTAG1 reference design with a 18.5cm *22.5 cm antenna as ST25DV-DISCOVERY-ANT-C1 . Our product is batteryless so we need to power it entirely by energy harvesting output.

Before making our custom board we tested on smarttag1 board , we have made a simple change in the provided firmware code for smarttag1 and added this line

 SMARTAG1_NFCTAG_WriteEHMode(SMARTAG1_NFCTAG_INSTANCE, 0x00);

after HWInitialization();

We got the expected behaviour and whenever we placed it near a android phone with nfc , we got the voltage from the energy harvesting pin between 3.3 to 3.6v .

But on our custom board when we install the same firmware, the ST android app detects the tag but we get the voltage output from the energy harvesting pin as only 0.1 to 0.3 volts . Can you please help us with this problem ? is this because of the antenna size ?

Hope my question is clear . thanks for your time

[JL. Lebon]

​Hello,

Yes, it is possible that the problem comes from the antenna.

Usually, the voltage ~0V at EH output means that there is not enough energy to power the tag and deliver EH.

The power transfer between reader and tag must be optimized in order to get enough energy to deliver EH.

For hat, several things could be done:

• maximize the coupling between reader's antenna and tag's antenna. This is usually achieved by proper alignment of antenna, and when antenna size are similar. A reader with a large antenna will not provide a lot of energy to a tag with a very small antenna.
• Tune both antennas correctly: maximizing coupling will de-tune antennas. It is recommended to tune antennas a little bit higher than 13.56MHz if coupling is high.
• Tune the tag's antenna to optimize power transfer: this can be achieved by having antenna with larger traces and less loops usually. A tuning capacitor can be added to tune correctly the new antenna. This will decrease a bit the communication performance, but you probably don't care as your reader is close to the tag in your application. What you can do quickly is to "cut" and "shortcut" your antenna, add the needed tunning capacitor and test again with this antenna with less loops if results are better.

You can also try to enable EH only "on-demand". By using SMARTAG1_NFCTAG_WriteEHMode(SMARTAG1_NFCTAG_INSTANCE, 0x00); you enabled EH permanently, as soon as the device has booted. Instead of doing this, you can use the EH_CTRL_Dyn register to control exactly when you enable/disable EH. This is done from the reader by sending a Write Dynamic Config command with value 0x01 at address 0x02.

Hope this helps.

[Speda.1]

thank you , I will follow your suggestions and report back . I have one more question , st has reference antenna designs , 14*14 mm (ANT7-T-ST25DV04K) , 18.5mm *22.5 mm antenna(ST25DV-DISCOVERY-ANT-C1) . do they support energy harvested output voltage of greater than 3.3V? or the size of antenna is too small to get a EH voltage greater than 3.3V?

thank you

[JL. Lebon]

​Hello,

In a general way, the EH is not designed to output more than 3.3V, even if it is possible to get a bit more with large antennas (class 1 45*75mm) at very strong RF field (>5A/m).

With small size antennas, it will be difficult to even reach 3.3V (specially with a smartphone as the reader) and probably not possible to get more than 3.3V.

Best regards.

[Speda.1]

thank you