NFC energy harvesting circuit configuration

[FWang.2]

Hello:

I am using the ST25 Dynamic NFC for energy harvesting applications in its batterless mode.

I think there is a power management unit in the circuit but without energy storage such as a capacitor, is that right? If so, I need to add an external capacitor for this purpose.

Could you help to explain how to choose the capacitor for energy storage and discharging for application?

Looking forward to hearing from you.

Kind regards

Fengzhou

[JL. Lebon]

Dear Fengzhou,

It is true that ST25DV does not embed a storage capacitor. The energy harvesting output is unregulated and unfiltered and its value is directly dependent on how much energy is received by the ST25DV.

So, at output of the ST25DV energy harvesting, depending on your application, you may want to add some filtering and a tank capacitor.

If you need clean voltage, you need to filter 27.12MHz (twice the 13.56MHz from the RF field since it is rectified).

The tank capacitor is used to limit voltage drop in case of peak of current surge from the powered device. Its value really depends on the current variation from the connected device.

You can find curves of what current the ST25DV can deliver versus energy harvesting voltage in the application note AN4913 https://www.st.com/resource/en/application_note/an4913-energy-harvesting-delivery-impact-on-st25dvi2c-series-behaviour-during-rf-communication-stmicroelectronics.pdf

For a given RF field power, the ST25DV can deliver a fixed power. So, the more current you draw, the less voltage you are given.

To calculate your capacitor, you should know the peak current consumption of your device, and choose it so that current drawn seen by the ST25DV upstream the capacitor is kept below the limit of your minimum possible voltage.

Best regards.

[FWang.3]

Dear JL.Lebon:

​

Thank you for your suggestion and these useful explanations.

​

I think the power directly from the RF field power is a good option but is limited by the existing configuration as well.

​

More energy might be requested later. I am still a bit confused about the calculation process for the capacitor: how do I compensate the charging and discharging time to meet the requirement of the electric design. Could you provide a calculation process to look at ? any reference design.

Looking forward to your reply.

Kind regards

Fengzhou

[TZhou.2]

Hi Fengzhou,

Nice to check your problem. I am also interested in it. Hopefully, we can discuss this issue. Do you mean the listener side should harvest much more energy from a reader? Thanks,

Thomas

[JL. Lebon]

Hello Fengzhou,

I don't have a reference design for this.

The calculation is basically a RC charge discharge calculation. You have to know the peak current drawn by your application, the duration of the surge, and the minimum operating voltage of your application. Based on this, you have to calculate the discharge curve of your capacitor, starting at the fully charged voltage and with the current during peaks. Then you have to adjust your capacitor value so that during the current peak the voltage does not go below your minimum operating voltage.

Best regards.

[davepatton]

Hi JL,

I am trying to use an STEVAL-SMARTAG2 board that has an ST25DV64KC but the EH by default is disabled.
How can I enable it?
Thanks,

Dave