Batteryless sensor node with RF energy harvesting and super cap

[Nikos A.]

Hi everyone,

We are currently working on a concept for designing a batteryless sensor node. The node will be equipped with the Powercast P2110B Powerharvester https://eu.mouser.com/datasheet/2/329/P2110B_Datasheet_Rev_3-3159474.pdf and a supercapacitor a sensor and low power MCU. Additionally, we have the P2110 evaluation board https://eu.mouser.com/datasheet/2/329/P2110_EVB_Rev_C_instructions_rev_1-3159512.pdf and the TX91501 transmitter https://www.mouser.com/datasheet/2/329/User-Manual-TX-915-01-Rev-A-4-1511111.pdf to assist with the development. It's worth noting that I have never integrated a supercapacitor into my designs before.

I have a few questions:

• On the evaluation board, can I exclusively use the supercapacitor to power up the sensor node, considering that the node will be designed to operate at ultra-low power? Or it should be combined with a battery?
• I've noticed that the supercapacitor discharges very quickly, even when I have disconnected the loads (S2 of the evaluation board). Is this normal behavior for such a small supercapacitor?
• In the timing diagram of the Powercast P2110B Powerharvester (page 3), it states that Vout (load supply) will be high when Vcap is 1.25V and will be set to low when Vcap is 1.02V. Doesn't this leave very little room for operation? Also, does the Vmax=1.25V limit the energy stored in the supercapacitor (E = 0.5CV^2)?
• On page 4, there is an equation provided for calculating the minimum required capacitor based on the load and the time of operation: C = 15 * Vout * Iout * ton. I'm having trouble understanding this formula. Based on my knowledge, shouldn't the minimum required capacitor be calculated as C = (I * t) / ΔV, where ΔV represents the allowable voltage drop across the supercapacitor?
  

Thank you in advance for your assistance.

[Nikos A.]

Anyone?