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I am harvesting energy from an NFC device using a tuned antenna on my PCB. Though this method I am able to generate about 3.05V. I would like to charge a super capacitor using the power harvested from the NFC device. To do this I have used the simple diode circuit provided here (and shown in Figure 1 below).

The problem I am facing is my circuit requires a minimum of 3V to operate within operating conditions, however with the added drop by typical diodes I believe there with be various situations where the generated voltage will drop below the required 3V. Are there any Diodes available which have ultra low voltage drops of less then 0.01V? and is that even possible?

Please note:

  • my system load will be < 5mA
  • The 3.05V generated was without a diode in the circuit

enter image description here

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    \$\begingroup\$ There is a problem that a lower forward voltage is going to bring with it larger reverse leakage currents. Probably you can tune the forward voltage as low as you like by choosing different metals in combination with different semiconductors in a Schottky diode. But you rarely see Vf below 0.2 V. Probably that is about the limit for getting useful rectification. \$\endgroup\$ – The Photon May 1 '16 at 1:54
  • \$\begingroup\$ I was afraid of this. I was perhaps ill have to use some kind of super efficient boost converter, unless some cleaver person can come up with a solution \$\endgroup\$ – user3095420 May 1 '16 at 2:21
  • \$\begingroup\$ The picture shows a solar cell. But you are actually using some kind of RFID thing, right? What is the resonant frequency? \$\endgroup\$ – mkeith May 1 '16 at 2:33
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    \$\begingroup\$ Maybe just use a tiny little ferrite core transformer and rectifier. \$\endgroup\$ – mkeith May 1 '16 at 3:08
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    \$\begingroup\$ Yes. That is why you add a rectifier after the transformer. To RECTIFY the AC to DC. I am not sure it will work. Whatever load you add to the antenna has to be chosen to maximize power transfer and also not spoil the resonance. \$\endgroup\$ – mkeith May 1 '16 at 3:37
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An ideal diode controller and MOSFET can be applied this situation -- the net effect is that of a Iload*Rds(on) voltage drop diode. Probably the simplest to apply would be Linear's LTC4412.

Dedicated supercapacitor charger ICs also likely would solve the issue, but would require careful specification.

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  • \$\begingroup\$ This solution looks like it would work although it would require me to make major modifications to my board layout. At this point in time probably my only option. \$\endgroup\$ – user3095420 May 1 '16 at 4:17
  • \$\begingroup\$ The LTC4412 would power up from the AC being rectified and the dc achieving 2.5 volts but where do you go from there - 13.56 MHz applied to the P channel device just would not work as a low volt drop peak rectifier. \$\endgroup\$ – Andy aka May 1 '16 at 9:42
  • \$\begingroup\$ @Andyaka -- there seems to be some confusion on the querent's part here -- if you can work with him to straighten it out, that'd help. \$\endgroup\$ – ThreePhaseEel May 1 '16 at 14:33
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    \$\begingroup\$ When did you become my manager dude? \$\endgroup\$ – Andy aka May 1 '16 at 14:36
  • \$\begingroup\$ @Andyaka -- my apologies if you took that as anything more than a suggestion. \$\endgroup\$ – ThreePhaseEel May 1 '16 at 15:02
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Check out the SM74611 Smart Bypass Diode from Texas Instruments.

Forward Voltage:
Vf[V] = 26mV @ 8A, Tj = 25°C

Other alternatives:

LX2400 Cool bypass switch (CBS) from Microsemi

Typical Forward Voltage
VF = 50mV @ 10A, Tamb = 85°C

SPV1001 Cool bypass switch (CBS) from STMicroelectronics

Vf[V] = 120mV @ 8A, Tj = 25°C
Vf[V] = 270mV @ 8A, Tj = 125°C

SBR30U30CT Super Barrier Rectifier from Diodes

Vf[V] = 190mV @ 2.5A, 125°C
Vf[V] = 250mV @ 5A, 125°C

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If you add few turns of wire to your antenna coil probably will get higher voltages and lower currents so you could employ Schottky diodes. Impedance matching is very important in harvesting RF energy. Some ferrite core could also help because will capture more energy. Energy required to switch a synchronous Mosfet rectifier at 13 MHz is probably more than the energy harvested.

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A MOSFET is better than any diode and it can be used if there is enough DC voltage to drive the gate. At low currents this MOSFET would be cheap and small. If you dont have suitable gate voltage then there are other options:

  • A Germanium diode will drop less than the Si Schottky.
  • A Ge Schottky would, in theory, be even better but I have not seen such devices.
  • There is a device called a "Back Diode" which I have not used but it could perform well.

Otherwise there are schemes that use depletion mode devices that run at very low voltages. When it comes to depletion mode it is easier to find J FETs than Mosfets.

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I faced a similar issue recently with a BLE device, and ended up choosing the MAX40200 "Ultra-Tiny Micropower, 1A Ideal Diode with Ultra-Low Voltage Drop". The specs can be seen here:

https://www.maximintegrated.com/en/products/analog/amplifiers/MAX40200.html

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