Low voltage drop rectifier for low voltages, how to?

[neazoi]

Hello,
I am trying to convert a low alternating voltage (1-2v) low current (<50mA) into DC, using a discrete components approach.
Low voltage drop diodes could be a solution, but I was wondering if a FET in depletion more can be used as a "diode" and if this will be of any good, compared to a real low voltage drop diode?

I have seen a way http://www.thetaeng.com/FETBridge.htm
Maybe if I use depletion mode complements I could avoid the extra diodes at all?

 

[erikl]

Maybe if I use depletion mode complements I could avoid the extra diodes at all?

Yes, but depletion FETs are "normally on", so they can't offer the required direction effect.

 

[neazoi]

Yes, but depletion FETs are "normally on", so they can't offer the required direction effect.

How about this one? http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm

 

[erikl]

How about this one? http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm

Read the text thoroughly:
"It only works for big enhancement voltage on the gate to drain and even then only within the maximum gate-source voltage."

For a bridge rectifier, this means you need a relatively high AC voltage (say: ≈6Veff to reach the 9V DC gate voltage for the BS250P MOSFET). But in this case you wouldn't mind so much about the voltage drop of a normal Si (Schottky) diode - or even a Ge diode, if temperatures allow for.

 

[neazoi]

Read the text thoroughly:
"It only works for big enhancement voltage on the gate to drain and even then only within the maximum gate-source voltage."

For a bridge rectifier, this means you need a relatively high AC voltage (say: ≈6Veff to reach the 9V DC gate voltage for the BS250P MOSFET). But in this case you wouldn't mind so much about the voltage drop of a normal Si (Schottky) diode - or even a Ge diode, if temperatures allow for.

If I step-up the voltage with a step-up converter and then use a Schottky diode to rectify it, then the Schottky will have the same voltage drop as if it operated in lower voltage, but now this drop will be only a small fraction of the higher voltage. How about this?

 

[erikl]

This is true. But ... the step-up converter will ruin the overall efficiency, I think :-(