AC to DC conversion without using diodes and no external supply

[rahulponna]

Hi,

Firstly the reason I have the constraints of not using diodes is that the
AC input voltage is very low (200mV) and even low voltage Schottkey diodes
have a forward drop of (230mV). And regarding external power supply, this
circuit is aimed at energy harvesting, so the aim is to do everything with
only the harvested energy.

Any ideas?


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[Paul Keinanen]

On Fri, 23 Jul 2010 03:37:03 +0000,

Hi,

Firstly the reason I have the constraints of not using diodes is that the
AC input voltage is very low (200mV) and even low voltage Schottkey diodes
have a forward drop of (230mV). And regarding external power supply, this
circuit is aimed at energy harvesting, so the aim is to do everything with
only the harvested energy.

Any ideas?

Use a transformer to increase the voltage to something usable before
rectification.

 

[amdx]

--
MikeK

Hi,

Firstly the reason I have the constraints of not using diodes is that the
AC input voltage is very low (200mV) and even low voltage Schottkey diodes
have a forward drop of (230mV). And regarding external power supply, this
circuit is aimed at energy harvesting, so the aim is to do everything with
only the harvested energy.

Any ideas?

What frequency?

 

[markp]

Hi,

Firstly the reason I have the constraints of not using diodes is that the
AC input voltage is very low (200mV) and even low voltage Schottkey diodes
have a forward drop of (230mV). And regarding external power supply, this
circuit is aimed at energy harvesting, so the aim is to do everything with
only the harvested energy.

Any ideas?


-------------------------------------

Linear Technology have an energy harvesting chip that might be what you
need, LTC3108. This includes a syncronous rectifier foe efficiency.

Mark.

 

[Jamie]

Hi,

Firstly the reason I have the constraints of not using diodes is that the
AC input voltage is very low (200mV) and even low voltage Schottkey diodes
have a forward drop of (230mV). And regarding external power supply, this
circuit is aimed at energy harvesting, so the aim is to do everything with
only the harvested energy.

Any ideas?

Not knowing your freq requirements... I would say using a precision
rectifier that can put gain on your weak signal will do..

http://en.wikipedia.org/wiki/Precision_rectifier

Looking at the "Improved Circuit" should give you some ideas.

You didn't say what was the max voltage coming in?

 

[Grant]

Not knowing your freq requirements... I would say using a precision
rectifier that can put gain on your weak signal will do..

http://en.wikipedia.org/wiki/Precision_rectifier

Looking at the "Improved Circuit" should give you some ideas.

You didn't say what was the max voltage coming in?

Did you miss the 'energy harvesting' point? There's chips designed
for this area, one announced recently and pointed out upthread by
another poster.

Grant.

 

[rahulponna]

rahulponna had written this in response to
http://www.electrondepot.com/electr...hout-using-diodes-and-no-external-484602-.htm
:

Jamie wrote:

I am looking at a frequency of 950MHz. the input voltage is 200mV (peak).
Also for precision rectifier I have to use external supply for the op amp.
I need to make all my electronics work with the harvested energy.


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[rahulponna]

rahulponna had written this in response to
http://www.electrondepot.com/electr...hout-using-diodes-and-no-external-484376-.htm
:

[email protected] wrote:


thanks for the suggestion. Can you please give me more info on how to use
back diodes for rectification? If you could point me to some links it
would be great. I am trying to google on usage of back diodes for
rectification.


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[rahulponna]

rahulponna had written this in response to
http://www.electrondepot.com/electr...hout-using-diodes-and-no-external-484404-.htm
:

amdx wrote:


950MHz


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[rahulponna]

rahulponna had written this in response to
http://www.electrondepot.com/electr...hout-using-diodes-and-no-external-484462-.htm
:

markp wrote:

Hey mark, I have already looked at the LT3108 chip.yes it's used for
energy harvesting, but it needs a dc input voltage. I plan to use that
chip, but before that I have to rectify my AC voltage.



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[Paul Keinanen]

On Sun, 25 Jul 2010 02:25:32 +0000,

I am looking at a frequency of 950MHz. the input voltage is 200mV (peak).
Also for precision rectifier I have to use external supply for the op amp.
I need to make all my electronics work with the harvested energy.

Since this is a fixed frequency, just feed the antenna power
(typically at 30-60 ohms) into a 950 MHz resonator and make an
additional high impedance connection into the resonator.

A 1/4 wave resonator with a magnetic probe close to the grounded
feeding the low impedance antenna power and a capacitive probe close
to the high end of the resonator to extract the power might do it.
Rectify the increased voltage in the conventional way e.g. with a
simple RF probe (series capacitor, diode to ground).

It is not at all self evident that any active rectifier circuit would
work at such high frequencies.

 

[Paul Keinanen]

On Sun, 25 Jul 2010 02:25:32 +0000,

I am looking at a frequency of 950MHz. the input voltage is 200mV (peak).

How is this measured ? Open circuit or loaded transmission line ?

200 mV peak is 140 mV rms and if that was unloaded voltage, the
voltage into a matched load would be 70 mV rms. Assuming 50 ohm
impedance levels, this is -10 dBm or 0.1 mW power available from the
source in matched condition.

If the original measurement was actually open circuit peak-to-peak,
the available power would be 25 uW.

 

[Paul Keinanen]

How is this measured ? Open circuit or loaded transmission line ?

200 mV peak is 140 mV rms and if that was unloaded voltage, the
voltage into a matched load would be 70 mV rms. Assuming 50 ohm
impedance levels, this is -10 dBm or 0.1 mW power available from the
source in matched condition.

If both the power source transmitter antenna as well as the energy
gathering antenna are simple omnidirectional Ground Plane antennas, at
950 MHz, the path loss of 1m distance (already in the far field for
that frequency) is 32 dB and at 10 m 52 dB and at 100 m 72 dB.

To get -10 dBm at the energy gathering antenna, the master transmitter
power would have to be +22 dBm (160 mW) at 1 m, +42 dBm (16 W) at 10 m
and +62 dBm (1600 W) at 100 m in free space. We are quickly talking
about power levels at which human exposure limits must be checked.

In order to transmit back any measurements to the master receiver,
co-located at the main transmitter site and assuming -120 dBm master
receiver sensitivity, the slave transmitter power would have to be -88
dBm @ 1 m, -68 dBm @ 10 m and -48 dBm @ 100 m.

At 100 m and assuming 100 % RF->DC->RF conversion efficiency, the
transmitter power would have to be -120 dBm + 72 dB + 72 dB = +24 dBm
(250 mW). Add to this the losses in the slave (perhaps 10-30 dB), so
clearly such distances (100 m) are not practical at these frequencies
and these kinds of antennas.

 

[markp]

markp wrote:

Hey mark, I have already looked at the LTC3108 chip.yes it's used for
energy harvesting, but it needs a dc input voltage. I plan to use that
chip, but before that I have to rectify my AC voltage.

Not true, the application circuit actually produces an AC source from DC by
using a resonant oscillator which is then synchronously rectified. The
device can be used with AC, see page 14: "Any source whose peak voltage
exceeds 2.5V AC or 5V DC can be connected to the C1 input through a
current-limiting resistor where it will be rectified/peak detected." In fact
it mentions vibrational sensors as a possible input source which is very
likely to be AC.

So it seems by placing a 1:12.5 step-up transformer from your 200mV input
you could produce a 2.5V AC signal suitable for use.

Mark.

 

[Paul Keinanen]

Not true, the application circuit actually produces an AC source from DC by
using a resonant oscillator which is then synchronously rectified. The
device can be used with AC, see page 14: "Any source whose peak voltage
exceeds 2.5V AC or 5V DC can be connected to the C1 input through a
current-limiting resistor where it will be rectified/peak detected." In fact
it mentions vibrational sensors as a possible input source which is very
likely to be AC.

So it seems by placing a 1:12.5 step-up transformer from your 200mV input
you could produce a 2.5V AC signal suitable for use.

Mark.

Apparently the internal oscillator is operated at 10-100 kHz, so it
would make sense to optimize the synchronous rectifier for that
frequency range too.

I very much doubt that the synchronous rectifier would be too useful
at 950 MHz, which the OP had in mind.

 

[Grant]

Not true, the application circuit actually produces an AC source from DC by
using a resonant oscillator which is then synchronously rectified. The
device can be used with AC, see page 14: "Any source whose peak voltage
exceeds 2.5V AC or 5V DC can be connected to the C1 input through a
current-limiting resistor where it will be rectified/peak detected." In fact
it mentions vibrational sensors as a possible input source which is very
likely to be AC.

So it seems by placing a 1:12.5 step-up transformer from your 200mV input
you could produce a 2.5V AC signal suitable for use.

At 950MHz?

Grant.

 

[Grant]

rahulponna had written this in response to
http://www.electrondepot.com/electr...hout-using-diodes-and-no-external-484602-.htm
:

Jamie wrote:

I am looking at a frequency of 950MHz. the input voltage is 200mV (peak).
Also for precision rectifier I have to use external supply for the op amp.
I need to make all my electronics work with the harvested energy.

[snip]

950MHz is a RFID "TAG" frequency.

!!

When I worked with them they were still down around 130kHz, time
flew. And they self power from the loop field, transmit during
the breaks in loop excitation. Were used back then for cattle,
dog and cat 'chipping'.

Easy to get power transferred 'cos it's a tuned circuit to
known frequency, and fairly short range -- rx inside or close
to loop antenna.

Grant.

 

[markp]

Apparently the internal oscillator is operated at 10-100 kHz, so it
would make sense to optimize the synchronous rectifier for that
frequency range too.

I very much doubt that the synchronous rectifier would be too useful
at 950 MHz, which the OP had in mind.

Yes, 950MHz is going to be probematical. I didn't see the 950MHz reply
before I wrote this, but the OP seemed to think the chip wouldn't work with
AC signals at all but it does.

Mark.

 

[markp]

Yes, 950MHz is going to be probematical. I didn't see the 950MHz reply
before I wrote this, but the OP seemed to think the chip wouldn't work
with AC signals at all but it does.

Mark.

Maybe putting one of these RF harvesters at the front end might help:
www.powercastco.com/PDF/P2110-datasheet.pdf

Mark.