I have an extremely low power application (recovering a small amount
of dc power from the audio output of a PC soundcard). I am trying to
key a transmitter when the soundcard puts out audio. It's actually a
self powered VOX, deriving all it's needed power from the audio output
of the soundcard. Right now, I'm almost succeeding, but need another
half volt or so to make the keying reliable.

I am using my laptop computer, which has only speaker out jacks. I am
feeding the speaker output into the low impedance side of an audio
transformer and taking the stepped up voltage from the secondary (8
ohm to 1K ohm impedance transformer). I am feeding the secondary into
a small signal schottky diode and filtering the rectified output to
get my dc power.

As it is now, it's barely usable. I have to really crank up the laptop
audio in order to get reliable keying.

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??

Thanks,

Al



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What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.



Albert writes:

I have an extremely low power application (recovering a small amount
of dc power from the audio output of a PC soundcard). I am trying to
key a transmitter when the soundcard puts out audio. It's actually a
self powered VOX, deriving all it's needed power from the audio output
of the soundcard. Right now, I'm almost succeeding, but need another
half volt or so to make the keying reliable.

I am using my laptop computer, which has only speaker out jacks. I am
feeding the speaker output into the low impedance side of an audio
transformer and taking the stepped up voltage from the secondary (8
ohm to 1K ohm impedance transformer). I am feeding the secondary into
a small signal schottky diode and filtering the rectified output to
get my dc power.

As it is now, it's barely usable. I have to really crank up the laptop
audio in order to get reliable keying.

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??

Thanks,

Al

On 06 Jun 2004 22:34:52 -0400, Sam Goldwasser
wrote:

What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

-

Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Al


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Albert wrote:

On 06 Jun 2004 22:34:52 -0400, Sam Goldwasser
wrote:


What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

-


Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Al

Why don't you just steal 5 volts off of a serial or USB port and
screw the audio rectification?

Bob



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Albert wrote:
It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Albert, take a high beta BJT which needs only 750mV and maybe 25uA to switch
lets say 15mA of collector current. You can also double the voltage of the
transformer (if it has isolated windings).
___
o----. ,---+-|--------+----|___|--+
)|( | | + 39k |
)|( | ### | +--------o
o----' '-+ | --- | |
| | |47u | |/
+-)-----------+ +-|
| | + |
| ### |
| --- |
| |47u |
+--|-------+---------------+--------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
Maybe you have to use another resistor to discharge the caps, if the
transistor doesn't turn off.

--
ciao Ban
Bordighera, Italy

In article ,
Bob Urz wrote:

Albert wrote:

On 06 Jun 2004 22:34:52 -0400, Sam Goldwasser
wrote:


What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

-


Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Al

Why don't you just steal 5 volts off of a serial or USB port and
screw the audio rectification?

Bob


Better yet, get one of those electronics hobbyist adaptors. They
connect to a port of your computer (SCSI, printer, serial, USB) and
provide you with analog and digital I/O on screw terminals. You can
write programs to control the I/O. They're commonly used for simple
machine control and data logging. Rat Shack even sold them for a
while...something like Hobo.

Why not use a bipolar transistor instead of a MOSFET? Then you only need
0.7 V or so to turn it on. Of course, you would need a high volue resistor
in series with the base to get a decent time constant. If you need the
current sensitivity, this could be followed by another transistor as an
inverter and then your MOSFET. Or, instead of the transistors, a voltage
comparator or op-amp?

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.




Albert writes:

On 06 Jun 2004 22:34:52 -0400, Sam Goldwasser
wrote:

What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

-

Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Thanks for all the suggestions.

I chose the mosfet because it has high input impedance. I'd rather not
go to a bjt because it will draw base current and will need a series
base current limiting resistor-hence the design becomes more
complicated because the time constant of the parallel rc network has
to be maintained.

Yes, I can use an op amp, provided I can get power for it from
somewhere. Yes, I can steal power from this or that computer port.
And, some radios even have low power dc voltages available on the
microphone connectors that could be used. The power needed is very
small, yes, I could power the circuit from batteries too.

But, not all radios have dc voltages on the microphone and some
computers don't have usb or serial ports to steal power from. I want
this thing to be universal in nature, meaning I can use it on any
radio/computer anywhere/anytime. This is my motivation for NOT
stealing computer/radio power or importing it from other sources.

Since it almost works now, my hope was to try and reduce the voltage
drop in the rectifiers by using mosfets as diodes.

Can anyone offer advice on the original question, which is…

Can I bias the base on all the time and feed and use the mosfet as a
near zero forward voltage drop rectifier?

Thanks,

Al


Albert, take a high beta BJT which needs only 750mV and maybe 25uA to switch
lets say 15mA of collector current. You can also double the voltage of the
transformer (if it has isolated windings).
___





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On a sunny day (Sun, 06 Jun 2004 22:28:43 -0400) it happened Albert
wrote in :

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.
How about a germanium diode?


Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??
If you are goin to use a small battery, why not use a small opamp!
CMOS opamp at 3V should take little power.
perhaps you could take power from the laptop par port?
JP

Can anyone offer advice on the original question, which is…

Can I bias the base on all the time and feed and use the mosfet as a
near zero forward voltage drop rectifier?

Sorry, a FET behaves like a controlled resistor, there is no diode
behaviour. A synchoneous rectifier is above your possibilities. I showed you
a great way of doubling your transformer voltage and you do not even see it!
Anyway there are also fets with lower threshold voltages, just last week
Winfield Hill answered a similar question, if you want to stick to a Mosfet.
What time constant do you need to switch that PTT after the voice has
finished? 1s.
Look you need anyway a bleed resistor so the 39k base resistor does that.
And the BJT costs only 5cent.


--
ciao Ban
Bordighera, Italy

"Albert" bravely wrote to "All" (06 Jun 04 22:28:43)
--- on the heady topic of "using mosfets as rectifiers?"

One method is to use a low power cmos opamp configured as an ideal AC
full wave rectifier and powered by a square 9V batt. If you don't want
to use a battery then you can easily get a little power from the pc by
using either the game port (+5V at pins 1,8,9,15), printer connector
(+5V 4.7Kohm pin 35, or data), and rs-232 (enable any handshake line).

Don't forget you need a time delay circuit in order to hold the key
down otherwise it will be switching on the peaks only, as you are
experiencing now BTW.

A*s*i*m*o*v


Al From: Albert
Al I have an extremely low power application (recovering a small amount
Al of dc power from the audio output of a PC soundcard). I am trying to
Al key a transmitter when the soundcard puts out audio. It's actually a
Al self powered VOX, deriving all it's needed power from the audio output
Al of the soundcard. Right now, I'm almost succeeding, but need another
Al half volt or so to make the keying reliable.

Al I am using my laptop computer, which has only speaker out jacks. I am
Al feeding the speaker output into the low impedance side of an audio
Al transformer and taking the stepped up voltage from the secondary (8
Al ohm to 1K ohm impedance transformer). I am feeding the secondary into
Al a small signal schottky diode and filtering the rectified output to
Al get my dc power.

Al As it is now, it's barely usable. I have to really crank up the laptop
Al audio in order to get reliable keying.

Al Can I use small signal mosfets to rectify the signal instead of the
Al schottky diode? This would gain my half a volt because the mosfet
Al would have very low voltage drop once it turns on.

Al Would it be ok to bias the gate positive all the time (with a small
Al button cell 3 volt battery) and just treat the mosfet like it was a
Al diode??

Al Thanks,

Al Al

.... Back when I was a boy, we sucked the air out of our own vacuum tubes!

On Mon, 07 Jun 2004 14:14:49 GMT, Jan Panteltje
wrote:

On a sunny day (Sun, 06 Jun 2004 22:28:43 -0400) it happened Albert
wrote in :

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.
How about a germanium diode?


Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??
If you are goin to use a small battery, why not use a small opamp!
CMOS opamp at 3V should take little power.
perhaps you could take power from the laptop par port?
JP

A voltage doubler with 2 schottkys driving a BC847c or such would
certainly work at average lsp output level. Used to key CW from a PC
that way.

....but somebody will undoubtedly advise to use a PIC or a 555 in due
time...:-)

--
- René

"Albert" wrote in message
news
On 06 Jun 2004 22:34:52 -0400, Sam Goldwasser
wrote:

What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

-

Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. .....................

You should be able to find a MOSFET that requires less voltage to turn on

Tam

If you use a MOSFET in a circuit to function similar to a diode in the
conducting direction, remember most MOSFETs have the built-in body diode
which will conduct in a direction opposite to what you want to rectify, so
this may not be a desired result.

If you say you're only about 0.5V away from something useable, why don't you
just change out your silicon diode for a germanium and regain about 0.4V?

Dave

"Albert" wrote in message
...
I have an extremely low power application (recovering a small amount
of dc power from the audio output of a PC soundcard). I am trying to
key a transmitter when the soundcard puts out audio. It's actually a
self powered VOX, deriving all it's needed power from the audio output
of the soundcard. Right now, I'm almost succeeding, but need another
half volt or so to make the keying reliable.

I am using my laptop computer, which has only speaker out jacks. I am
feeding the speaker output into the low impedance side of an audio
transformer and taking the stepped up voltage from the secondary (8
ohm to 1K ohm impedance transformer). I am feeding the secondary into
a small signal schottky diode and filtering the rectified output to
get my dc power.

As it is now, it's barely usable. I have to really crank up the laptop
audio in order to get reliable keying.

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??

Thanks,

Al



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starfire wrote...

If you use a MOSFET in a circuit to function similar to a diode
in the conducting direction, remember most MOSFETs have the
built-in body diode which will conduct in a direction opposite
to what you want to rectify, so this may not be a desired result.

This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions, one can arrange to simply turn the FET around.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

In article ,
Winfield Hill wrote:
[.. mosfet rectifier ..]
This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions, one can arrange to simply turn the FET around.

I know they do conduct in both directions, but how can we say they are
happy about it. They may be disgusted.

Make sure you use a fairly modern MOSFET that has a spec for the recovery
of the diode. If you build the recifier with a VN10, there will be a
large recovery current when the unit is first powered up.

When the load side is at zero volts, the body diode of the MOSFET does the
rectifying. On MOSFETs with no spec for the diode recovery, conduction in
the body diode stores lots of charge in the part. When the voltage
reverses, this charge tends to bias the parasitic NPN.

--
--
forging knowledge

On Sun, 06 Jun 2004 22:28:43 -0400 Albert wrote:

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

No, because the MOSFET has an intrinsic reverse diode built into it.
You could certainly make it give you a very low forward voltage drop,
but it would still conduct in the reverse direction, too, but with the
standard silicon PN junction 0.7V drop.

Besides, where would you get the 10 volts you'd need to turn the
MOSFET fully on?

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

"Albert" bravely wrote to "All" (07 Jun 04 09:39:42)
--- on the heady topic of " using mosfets as rectifiers?"

Al From: Albert
Al Can anyone offer advice on the original question, which is

Al Can I bias the base on all the time and feed and use the mosfet as a
Al near zero forward voltage drop rectifier?

Fets tend to conduct in both directions (the source interchanges with
the drain to some extent) so this makes them ill suited to behave like
a rectifier.

A*s*i*m*o*v

.... I transmit, therefore I ham!

Ken Smith wrote...

Winfield Hill wrote:
[.. mosfet rectifier ..]
This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions, one can arrange to simply turn the FET around.

I know they do conduct in both directions, but how can we say they are
happy about it. They may be disgusted.

Not at all, operating as switches they are _pefectly_ happy, and remain
so up to some maximum current, which is vey high, but low enough so the
voltage drop across the FET is under say 200mV. We're talking amps here.

Make sure you use a fairly modern MOSFET that has a spec for the recovery
of the diode. If you build the recifier with a VN10, there will be a
large recovery current when the unit is first powered up.

When the load side is at zero volts, the body diode of the MOSFET does the
rectifying. On MOSFETs with no spec for the diode recovery, conduction in
the body diode stores lots of charge in the part. When the voltage
reverses, this charge tends to bias the parasitic NPN.

This is a somewhat theoretical discussion, because this isn't the
best way to solve the O.P.'s query, but were one to use MOSFETs as
signal rectifiers, the issue of stored charge wouldn't be an actual
problem either. That's because long before the diode would become
turned on, the circuit's comparator would have driven the gate and
turned on the FET, keeping the reverse-voltage drop across the FET
very low, which after all is exactly the job of an active rectifier.
This technique is commonly used in high-efficiency low-voltage SMPS.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

In article ,
Winfield Hill wrote:
[..I wrote ..]
When the load side is at zero volts, the body diode of the MOSFET does the
rectifying. On MOSFETs with no spec for the diode recovery, conduction in
the body diode stores lots of charge in the part. When the voltage
reverses, this charge tends to bias the parasitic NPN.

This is a somewhat theoretical discussion, because this isn't the
best way to solve the O.P.'s query, but were one to use MOSFETs as
signal rectifiers, the issue of stored charge wouldn't be an actual
problem either. That's because long before the diode would become
turned on, the circuit's comparator would have driven the gate and
turned on the FET, keeping the reverse-voltage drop across the FET
very low, which after all is exactly the job of an active rectifier.
This technique is commonly used in high-efficiency low-voltage SMPS.

Your understanding of the OP's situation must be different than mine. I
though that the only way his circuit got power was via the rectifier.
When power is first applied to the system, there is no voltage to run the
comparitor from and hence it can't be controlling the FET and hence the
body diode must do the rectifying until enough voltage is produced to run
the comparitor.

--
--
forging knowledge

In article ,
Jim Adney wrote:
On Sun, 06 Jun 2004 22:28:43 -0400 Albert wrote:

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

No, because the MOSFET has an intrinsic reverse diode built into it.

See elsewhere in this thread.


Besides, where would you get the 10 volts you'd need to turn the
MOSFET fully on?

If you pick your MOSFET carefully, 3V is enough to drive the gate.
--
--
forging knowledge

Thanks Asimov, that was exactly the info I needed!

So the zero dropmosfet can only be used as a rectifier IF teh bias
voltage is switched on and off when the input goes in the desired
direction!

Awesome, I think you again.

A



Fets tend to conduct in both directions (the source interchanges with
the drain to some extent) so this makes them ill suited to behave like
a rectifier.




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Jim, Win and Ken,

Yes, you have uncovered the dilema and understand my question. Thanks
to you all!

I've already found (and ordered) a 2.2 volt turn on mosfet for my
switching transistor. This should give me the the head room I need to
drive the gate with a lower voltage.

With regard to the mosfet as a diode issue, I understand what you all
are saying.....without a means to tell the mosfet when to conduct, it
will conduct in either direction. Since I don't alot of extra power to
generate the signals to control the mosfets conduction periods, this
option appears closed to me.

My shottky diodes are already dropping just under 4 tenths of a volt,
so switching to a ger diode gets me very little and might not help at
all as some of them have very high leakage currents.

I also note that Maxim makes some single supply op amps that operate
on very low voltage and draw microamps of supply current. Perhaps a
small battery can be used to power a 3X amplifier since the load
impedance of my mosfet switch is high, it really doesn't need alot of
power or a high fidelity audio amp!

Again, thanks to you all. I've learned alot from this discussion.

A

On Mon, 07 Jun 2004 23:02:38 -0500, Jim Adney
wrote:

On Sun, 06 Jun 2004 22:28:43 -0400 Albert wrote:

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

No, because the MOSFET has an intrinsic reverse diode built into it.
You could certainly make it give you a very low forward voltage drop,
but it would still conduct in the reverse direction, too, but with the
standard silicon PN junction 0.7V drop.

Besides, where would you get the 10 volts you'd need to turn the
MOSFET fully on?

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------



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Ken Smith wrote...

Winfield Hill wrote:
[..I wrote ..]
When the load side is at zero volts, the body diode of the MOSFET does
the rectifying. On MOSFETs with no spec for the diode recovery,
conduction in the body diode stores lots of charge in the part. When
the voltage reverses, this charge tends to bias the parasitic NPN.

This is a somewhat theoretical discussion, because this isn't the
best way to solve the O.P.'s query, but were one to use MOSFETs as
signal rectifiers, the issue of stored charge wouldn't be an actual
problem either. That's because long before the diode would become
turned on, the circuit's comparator would have driven the gate and
turned on the FET, keeping the reverse-voltage drop across the FET
very low, which after all is exactly the job of an active rectifier.
This technique is commonly used in high-efficiency low-voltage SMPS.

Your understanding of the OP's situation must be different than mine.
I though that the only way his circuit got power was via the rectifier.
When power is first applied to the system, there is no voltage to run the
comparitor from and hence it can't be controlling the FET and hence
the body diode must do the rectifying until enough voltage is produced
to run the comparitor.

If that's true, the FET should be paralleled with a Schottky diode.
For that matter, did anyone suggest he simply use Schottky diodes?

But to address your point about diode reverse-recovery time, the
intrinsic diodes in power MOSFETs have characteristics much like
ordinary rectifier diodes. In some high-speed SMPS applications
this can be an issue, where one wants fast-recovery (10s of ns)
instead. It wouldn't likely be an issue here. Once again, using
FETs isn't a very good way to attack the problem, but if they were
to be used, the most painful issue might be their high capacitance.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

In article ,
Winfield Hill wrote:
[...]
But to address your point about diode reverse-recovery time, the
intrinsic diodes in power MOSFETs have characteristics much like
ordinary rectifier diodes.

See if you can find a VN10KM (Siliconix) laying around in your junk box
and measure its recovery time.

Many years ago, I worked on the design of a system that used a VN10KM to
drive a long line. When the big stuff switched the MOSFET was fed a
reverse current spike. The resulting recovery time was many mS long. I
ended up adding a diode to the design to keep the current out of the
device.

IIRC, the data sheet for the part in question didn't show a body diode and
there was no spec for it.
--
--
forging knowledge

Ken Smith wrote...

See if you can find a VN10KM (Siliconix) laying around in your junk
box and measure its recovery time.

I've got a few. I'll do it, someday. :)

Many years ago, I worked on the design of a system that used a VN10KM
to drive a long line. When the big stuff switched the MOSFET was fed
a reverse current spike. The resulting recovery time was many mS long.
I ended up adding a diode to the design to keep the current out of the
device.

It's charge that has to be removed. You must've had almost no current,
so t = q / i was very long.

IIRC, the data sheet for the part in question didn't show a body diode
and there was no spec for it.--

Yep, *all* VMOS parts have body diodes.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

In article ,
Winfield Hill wrote:
Ken Smith wrote...

[.. VN10KM recovery ..]

It's charge that has to be removed. You must've had almost no current,
so t = q / i was very long.

IIRC: 6mA give or take a bit. The MOSFET pulled to the -15V rail. There
was, I think, a 4.7K to the +15V. I think I may still have the schematic
at work. I'll have to take a look this has me curious about it.


IIRC, the data sheet for the part in question didn't show a body diode
and there was no spec for it.--

Yep, *all* VMOS parts have body diodes.

I think that Siliconix didn't think it was important enough to mension or
perhaps they measured the recovery and decided just not to bring the
subject up.

--
--
forging knowledge

In article ,
Ken Smith wrote:

I think that Siliconix didn't think it was important enough to
mension or perhaps they measured the recovery and decided just
not to bring the subject up.

I got bitten by Siliconix when trying to use the
VN10KM as a direct replacement for a JFET bipolar
analogue switch. The presence of a shunt body diode
is not acknowleged on any Siliconix VMOS data sheet,
but there is a mention in one of their app notes.

A similar situation exists with Zetex DMOS Fets.
There is no explicit mention of a body diode in the
process.... you have to notice that some data sheets
carry an added a Source-Drain Diode spec.

--
Tony Williams.

Ken Smith wrote...

Winfield Hill wrote:
Ken Smith wrote...

[.. VN10KM recovery ..]

It's charge that has to be removed. You must've had almost no current,
so t = q / i was very long.

IIRC: 6mA give or take a bit. The MOSFET pulled to the -15V rail.
There was, I think, a 4.7K to the +15V. I think I may still have the
schematic at work. I'll have to take a look this has me curious about it.

Please do. The setup and source of diode charge and discharge currents
isn't clear from your description.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

Tony Williams wrote...

Ken Smith wrote:

I think that Siliconix didn't think it was important enough
to mension or perhaps they measured the recovery and decided
just not to bring the subject up.

I got bitten by Siliconix when trying to use the VN10KM as
a direct replacement for a JFET bipolar analogue switch. The
presence of a shunt body diode is not acknowleged on any
Siliconix VMOS data sheet, but there is a mention in one of
their app notes.

A similar situation exists with Zetex DMOS Fets. There is
no explicit mention of a body diode in the process.... you
have to notice that some data sheets carry an added a
Source-Drain Diode spec.

The rule for all these parts is there's an intrinsic diode,
even if it's not mentioned. We all know that now, or should,
but in the early days, with exotic-looking names, numbers and
descriptions, this reality wasn't so obvious.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

In article ,
Tony Williams wrote:
In article ,
Ken Smith wrote:

I think that Siliconix didn't think it was important enough to
mension or perhaps they measured the recovery and decided just
not to bring the subject up.

I got bitten by Siliconix when trying to use the
VN10KM as a direct replacement for a JFET bipolar
analogue switch. The presence of a shunt body diode
is not acknowleged on any Siliconix VMOS data sheet,
but there is a mention in one of their app notes.

You also had to watch out for the gate protection zener. It had a *noisy*
leakage current that went to the channel when it was on.


--
--
forging knowledge

On Tue, 08 Jun 2004 11:43:07 -0400 Albert wrote:

So the zero dropmosfet can only be used as a rectifier IF teh bias
voltage is switched on and off when the input goes in the desired
direction!

Not quite true. Asimov's response was for FETs which, as he described,
are fairly symmetric, but your question was for MOSFETS, which are not
at all symmetric.

MOSFETs won't work for a completely different reason. See my other
post.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On 7 Jun 2004 18:43:19 -0700 Winfield Hill
wrote:

starfire wrote...

If you use a MOSFET in a circuit to function similar to a diode
in the conducting direction, remember most MOSFETs have the
built-in body diode which will conduct in a direction opposite
to what you want to rectify, so this may not be a desired result.

This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions,

That still keeps it from being much of a rectifier, however.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On 8 Jun 2004 10:37:53 -0700 Winfield Hill
wrote:

If that's true, the FET should be paralleled with a Schottky diode.
For that matter, did anyone suggest he simply use Schottky diodes?

I may be wrong, but I think both you guys have missed the point of his
question. He stated that he is already using Schottkys. He wants
something with less voltage drop. He asked about MOSFETS.

I believe he was hoping that he could use some kind of active
rectifier circuit using driven MOSFETs so that there would be NO
forward voltage drop. I suspect that he has no knowledge of the
reverse intrinsic diode (which makes this approach a complete looser)
and hadn't yet thought about how he might come up with the voltage
necessary to turn the MOSFETs on.

The short answer is that MOSFETs are not the solution he was hoping
for.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On Tue, 08 Jun 2004 12:06:05 -0400 Albert wrote:

I've already found (and ordered) a 2.2 volt turn on mosfet for my
switching transistor. This should give me the the head room I need to
drive the gate with a lower voltage.

I don't follow this stuff much, so I didn't know that such things
existed. I still have to ask: Is 2.2V where the turn-on starts, or
where it is really fully on?

Thinking about this, I gather that you intend to run these MOSFETs
backwards, so that the "rectifier" is on when it is parallel to the
body diode and off when in the usual forward direction. Does that
work? I never considered the possibility....

My shottky diodes are already dropping just under 4 tenths of a volt,
so switching to a ger diode gets me very little and might not help at
all as some of them have very high leakage currents.

In my limited experience, Schottkys have a lower forward drop than
even Ge.

I don't know your application, but I wonder if you could transformer
couple your signal and boost it a bit, just enough to give you the
extra voltage you need to keep this job simple.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

In article ,
Winfield Hill wrote:
[... mosfet as rectifier ..
look up
Please do. The setup and source of diode charge and discharge currents
isn't clear from your description.

I have someone else checking their records. I didn't turn up a copy of
the schematic. I think I, remember where the charge current came
from.


Remote preamp Long cable Main unit
.................. +15 .....................
+15V .--------------------------.
! . GND .
/ .--------------------------.
\ . .
/ . .
\ . Signal in question .
!----------------------------------------- Reciever
!- . .
!- Q1 . .
!- . -15 .
! .--------------------------.
-15V . .
.................. .....................


With Q1 on, the signal is at -15V.
With Q1 off the signal is at +15V
This works fine until we add some more info.

There is some big currents switching in the pre-amp section. Q1 turns on
just as the other thing switches off and remains on for a few mS. The
other circuit switching causes the preamp's GND to jump vs the ground in
the main unit. The combination of the cable capacitance and the receiver
circuit caused Q1's current to flow backwards. Either the bias on Q1
wasn't enough to keep the voltage below 0.7V or the current was backwards
just when Q1 was supposed to go off.

The cable could be as much as a mile long. The large signalling voltage
swings were needed to deal with the ground differences.


--
--
forging knowledge

In article ,
Jim Adney wrote:
On 8 Jun 2004 10:37:53 -0700 Winfield Hill
wrote:

If that's true, the FET should be paralleled with a Schottky diode.
For that matter, did anyone suggest he simply use Schottky diodes?

I may be wrong, but I think both you guys have missed the point of his
question.

You may be assuming incorrectly that we are aiming at his point. We are
talking about using MOSFETs as recifiers and one strange case I had some
years back.

He stated that he is already using Schottkys. He wants
something with less voltage drop.

If you use a MOSFET as a rectifier, you get less drop than a Schottky.
The sticking point is that until you've rectified some power you don't
have the voltage needed to make the gate drive for the MOSFET. We solve
this by putting the Schottky in parallel with the MOSFET. Initially, the
Schottky does the work and gets the voltage part way up. Once enough
voltage has been made to run it, the MOSFET takes over the work.

[....]
forward voltage drop. I suspect that he has no knowledge of the
reverse intrinsic diode (which makes this approach a complete looser)

No, all you have to do is flip the Source and Drain legs of the MOSFET and
the idea works just fine, assuming the gate drive is there as needed.


--
--
forging knowledge

In article ,
Jim Adney wrote:
On 7 Jun 2004 18:43:19 -0700 Winfield Hill
wrote:

starfire wrote...

If you use a MOSFET in a circuit to function similar to a diode
in the conducting direction, remember most MOSFETs have the
built-in body diode which will conduct in a direction opposite
to what you want to rectify, so this may not be a desired result.

This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions,

That still keeps it from being much of a rectifier, however.


10V Push button
0.1Hz AC /
--------------/ O-------------- DC load

If I push the button only while the AC input is positive, I've made a
rectifier. The switch would conduct in both directions if I just held the
button.

--
--
forging knowledge

In article ,
Jim Adney wrote:
[...]
Thinking about this, I gather that you intend to run these MOSFETs
backwards, so that the "rectifier" is on when it is parallel to the
body diode and off when in the usual forward direction. Does that
work? I never considered the possibility....

Works good. Many DC-DC converters that run 1.8V logic use this method to
make a lower drop than a Schottky.

At 1.8V, 0.4V means you lose 22% of the power in the diode.


--
--
forging knowledge

On Thu, 10 Jun 2004 03:26:48 +0000 (UTC), the renowned
(Ken Smith) wrote:

In article ,
Jim Adney wrote:
[...]
Thinking about this, I gather that you intend to run these MOSFETs
backwards, so that the "rectifier" is on when it is parallel to the
body diode and off when in the usual forward direction. Does that
work? I never considered the possibility....

Works good. Many DC-DC converters that run 1.8V logic use this method to
make a lower drop than a Schottky.

At 1.8V, 0.4V means you lose 22% of the power in the diode.

Just Google on "synchronous rectification" for *lots* of info.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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