Searching \ for '[EE]: FETS again, as a regulator?' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/index.htm?key=fets+again+regulator
Search entire site for: 'FETS again, as a regulator?'.

Exact match. Not showing close matches.
PICList Thread
'[EE]: FETS again, as a regulator?'
2003\02\19@154133 by Micro Eng

picon face
I'm looking at a schematic of a power supply where they they 1.8V and
'convert' to 1.2V, yet a single FET is being used, where the gate is being
driven by an opamp.  I am assuming that the voltage on the gate via the
opamp is being tweaked to adjust the voltage on the output. I suppose this
is similar to a PWM controller used in switchers that drive the gates to
buck the voltage.  True?  In other words, is this what is happening, in a
nutshell?

The FET being used is a FDP7030BL, logic level N channel device.

In looking over the datasheet, I don't really see exactly how this works.
From the other discussions I was led to believe that as you trigger the part
the amount of current flow based on the gate voltage. But in order to make a
voltage regulator out of it, wouldnt the load have to be well defined?  In
other words, an open circuit would present no load, thus no current flow.

So, if the voltage on the gate was set to a static value, how would this
create the voltage regulation?



_________________________________________________________________
The new MSN 8: advanced junk mail protection and 2 months FREE*
http://join.msn.com/?page=features/junkmail

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email spam_OUTlistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body

2003\02\19@163503 by Ned Konz

flavicon
face
On Wednesday 19 February 2003 12:33 pm, Micro Eng wrote:
> I'm looking at a schematic of a power supply where they they 1.8V
> and 'convert' to 1.2V, yet a single FET is being used, where the
> gate is being driven by an opamp.  I am assuming that the voltage
> on the gate via the opamp is being tweaked to adjust the voltage on
> the output. I suppose this is similar to a PWM controller used in
> switchers that drive the gates to buck the voltage.  True?  In
> other words, is this what is happening, in a nutshell?
>
> The FET being used is a FDP7030BL, logic level N channel device.
>
> In looking over the datasheet, I don't really see exactly how this
> works. From the other discussions I was led to believe that as you
> trigger the part the amount of current flow based on the gate
> voltage. But in order to make a voltage regulator out of it,
> wouldnt the load have to be well defined?  In other words, an open
> circuit would present no load, thus no current flow.
>
> So, if the voltage on the gate was set to a static value, how would
> this create the voltage regulation?

If they're driving the gate from an op amp, this is probably a circuit
where the 1.2V output is fed back to the inverting input of the op
amp, and a 1.2V reference is being fed to the non-inverting input.

The op amp will turn on the FET as hard as is necessary to make the
output stay at 1.2V.

--
Ned Konz
http://bike-nomad.com
GPG key ID: BEEA7EFE

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

2003\02\19@195200 by Peter L. Peres

picon face
The voltage is set by the Vgs thershold voltage, with a precision that
depends on the gain of the device (high gain = high precision).

Peter

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listservspamKILLspammitvma.mit.edu with SET PICList DIGEST in the body

2003\02\19@204024 by Russell McMahon

face
flavicon
face
> I'm looking at a schematic of a power supply where they they 1.8V and
> 'convert' to 1.2V, yet a single FET is being used, where the gate is being
> driven by an opamp.  I am assuming that the voltage on the gate via the
> opamp is being tweaked to adjust the voltage on the output. I suppose this
> is similar to a PWM controller used in switchers that drive the gates to
> buck the voltage.  True?  In other words, is this what is happening, in a
> nutshell?

If there is no inductor involved then the FET is used in a linear mode as a
resistor.
When turned partially on the Drain-Source channel looks like a resistance.
Varying Vgs varies this resistance.
The op amp is connected so as to compare the output voltage with the desired
voltage.
When the output voltage is too low (FET resistance too high) the Opamp
output rises and turns the FET on more to decrease its resistance and
thereby increase the output voltage.

Without any feedback the above would oscillate between FET hard on and hard
off with the opamp acting as a comparator. With feedback the amplifier has
finite gain and the system reaches a stable point where the FET gate is held
at the right voltage.

The beauty of this scheme is that the exact characteristics of the FET do
not need to be known in advance and they may change with eg temperature and
be adjusted for. The feedback cct always drives the FET to the desired
output point.
Which is what feedback is all about.



           Russell McMahon

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam.....mitvma.mit.edu with SET PICList DIGEST in the body

2003\02\20@082412 by Micro Eng

picon face
Now it makes sense...using the feeback to force the regulation.

Next question is...related to such a design, and the answer might be there
are always a dozen ways to get to the end....

Is there any sort of advantage to using this type of design for voltage
regulation over say a small LDO or other type of LinearTech or Maxim part
that is designed for a voltage regulator? Maybe its cost...and thats an
issue...


_________________________________________________________________
MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*.
http://join.msn.com/?page=features/virus

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspam_OUTspamTakeThisOuTmitvma.mit.edu>

More... (looser matching)
- Last day of these posts
- In 2003 , 2004 only
- Today
- New search...