Re: Designing a high current (10A) voltage buffer
- From: John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 28 Jul 2007 16:42:07 -0700
On Sat, 28 Jul 2007 13:20:15 -0700, Michael <nleahcim@xxxxxxxxx>
wrote:
On Jul 28, 1:52 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
You won't need the derivative term, and you can combine the integral
and proportional into one opamp by using a series r+c as the feedback
element.
Make the gate resistor 33 ohms maybe; it keeps the fet from doing RF
oscillations on its own.
John
Why skip the D? I mean, it doesn't hurt anything - when properly
tuned, it only helps...
It can be a huge noise amplifier, and gets quirky in a lot of
situations.
On another note - something's been bothering me - would it be possible
to make just a proportional controller?
Sure. But the opamp isn't ideal, and its gain will start to droop at
some frequency, so it's never going to be pure proportional. And a lot
depends on the load... if it's a pure resistor as shown, good, but any
load capacitance complicates life, and the fet itself has capacitance.
So the proportional gain may have to be very low to keep the loop
stable, so then you's need some integral to get accuracy.
This ain't simple. Keep in mind that the transcondance of the fet will
vary with current, so the loop dymamics aren't fixed. The safe thing
to do is to make a *slow* P+I controller, slow as in low KHz loop
bandwidth, if you can tolerate that.
I feel like if I set the I and
D gain terms to zero this thing would oscillate terribly, but looking
back through my control systems textbook P controllers are shown to be
stable, though they can have a steady state error.
So - where'd you get 33 from? That was one of those values that I just
didn't have a clue about...
Seems to keep most mosfets from becoming RF oscillators. Bigger values
work too, but may slow down the fet gain and complicate the loop even
more.
John
.
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