Re: Feed Forward / Slop compensation

On Feb 21, 8:36 am, reggie <veggie...@xxxxxxxxxxxxxx> wrote:
--------quote-----------------------------
Feedforward is a method used to reduce the effect of a disturbance on
the output without running into trouble with oscillations.
-------- end quote-----------------------------

1) Don't you mean input not output otherwise it would be feed back?
Could you give me a circuit example? Sorry its hard with no attachment
possibility on this forum, could you provide me with a link to a
feedforward circuit diagram?

I should have said "effect of a disturbance somewhere as seen on the
output". The "on the output" applies only to where you look for the
effect.


--------quote-----------------------------
Slope compensation is a method used to prevent oscillations in
converters such as the classic booster where the inductor current is
continuous. ACII art:
L1 D1
---)))))----+---->!------+-- Output
! !
O Q1 === C1
/ !
! GND
GND
While Q1 is on, current builds up in L1 but no current goes through
D1. This means that turning on Q1 sooner or for longer to increase
the output voltage actually reduces the output voltage in the short
term. This can lead to oscillation in any servo loop which has a
high
bandwidth. In a "voltage mode" PWM, this is prevented by reducing
the
bandwidth.
-------- end quote-----------------------------

Have you not just described Right half plane zero in your continuous
current boost circuit not slope compensation?

I explained the problem slope compensation is fixing. I was trying to
make clear where the problem is. It is in the fast feedback path of
the current sense not in the voltage feedback path.

Continuous current Boost
and fly back converters can only be stabilised by heavily damping the
voltage element of the feedback loop, be it current mode control or
voltage mode control.

You have missed the point. The current mode control loop without
slope compensation will oscillate even if you integrate the voltage
path to death. The current comparitor to the pass transistor is a
fast feedback path.


-------- quote-----------------------------
Now imagine that the pilot suddenly opens the throttles
-------- end quote-----------------------------

The Line in the PSU is the throttle, yes? Are you a piolet and
engineer?!

The Line input is like the throttle but the Line input is much less of
a problem than the throttle.

I don't want to get on the plane regardless of which seat. As a
result becoming a pilot wasn't really an option.


-------- quote-----------------------------
Feed forward in this case is an input taken from the throttle setting
The Line deviation in a PSU
-------- end quote-----------------------------

I think this is the same as below, from my first post:
-------- quote-----------------------------
In another document (circuit diagram in fact) they pull up and down
on
the current sense pin magnitude depending on the rectified line
voltage
-------- end quote-----------------------------

From what you have said I think feed forward takes a quantity (line
voltage) and compensates the fast current control system before the
slow voltage control system knows what has happened, this has the
effect of changing the slope of the primary current, but using a real
quantity as apposed to a fixed ramp graphical method of controlling
the primary currents slope.

Am I making any sense?

You are making sense but you still seem to have topics confused. Put
slope compensation in one room and feedforward in a different one.
The feedforward usually won't be changing the ramp. It normally will
change the other leg of the current sense comparitor. Since the
comparitor looks at the difference, instead of raising one you can
lower the other.


.

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