Buck converter with external error amp and negative feedback?

[Flyback]

Hello,

I have implemeneted an external error amp with this 12V, 10A buck converter with an op amp (U3) followed by an NPN (Q1).

The error amplifier feedback is taken into the "non-inverting input" of the op amp due to the fact that the NPN (Q1) does an inversion.

Is this OK?......i always thought negative feedback had to go to the inverting input of the error amp?
(the error amp is external as this buck will be paralleled with ten other bucks all feeding the same 100A load...just one error amp feeds all controllers)

Schematic (also provided in pdf)
https://i39.tinypic.com/317ccx5.jpg


LTspice sim also attached

 

[ronsimpson]

I think using the non-inverting input is not commonly done because the gain can not go below one.

The TL494 is like that. I never had a problem with it. See Figure 20 in the attached.

 

[Flyback]

I think using the non-inverting input is not commonly done because the gain can not go below one.

thanks but are you sure....the shown error amplifier is , i thought, entirely equivalent to an inverting amplifier, gain = RF/RI. ?

Thanks for TL494, the TL431 also has the feedback going to the non-inverting input, and a transistor output stage....

https://www.electro-tech-online.com/custompdfs/2013/04/TL431A.pdf
(pic on top of page 2)

.....i really think that it is the same as an inverting amplifier, exactly the same, surely?

 

[ronsimpson]

You know spice. Do a frequency sweep of two error amplifiers. (inverting and non-inverting The non-inverting amplifier can not have a gain less than 0db. The inverting amp will have a high gain at DC and low frequencies and the gain will hit zero at the same spot but continue negative.

 

[Flyback]

The non-inverting amplifier can not have a gain less than 0db

yes but the error amplifier in the top post is "inverting"

 

[moffy]

It is inverting if the phase shift through the amp is 180 degrees, as you already assume. Since the transistor inverts the output of the op amp it needs to go to the +ve input to be stable, i.e. total phase shift 180 (I am ignoring the phase shift introduced by the passives for simplicity).

 

[Flyback]

thanks, this is fascinating to me, that the (+) input can get the feedback and yet its overall negative feedback...

 

[ronsimpson]

yes but the error amplifier in the top post is "inverting"

Yes you are right. By inverting the output you invert the inputs.

There is a topic about non-inverting error amplifiers. I got off on the wrong subject. (inverted output) I did not get off on the right subject.

Edit:
The TL431 is drawn a little confusing. The 'transistor' is inside the amplifier, not on the outside as drawn.