Electronics > Power/Renewable Energy/EV's

Feedback Loop: Calling a spade a spade

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Hi, So anyway, On the subject of this post, it is still considered that feedback loop docs on the web are very poor.....

As such, Why is it that the Feedback loop books/docs do not properly explain how to do feedback loop
calculation for eg an offline flyback?
The first job is to convert the Flyback to its "equivalent buckboost".
Best to "refer everything to secondary" in this process. (will not explain why here)

So basically you find that buckboost which has the ......
a) same duty cycle
b) same vout
c) same iout (max)

The inductor of this "equivalent buckboost" will be equal (in inductance) to the secondary inductance of the flyback.

You simply find the "effective input voltage" of the buckboost which allows you these requirements.
Then you have your "equivalent buckboost". You can then shovel its values into the Power Stage transfer function
of the feedback loop.

So, Supposing its in CCM, You then subject it to the power stage transfer function on page 9, fig 8 of TN203...


....But in fig 8, we are told that "RMAP" is "PWM ramp voltage divided by the corresponding sensed current".
-But This just isnt a good explanation, its not just the amplitude of the "ramp" bit...its the actual peak current
value that goes into the calculation of RMAP.

Also, we have to remember that we converted our flyback to a (equivalent) buckboost, and in many cases the amplitide of the sensed current will now be much bigger than it
was when we had the original flyback. However (and this isnt explained anywhere on the web)...you must still calculate this bit as if you were still
working with the original flyback, and not the "buckboost equivalent". So in other words, the RMAP will be actually be the value of the
primary sense resistance
used in the flyback's FET source connection. But the docs dont tell you this....they leave students for dead by telling them to convert to the
"equivalent buckboost", and then leaving them lost from there on.

The docs also dont tell you that you must put in the attenuation factor that is exhibited by the attenuation that the output voltage of the error
amplifier gets exposed to in order to give the peak voltage into the PWM comparator. (usually about 0.3 or so) And of course, this is from the original flyback situation, since
this situation didnt really exist at all with the "equivalent buckboost" situation, because there never really was an "equivalent buckboost" on the

Obviously your optocoupled error amp calculation will take the form of page 19 of the TND381 App note...


So we see students getting well lost with all this, and we have no texts or docs to refer them to...do you know of any?

Andy Chee:

--- Quote from: Faringdon on February 15, 2024, 09:58:23 pm ---So we see students getting well lost with all this, and we have no texts or docs to refer them to...do you know of any?

--- End quote ---
I found Christophe Basso's books gripping reading:



usual FTTS garbage. only FTSS has this issues.

Problem is psyc not engineering.

Farringdon Read  a book recommend "Control System Theory" DelToro, etc. 1971


Thanks, i cant find that book  to order online.
I have the two Basso books  that Andy Chee kindly recomends....i am of the impression that these two are the best out there on this kind of subject?
One of them has fallen to bits from such high useage.

 Principles of Control Systems Engineering
 Del Toro, Vincent
 Parker, Sydney R.
 New York et al.: McGraw-Hill Book Co., 1960

Any university engineering library should have a copy.

Happy reading !

Del Toro taught my CCNY course about 1971



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