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Feedback Loop: Calling a spade a spade

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Faringdon:
Thanks...

Do you agree, page 6 of the document “Practical Feedback Loop Design considerations for switch mode power supplies” is  somewhat general and non-commital in its attire?

Practical Feedback Loop....
https://www.pdffiller.com/jsfiller-desk10/?requestHash=c955daa6377db4d605d6ae3210244f1cf38b021219fe93e1df94e97d016d993b&projectId=1362206773&loader=tips&MEDIUM_PDFJS=true&PAGE_REARRANGE_V2_MVP=true&isPageRearrangeV2MVP=true&jsf-page-rearrange-v2=true&jsf-new-header=false&LATEST_PDFJS=false&routeId=2bc220699173ef97f6b3d8196f006eac&mode=force_choice#846a5be8c7734ec4a3736531a2e1f463


…..I am referring to equation (18) of page 6, ie, the value “Km = delta IDSpk/delta VEA”.
This “Km” value occurs in the Power stage transfer function of the BuckBoost in equation (21) of the same page.

However, the equation (18) is incomplete in its attire. So too is the diagram below it in figure (10).
The “Modulation Gain” that’s being referred to is often  actually called as  “R/Ri”

Where:
R = Load resistance
Ri = Current sense resistor in source of FET.

There is usually another gain associated with the modulator (actually an attenuation), which is the factor by which the “VEA” error amplifier output voltage is divided down before it is taken into the PWM comparator. Usually a factor of 0.25 or so. However, the page does not actually mention this.

The document TN-203...(at the bottom of page 9, for buckboost...)
“Voltage mode, current mode (and hysteretic control)”, as follows
https://www.microsemi.com/document-portal/doc_view/124786-voltage-mode-current-mode-and-hysteretic-control

..Gets the Modulation gain correct, they however, for whatever reason, call the Sense resistor, the “RMAP” …ie, the “PWM ramp voltage divided by the ramp current”…….this is Ohms Law, and is in fact the sense resistance , Ri.

However, they unfortunately don’t make any reference to the gain (attenuation) involving the dividing down of the error amplifier output voltage.

Do you know why none of these papers can “call a spade a spade”?

mtwieg:

--- Quote from: Faringdon on September 26, 2023, 07:16:01 pm ---…..I am referring to equation (18) of page 6, ie, the value “Km = delta IDSpk/delta VEA”.
This “Km” value occurs in the Power stage transfer function of the BuckBoost in equation (21) of the same page.

However, the equation (18) is nonsense. So too is the diagram below it in figure (10).
--- End quote ---
It's not nonsense. Km is simply defined as the change in peak current per change in error amplifier output voltage.


--- Quote ---The “Modulation Gain” that’s being referred to should actually be “R/Ri”

Where:
R = Load resistance
Ri = Current sense resistor in source of FET.
--- End quote ---
Substituting this for Km would give wrong results (unless you change something else in the equations).


--- Quote ---There is usually another gain associated with the modulator (actually an attenuation), which is the factor by which the “VEA” error amplifier output voltage is divided down before it is taken into the PWM comparator. Usually a factor of 0.25 or so. However, the page does not actually mention this.

The document TN-203...(at the bottom of page 9, for buckboost...)
“Voltage mode, current mode (and hysteretic control)”, as follows
https://www.microsemi.com/document-portal/doc_view/124786-voltage-mode-current-mode-and-hysteretic-control

..Gets the Modulation gain correct, they however, for whatever reason, call the Sense resistor, the “RMAP” …ie, the “PWM ramp voltage divided by the ramp current”…….this is Ohms Law, and is in fact the sense resistance , Ri.

However, they unfortunately don’t make any reference to the gain (attenuation) involving the dividing down of the error amplifier output voltage.

Do you know why none of these papers can “call a spade a spade”?

--- End quote ---
What your describing is indeed how a lot of peak current mode control chips operate, but certainly not the only way. How exactly these chips operate internally is beyond the scope of this review paper. It's treating the PWM modulator as a black box with some effective transconductance, which it calls Km. Just because it doesn't express this transconductance in the exact terms you're accustomed to doesn't make it nonesense.

jonpaul:
FTTS: Your posts are an infinite and unstable feedback loop.
  get some psyc counceling.  Will solve all your problems!

And save our time and bandwidth,

j

Faringdon:

--- Quote ---Just because it doesn't express this transconductance in the exact terms you're accustomed to doesn't make it nonesense.
--- End quote ---
Thanks.
Thanks, ayk, they have omitted the inevtable scaling factor that is used to scale the VEA, the error amplifier output voltage...this is an important feedback loop parameter, i believe you would agree.
As you well know, The VEA gets scaled before going to pwm comparator...then there is another "effective scaling factor" due to the fact the user may use a low value sense res, and at max load it may not peak at the threshold, but some way below it...this imprtant scaling, is omitted.....i am sure Choi knows it...but he doesnt say it.....the other paper fails to spell this out...but gives Ri, which accounts for it anyway

mtwieg:

--- Quote from: Faringdon on September 28, 2023, 06:05:35 am ---
--- Quote ---Just because it doesn't express this transconductance in the exact terms you're accustomed to doesn't make it nonesense.
--- End quote ---
Thanks, ayk, they have omitted the inevtable scaling factor that is used to scale the VEA, the error amplifier output voltage...this is an important feedback loop parameter, i believe you would agree.
--- End quote ---
No.

--- Quote ---As you well know, The VEA gets scaled before going to pwm comparator...
--- End quote ---
And in some cases the current sense voltage is amplified instead (or a combination of amplifying one and attenuating the other). And usually there is some offset thrown in somewhere before the PWM comparator. Regardless of how the internal circuitry works, you ultimately get some roughly linear relationship between error amplifier voltage and peak current. For the purpose of this review paper, that's what matters. There's no reason for them to look inside the black box.

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