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Dennis Feucht

Seemingly Simple Circuits, Part 1: The Current-Mode Switching Converter

Dennis Feucht
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D Feucht
D Feucht
12/28/2014 11:33:12 PM
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Re: compensation network
"... current and voltage feedback modes."


A mode is a change in circuit structure. The analysis of this series is for the peak (or valley) current-controlled converter with an outer voltage control loop. There is such a thing as a voltage controlled converter, wherein the output voltage controls the PWM duty ratio, but that is a different control scheme than the one being developed here. In the scheme developed here, the duty ratio is controlled by the peak current, which controls the converter power-stage output current, which affects the voltage, which is fed back and affects the peak current level that affects the duty ratio, thus closing the loop.

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amrutah
amrutah
12/27/2014 6:26:50 PM
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compensation network
Dennis, Thanks for the converter series... Very eager to learn and understand current and voltage feedback modes. Will hope to learn the compensation for these feedback networks.

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D Feucht
D Feucht
12/18/2014 9:06:06 AM
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Re: Help or Hinder
Ken,

I find it vastly simplifying to formulate the circuit analysis with the nested loops. So have major contributors in the past. Ridley's breakthrough paper of the early 1990s does this. By separating the current and voltage control loops, an example of problem reduction occurs, by decomposing it into two separate control problems to be solved: first the inner current loop, then the outer loop. The current loop is thus a block in the overall voltage loop, and as we will see in later parts of the article, it affects behavior around the Nyquist frequency - relatively high-frequency behavior relative to the switching frequency. The voltage loop bandwidth is much below this, and consequently separates in frequency the effects of each of the loops.

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kencoffman
kencoffman
12/17/2014 12:12:19 PM
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Help or Hinder
Dennis, I wonder if categorizing the V feedback and I feedback into categories helps in understanding, or hurts. What I'm thinking: if the V feedback and I feedback add together in the same direction, that's simply a gain modulation thing. If, in any case, the I feedback subtracts from the V feedback, well, that's a gain degradation. I'm not explaining my thougth very well. Regardless, I'm not sure if the feedback is so complex and variable depending on operating state, that a simplification aids proper analysis or leads off into the weeds.

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uchiha
uchiha
12/17/2014 12:01:12 PM
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Re: A Matter of Some Debate
enjoyed the discussion too.although the content is comlocated the effective understanding will help to sort out the matters easily

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uchiha
uchiha
12/17/2014 11:58:51 AM
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Re: Good that you picked this one
Understanding will be effective if it is done in the correct way. Because it can increase the effectiveness of the application. Wrong understanding may cause for a loss.

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Victor Lorenzo
Victor Lorenzo
12/16/2014 2:48:59 PM
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Re: Good that you picked this one
Thanks Dennis for bringing us this series. This converter represents one of those apparently simple circuits that become surpricingly complex to fully characterize and understand. in converters theory and design we must go far beyond calculating some operating points and some time constants and doing some SPICE simulations.

I am looking forward to descending for the next layer.

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DaeJ
DaeJ
12/11/2014 8:59:02 PM
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Re: Good that you picked this one
To analyze this circuit, engineer could simulate this circuit in the pspice. Once fully understanding it, this circuit could be used to any application. Possibly, as one of application, current might be measured cross Rs. It could be imaged that this circuit could be used in the measurement tool.

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D Feucht
D Feucht
12/11/2014 8:17:54 PM
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Re: Good that you picked this one
Nav-

I hope that I do not disappoint you in that "control op-amp circuitry" is usually found in the outer voltage-regulating loop. To understand it - a continuous, unsampled part of the overall converter - ordinary classical control theory of the kind that is presented in undergrad active-circuits textbooks is sufficient. At least the principles are there, and can be found more clearly explained in most introductory control theory textbooks.

But the inner peak-current control loop - that is more complicated, though it has few parts. This series is about it. Perhaps in the future I will do a series on ordinary feedback control because its application to circuits can be explained  more clearly than it is in most textbooks. I have not written articles about it because I already did in my book, Designing Amplifier Circuits, at

sci.presswarehouse.com/Books/SearchResults.aspx?str=Feucht

The dynamics of feedback loops continues to be developed in the succeeding volumes, especially in Designing Dynamic Circuit Response.

NOTE OF CORRECTIONS: Two name typos in my previous entry:

"Tom Hegarty" should be Tim Hegarty

"Free Lee" should be Fred Lee. My apologies to both persons for my bad typing.

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Navelpluis
Navelpluis
12/11/2014 2:13:11 AM
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Newbie
Good that you picked this one
Hi Dennis, just to encourage you: Good that you picked this one. Indeed it is a nasty one. Peeling the circuit off shell by shell is the only way to really understand this one. For younger engineers out there a *must follow*, I would say. I am sure you have a good method about to get the control opamp circuitry explained, can't wait ;-)

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