Current mode or Voltage mode SMPS?

[marginal]

Hello,

The following article states that current mode control is best, period:- (see first paragraph, left of man's picture)

http://www.powersystemsdesign.com/design_tips_june07.pdf

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The following article states that sometimes current mode is best, and sometimes voltage mode is best:-
(Click "Show the answer" for question 3 )

http://www.powerint.com/en/community/documents-quizzes/puzzler-7#answer2

Do you know who is right ?

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Also, with a forward converter, how can one have current mode control -since the current doesn't actually "ramp" during a switch ON cycle ?

 

[cabraham]

They are both right. CMC is better, but "better" may not always be needed. Some applications are fine w/ VMC.

Power Int. makes some valid points but I have issues with a few of their points. Regarding their "lossless sensing", they use the inherent Rdson of the internal power FET as a current sense element. The problem with this approach is that Rdson is not a well controlled parameter. It varies w/ temp & gate-to-source voltage. A low valued current sense resistor adds a little more loss but is much more precise & reliable as the ohm value is the controlled parameter. It has small temp coefficient.

Also another issue I have is that they claim that CMC has difficulty under light load to no load conditions. Like most pronouncements, this opinion does have some merit. If NO slope compensation is used, then the sensed current waveform under no load is too small to predictably control the servo loops. But, slope compensation should ALWAYS be used with peak CMC. Even if the duty factor never approaches 50%, slope comp provides other benefits. One such benefit is correction of peak to average error over varying duty factors. Another is that at no load conditions, the slope comp ramp provides control of the servo loop. The slope comp is resistively summed with the sensed current and fed into the control IC. Under no load, even if the sensed current signal vanishes, the slope comp provides the ramp to control the loop. It operates like VMC in these circumstances.

CMC is harder to implement due to its complexity. People in the learning stage should use VMC. Once mastered, then CMC can be attempted.

With forward converters, the primary current does indeed ramp during the on time. Have I helped? BR.

Claude