+ Post New Thread
Results 1 to 13 of 13
  1. #1
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Hello,
    We have just designed a SMPS (48Vin to 1V5 at 95A) using Vicor Modules as in the attached. The PRM48’ is the SMPS and the VTM48’ is a “Voltage Transformation Module”. (with NP:NS = 24:1)
    The PRM48’ switches at 1MHz.

    On page 38 of the PRM48’ datasheet they give the calculation for the feedback loop. I included a zero for the capacitor’s ESR to this and made out the bode plots as in the attached excel document. It is very easy to select feedback compensation components which give superb gain and phase margin at crossover frequencies running up to nearly 100kHz. This seems too good to be true.

    For example, the attached schematic gives a crossover frequency of 53kHz and a phase margin of 118 degrees. The gain margin is superb too, as can be seen.

    Is this too good to be true?


    VTM48EF020T080A00 datasheet:-
    http://cdn.vicorpower.com/documents/...020_080A00.pdf

    PRM48JH480T250A00 datasheet:
    http://cdn.vicorpower.com/documents/...T250A00-ds.pdf

  2. #2
    Advanced Member level 5
    Points: 40,641, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,551
    Helped
    1926 / 1926
    Points
    40,641
    Level
    49

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Since they mention several times that the "K factor" is
    24 (and evidently fixed?) you have to wonder whether
    there is in fact a control loop at all, to have stability
    concerns. This may be a simple fixed duty chopper and
    nothing but?


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    the VTM is indeed a fixed duty module, (an electronics transformer with ratio 24:1) but the PRM48' module is definetely with a feedback loop as in the pdf of the top post.



  4. #4
    Advanced Member level 5
    Points: 20,841, Level: 35
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,398
    Helped
    1213 / 1213
    Points
    20,841
    Level
    35

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Not sure why one should be surprised at fc at just a tenth of the switching frequency. Adding the voltage transformers will likely degrade the available bandwidth substantially, though.


    1 members found this post helpful.

  5. #5
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Thanks, you say that adding in the voltage transformers (VTM48’ modules) will degrade the bandwidth, but the feedback loop calculation spoken of in the top post includes those VTM48’ modules…….these modules, as you know, have to be accounted for in the power stage transfer function….and we have to refer the capacitance at the output of these VTM48’ modules to the modules primary side…..also, the load resistance itself has to be referred to the primary side of the VTM48’ module.

    So anyway the wonderful feedback loop properties that I spoke of does include those VTM48’ modules, so I am not sure what you mean when you say that they will degrade the bandwidth.

    And yes, whilst i appreciate that 53khz (for crossover frequency) is well below 0.1*f(sw) in this case, , its still a relatively marvellous crossover frequency.....thinking as you know of a typical opto based mains power supply which would be around 1khz.



  6. #6
    Advanced Member level 5
    Points: 20,841, Level: 35
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,398
    Helped
    1213 / 1213
    Points
    20,841
    Level
    35

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Your spreadsheet seems to imply you think the combined power stages will have an overall transfer function dominated by one pole and one zero. Don't you think that's a bit optimistic for two converters cascaded in series?

    50kHz is pretty high by typical standards, though I'm guessing that if they switch at 1MHz then they're meant for non-typical applications.



    •   AltAdvertisement

        
       

  7. #7
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    The VTM48 module is a fixed ratio voltage converter and so the feedback loop for that is not so complicated. The external error amplifier drives into the PRM48 module.

    - - - Updated - - -

    we have set our system up like the schematic on the bottom of page 2 of the PRM48' datasheet...

    http://cdn.vicorpower.com/documents/...T250A00-ds.pdf

    Your spreadsheet seems to imply you think the combined power stages will have an overall transfer function dominated by one pole and one zero.
    Page 36 of the datasheet linked in this post says exactly that....they show the dominant pole....as you know, since we do not know what is actually inside the PRM48' module, we have to just accept what they tell about it regarding the power stage small signal transfer function.



  8. #8
    Advanced Member level 5
    Points: 20,841, Level: 35
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,398
    Helped
    1213 / 1213
    Points
    20,841
    Level
    35

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Quote Originally Posted by treez View Post


    Page 36 of the datasheet linked in this post says exactly that....they show the dominant pole....
    That's just for the PRM part. If it's a current mode controlled supply (the datasheet does not specify), then a single pole may be a reasonable approximation. But the VTM parts are described as constant duty cycle, so they should be expected to have at least a conjugate pole response. The AC model in the datasheet is obviously not plausible, as it suggests a bandwidth higher than the converter's switching frequency.



  9. #9
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    The AC model in the datasheet is obviously not plausible, as it suggests a bandwidth higher than the converter's switching frequency.
    Thanks do you mean the AC model on page 29 (figure 24) of the PRM datasheet?.....
    PRM datasheet:-
    http://cdn.vicorpower.com/documents/...T250A00-ds.pdf

    - - - Updated - - -

    But the VTM parts are described as constant duty cycle, so they should be expected to have at least a conjugate pole response
    Thanks, but the VTM48' module can be simply considered in the feedback loop calculation as a simple ideal transformer with NP/NS = 24/1
    ..We dont need to consider any poles or zeros in relation to the VTM module and the feedback loop calculation. That is, the feedback loop calculation of the schematic which is like the bottom diagram on page 2 of the PRM48' datasheet...i appreciate the poles and zeros of the VTM48' module are there...but we neednt consider them as the below extols....

    Page 12 of the VTM48’ datasheet explains that partly because the VTM48’ is switched at such high frequency, we can ignore the poles and zeros associated with it, as they will be well beyond our crossover frequency..

    The VTM48’ module can be treated simply in the feedback loop as an (almost) ideal transformer…

    As page 14 of the VTM48’ datasheet says…
    The SAC topology bases its performance on efficient transfer
    of energy through a transformer without the need of closed
    loop control. For this reason, the transfer characteristic can be
    approximated by an ideal transformer with some resistive drop
    and positive temperature coefficient.
    …so we can see that the VTM48’ is very easy to handle from the feedback loop point of view…just an “ideal transformer” (almost) in the power path..



    The following from page 13 of the VTM48’ datasheet confirms that the VTM can just be considered as a simple transformer component…
    The overall AC impedance varies from model to model. For most models it is dominated by DC ROUT value from DC to beyond 500 KHz.
    …and the crossover frequency of the PRM/VTM converter is going to be well under 500khz, so the VTM48’ module can be thought of as simply an “ideal transformer” in the power path….meaning it is very simple to handle from the feedback loop calculation point of view.
    ..All we need to do is refer the capacitance and the load Resistance at the secondary of the VTM48 module to the primary of the VTM48’ module….As such, the VTM48’ module introduces no real complexity to us in the feedback loop calculation..

    VTM48’ datasheet
    http://cdn.vicorpower.com/documents/...020_080A00.pdf

    PRM48' datasheet
    http://cdn.vicorpower.com/documents/...T250A00-ds.pdf
    Last edited by treez; 1st August 2016 at 17:30.



  10. #10
    Advanced Member level 5
    Points: 20,841, Level: 35
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,398
    Helped
    1213 / 1213
    Points
    20,841
    Level
    35

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Quote Originally Posted by treez View Post
    Thanks do you mean the AC model on page 29 (figure 24) of the PRM datasheet?.....
    PRM datasheet:-
    http://cdn.vicorpower.com/documents/...T250A00-ds.pdf
    No I'm referring to figure 14 in the VTM datasheet.

    Thanks, but the VTM48' module can be simply considered in the feedback loop calculation as a simple ideal transformer with NP/NS = 24/1
    ..We dont need to consider any poles or zeros in relation to the VTM module and the feedback loop calculation. That is, the feedback loop calculation of the schematic which is like the bottom diagram on page 2 of the PRM48' datasheet...i appreciate the poles and zeros of the VTM48' module are there...but we neednt consider them as the below extols....

    Page 12 of the VTM48’ datasheet explains that partly because the VTM48’ is switched at such high frequency, we can ignore the poles and zeros associated with it, as they will be well beyond our crossover frequency..

    The VTM48’ module can be treated simply in the feedback loop as an (almost) ideal transformer…

    As page 14 of the VTM48’ datasheet says…

    …so we can see that the VTM48’ is very easy to handle from the feedback loop point of view…just an “ideal transformer” (almost) in the power path..
    I see no reason to take the datasheet's claims at face value, without some empirical plots of its frequency response or a detailed schematic. The notion that a power supply's AC response is dictated by inductances <1nH is just nonsense to me.



  11. #11
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Thanks, i mean i can certainly see your point...for the VTM though , i can believe that it can be considered as just a transformer in our application which has crossover at ~60khz and no higher, partly due to the really fast switching frequency, and the act that its not regulated as such...just puts vout at 1/24th of vin.



    •   AltAdvertisement

        
       

  12. #12
    Advanced Member level 5
    Points: 34,256, Level: 45
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,117
    Helped
    514 / 514
    Points
    34,256
    Level
    45

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Woops I am guity of dullardness…those error amp poles and zeros would be unlikely to allow that high crossover, I put a typo in the excel document…..but still we are curious about the equation on page 36 of the PRM datasheet being the crux of the control-to-output transfer function.
    Corrected excel sheet is attached, but still we would like double confirmation of the main pole frequency of page 36.
    Its interesting that the PRM is called a “buckboost” regulator, and buckboosts have a right half plane zero which limits your crossover frequency selection…but the PRM datasheet says nothing of a right half plane zero. Why not?



  13. #13
    Advanced Member level 5
    Points: 20,841, Level: 35
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,398
    Helped
    1213 / 1213
    Points
    20,841
    Level
    35

    Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?

    Quote Originally Posted by treez View Post
    Woops I am guity of dullardness…those error amp poles and zeros would be unlikely to allow that high crossover, I put a typo in the excel document…..but still we are curious about the equation on page 36 of the PRM datasheet being the crux of the control-to-output transfer function.
    Corrected excel sheet is attached, but still we would like double confirmation of the main pole frequency of page 36.
    I'm not sure what sort of "confirmation" you can expect, unless you think someone here has direct experience with these modules. Without hands-on experience you only have the datasheet, take it or leave it.
    Its interesting that the PRM is called a “buckboost” regulator, and buckboosts have a right half plane zero which limits your crossover frequency selection…but the PRM datasheet says nothing of a right half plane zero. Why not?
    It seems the term "buckboost" has been adapted to converters which can function either as a buck or boost (but never a buckboost in the typical sense) using an H bridge of FETs. So the transfer function will greatly depend on the operating point of the converter. The datasheet does not show this, probably because they assume nobody will try to design for a crossover frequency high enough for it to matter.


    1 members found this post helpful.

--[[ ]]--