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29th July 2016, 17:33 #1
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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/...T250A00ds.pdf

29th July 2016, 18:06 #2
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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?
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29th July 2016, 18:14 #3
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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.

29th July 2016, 23:19 #4
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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.
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31st July 2016, 19:33 #5
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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.

31st July 2016, 23:53 #6
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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 nontypical applications.

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1st August 2016, 06:36 #7
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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/...T250A00ds.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.

1st August 2016, 08:15 #8
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Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?
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.

1st August 2016, 17:24 #9
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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.
PRM datasheet:
http://cdn.vicorpower.com/documents/...T250A00ds.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
..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.
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.
..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/...T250A00ds.pdfLast edited by treez; 1st August 2016 at 17:30.

2nd August 2016, 14:55 #10
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Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?
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..

2nd August 2016, 20:22 #11
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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.

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7th August 2016, 19:22 #12
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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 controltooutput 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?

8th August 2016, 18:40 #13
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Re: Vicor modules give brilliant feedback loop dynamics....too good to be true?
I'm not sure what sort of "confirmation" you can expect, unless you think someone here has direct experience with these modules. Without handson 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?
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