FreeLists / si-list / [SI-LIST] Re: VRM

[Istvan Novak <istvan.novak@xxxxxxxxxxx>]

Mohamad,

Unfortunately VRM models are not easy to get.  You usually can get 
models for controllers of VRMs, but this helps you only if you build 
your own VRM or know the entire schematics of the VRM you use.  If you 
use or plan on using off-the-shelf DC-DC converters, you may very well 
need to do the characterization yourself and from the characterization 
data, create your own model.  You can do the characterization in the 
time domain with an electronic test load that you can modulate, or do 
small-signal characterization in the frequency domain with a suitable 
Vector Network Analyzer setup.  In either way you need to remember that 
the converter has an active loop in it, where your external output 
capacitors are within the feedback loop.  This means that your best 
chance to get the characterization correct is to use approximately the 
same capacitors that you intend to use with the converter in your 
application.  For this reason sometimes you need to repeat the 
characterization with different sets of output capacitors.

You can get some ideas about the small-signal frequency domain 
characterization from
"Frequency Domain Power Distribution Measurements - An Overview" posted 
on http://home.att.net/~istvan.novak/papers.html.  You may also find 
paper 11-WP1 from DesignCon 2007 interesting: "Characterization and 
Output Filter Optimization Methods for Voltage Regulator Modules".  It 
describes a time-domain approach.

Once you have the characterization data, you will have to fit your 
selected model onto the data.  In the simplest for, a single inductor or 
series inductor-resistor may.  Dr. Howard Johnson described a simple 
three-element model in his "Voltage Regulator Droop" article, EDN, 
September 14, 2006.  These simple models have limited bandwidth and they 
usually severely overestimate the peaking between the converter and 
external capacitors.  If you want more accurate, but still easy-to-use 
linearized small-signal models, you can follow the procedures in Section 
9.2 of the book "Frequency-Domain Characterization of Power Distribution 
Networks".  All of these models would run equally in transient or AC 
simulations.

BTW: thanks everyone giving feedback on the above book; now in addition 
to the extended table of contents, an errata sheet is also posted at: 
http://home.att.net/~istvan.novak/Novak-Miller_TOC_v4.pdf.

Regards,

Istvan Novak
SUN Microsystems


Mohamad Haghtalab wrote:
>  
>   Hi all
>    
>   for PI(power integrity) simulation I need VRM parameters of voltage 
> regulators in my PCBs
>   How can I extract them?
>    
>   Thanx for your help
>        
> ---------------------------------
> Be a better friend, newshound, and know-it-all with Yahoo! Mobile.  Try it 
> now.
>
>   


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