Boris,
The VRM is a nonlinear device so if you want to model it as a linear circuit
using a simple RL circuit you may want to extract the R and L from an impedance
frequency profile measured at similar load currents that match the system power
consumption. I have done this using PIScanner for FPGA systems and I've got
different impedance frequency profiles at different current loads and different
PDN structures. For example, the VRM loop bandwidth and location of the VRM
voltage sense point make a significant difference. If you do this measurement
taking in consideration all involved factors your RL model may be more
accurate. A simple "small-signal" AC simulation on a VRM linearized model
typically does not produce an accurate result. I have documented some details
of my experiments in app notes on this web site http://piscanner.com/support if ;
you or anyone else is interested.
Regards,
Cosmin
http://www.piscanner.com
On Monday, February 21, 2022, 06:42:43 AM PST, Boris Bakshan
<bbakshan@xxxxxxxxx> wrote:
Thank you Charles,
I will go ahead and start reading all references. The AC analysis process
that you mentioned, can this be done with Simplis?
On Mon, Feb 21, 2022 at 4:13 PM Charles Hymowitz <charles@xxxxxxxx> wrote:
Boris
You would perform an AC analysis and look at the output impedance (AC 1
source at the output). In some cases, the output capacitance you use can
mask the impedance of the regulator so you need to be careful. Always plot
the impedance in the on and off states to see what is passive and what is
part of the active loop. However, an RL model is often not sufficient to
model many regulators, especially those with multiple internal loop.
The other, and probably more important issue is that vendor models are
generally not accurate for output impedance. We generally get our own data
and make our own models. TI has started to be better at this, but almost
all the other VRMs we have tested from all manufacturers have not matched
the model. Most were not even close, especially older regulators.
Here is some material we have published about a better approach which is
to use a state space average model if your simulator allows it. If it only
allows an RL that could be an accuracy limitation or you will have to
change the RL at each current you want to simulate.
Learn about Power Supply/VRM Modeling for Switchers and Linears
VRM Modeling: A Strategy to Survive the Collision of Three Worlds
https://www.signalintegrityjournal.com/articles/1017-vrm-modeling-a-strategy-to-survive-the-collision-of-three-worlds
Measurement Based VRM Modeling, 2017 IEEE 21st Workshop on Signal and
Power Integrity (SPI) https://ieeexplore.ieee.org/document/7944009
https://www.picotest.com/downloads/Measurement-Base-VRM-Model-Tutorial-Final-2017.pdf
Video: Measurement-Based, Harmonic Balance VRM Model
https://www.picotest.com/blog/video-measurement-based-harmonic-balance-vrm-model/
Characterizing and Selecting the VRM
https://www.signalintegrityjournal.com/articles/831-characterizing-and-selecting-the-vrm
Characterizing the VRM
https://event.on24.com/eventRegistration/EventLobbyServlet?targetþg20.jsp&partnerrefÿD3SS&eventid
52277&sessionidÿkeyâ°E0C982E79AD60E6B066ACF5CBEFA78®Tagÿourcepageþgister
Measuring and Interpreting Impedance Data
https://event.on24.com/eventRegistration/EventLobbyServlet?targetþg20.jsp&referrerÿtps%3A%2F%2Fwww.signalintegrityjournal.com%2Fevents%2F99-measuring-and-interpreting-impedance-data&eventid
49545&sessionidÿkeyÃ37B34BE8615E5EA7FEB1101C3D0BF8®Tagÿourcepageþgister
Charles Hymowitz - Managing Director
AEi Systems
Charles@xxxxxxxx
(310) 216-1144
(310) 863-8034 (M)
http://www.aeng.com - Analytical Heavy Lifting
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf Of Boris Bakshan
Sent: Monday, February 21, 2022 5:43 AM
To: SI-LIST <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Extracting R-L model of a VRM
Dear SI-Listers,
I have a Simplis model of a VRM and I would like to extract the active
output inductance & resistance for use in a simple series R-L regulator
model.
Is there an easy way to extract it in Simplis or with a different EDA tool?
Thanks a lot!
Boris
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