[SI-LIST] Re: small signal AC model for Current mode DC_DC converters, for full PDN simulation

Istvan:
Regarding the RLC lumped models for decoupling caps, to avoid errors in 
Electrolytic capacitors (Tantalum/Aluminum and polymers of both) it is 
helpful to plot a more complex model that considers the RC ladder effect 
(to mimic the capacitance loss with the increased frequency). Also for low 
ESL<1.2nH it is important to include a RL network to mimic decaying ESL 
with increasing frequency.

You can import the Electrolytic models that takes RC ladder effect into 
consideration from the site: 
http://www.kemet.com/kemet/web/homepage/kechome.nsf/weben/kemsoft

Instructions:
http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/KEMETSpiceman361x.pdf/$file/KEMETSpiceman361x.pdf
 

 
Kind Regards
Allen Mayar



Be gentle with your ecosystem, print only when necessary


 



"Istvan Nagy" <buenos@xxxxxxxxxxx> 
Sent by: si-list-bounce@xxxxxxxxxxxxx
03/22/2009 06:22 PM

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Subject
[SI-LIST] small signal AC model for Current mode DC_DC converters, for 
full PDN simulation






Hi

I would like to model a power distribution system in frequency domain with 
a 
spice-like AC analysis, with all elements of it, together.
For decoupling capacitors, I can use RLC lumped models, for IC pins 
something similar, for power planes and package planes, I can use an 
electromagnetic simulator to create a touchstone file that I can import 
into 
the program (QUCS, or Agilent ADS).
But, for the voltage regulator, I think I have to create a small signal 
model to include in the simulation as a subcircuit. This model must have 
the 
same output impedance versus frequency response, as the original DCDC 
converter has, nothing else hast to be the same (switching circuits are 
not 
needed in the model, nor correct voltage levels). I dont want to model the 

DCDC converter with a single inductance or similar model, but put the 
whole 
control loop equivalent circuit into the simulation. the whole thing is 
described here: 
http://www.buenos.extra.hu/download/PowerIntegrityDesign_prj.rar (there is 
a 
pdf in it, and some circuit files) The "circuit" is all the models and 
elements of the PDN together.

For a voltage mode converter, I think it is quiet straightforward how to 
make the equivalent circuit ( 
http://www.buenos.extra.hu/download/voltagemode.jpg ), but for a current 
mode converter, it is trickier. My assumption was this: I read somewhere 
that a current mode converter has a double pole at the half the switching 
frequency, and a single pole at the 1/(2*TT*R_load*C_out). So, based on 
this, I made an equivalent circuit which has the same poles and no zeroes, 

so the same transfer function: 
http://www.buenos.extra.hu/download/currentmode.jpg The circuit element 
parameters are automatically calculated based on the provided switching 
frequency, load current, some other elements are coming from the original 
schematics, like the compensation RC networks... Maybe this way of 
modelling 
is not perfect for this purpose, but I think it's better than just using a 

single inductor (or an RLC model) for representing the whole DC/DC, or 
than 
guessing about the transient response.

Is this model correct, or if not, how should I make it to be correct?
I am not shure in this part.

regards,
Istvan Nagy
CCT, UK

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