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

You may be losing yourself in unnecessary detail and may be missing the 
important parts of the big picture.

In most circumstances stabilization is realized by closing the loop well 
below 1/2 the switching frequency and the contribution of the double 
pole to the overall stabilized result may be ignored.  You should 
remember that in most designs, the bulk caps dominate from the mid audio 
range.  The two branch representation of a VRM is usually adequate:  one 
L/R for small signal and one for large signal.  If you miss the large 
signal response you could be unpleasantly surprised by what happens when 
you see a large load transient. 

I recommend you go ahead and build up the response including the double 
pole at F/2 with the bulk caps, and then compare that response with a 
simpler two branch model also with the bulk capacitors.  Do that first 
for small signal to convince yourself that the simpler model is 
adequate.  Then do it for a large transient to see where the two branch 
model shows you response you want to know about.

Steve.

Istvan Nagy wrote:
> 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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