I've used SPICE for years to analyze the loop gain (gain and phase margin) of buck DC/DC converters and I am certain Spectre can do the same. However, I suspect you meant to ask about SpectreRF. To use Spectre, you would have to use a state space averaged model. The state space averaged model replaces the switch with an ideal DC/DC transformer. The more interesting approach, and probably the one you are asking about, is to apply SpectreRF directly to the time-varying model, perhaps even to the device level model. I think it would work for a pwm converter with a fixed switching frequency. I have always wanted to try it but never had the opportunity. I am not so sure about a resonant converter or any converter where the frequency can vary.

I have not tried this, but I think you should be able to use the method described in the article http://ieeexplore.ieee.org/xpl/abs_free.jsp?arNumber=900125 by simply replacing the AC sources with PAC sources. The article explains how Spectre's stb analysis is working.

Ken, how about implementing a pstb analysis in SpectreRF?

Richard,
I hope this is readable. I had some trouble with the web page.

I agree with you that parasitics affect the analysis but I ususally do the state space averaged models anyway because: (1) the more complex the system, the more I learn about it by building a state space averaged model;(2) I rarely know the parasitics, especially those of external magnetics, accurately enough to justify skipping the state space averaged model.

I will post this message in pieces to circumvent some of the problems I'm having with this website.

The above macro model is only good for the continuous conduction mode; it does not work if the inductor current periodically drops to zero amps. However, there are marcomodels that automatically switch between continuous and discontinuous state space averaged models on the fly.  I will list a reference by Chen and Rodriquez later.

For a simple voltage model converter, the duty cycle is proportional to the error amplifier output. For a current mode converter, duty cycle is linear combination of output voltage and inductor current.

The nonlinear state space averaged macro models simulate large signal transient behavior and SPICE automatically linearizes them for small signal AC analysis (gain and phase margin). To break the loop for gain and phase margin analysis, insert a DC voltage source in series with the duty cycle. Set the DC voltage to zero and make the AC magnitude unity. Run an AC analysis. The magnitude of the loop gain equals the magnitude of the ratio of the voltage on duty cycle side of the DC source to the voltage on the other side. The phase margin equals the phase of the same ratio.

I forgot to comment on the accuracy of state space averaged models. The state space averaged models start to break down between 1/10 and 1/2 the switching frequency. Also, as you said, parasitics can cause inaccuracies. For example, the state space averaged model can not simulate the effects of glitches in the sensed inductor current caused by the catch diode's reverse recorvery transient.

There are a few papers on sampled data effects that extend the accuracy of the state space model right up to half the switching frequency but since it is risky to push the loop gain's cross over frequency out that far, such papers are mainly of academic interest.

Most of the time, I found the state space averaged models predicted phase margins to within 5 to 20 degrees of measurements, depending on how well I knew the true parameter values.

For simulating power electronics I prefer SPICE. Matlab could do it but it is much easier to draw out the schematic for SPICE than to derive the circuit equations for matlab and re-invent the integration wheel.

As for state space averaging versus SpectreRF, I think it would be far easier to explain state space averaging than SpectreRF. I would think there are just as many ways, if not more, to get the SpectreRF simulation wrong as there are to get the state space averaged model wrong.  In the power electronics arena, state space averaging is a proven method of analysis that has been thoroughly documented over the last 30 years. I am not aware of any documentation about applying SpectreRF to power electronics. Ideally, one would use both state space averaging and SpectreRF and make sure the two results agreed. In my opinion, new methods should never make old methods obsolete, they should  instead provide an extra level of confidence. If I were the customer or technical reviewer, I would not trust an analysis from an engineer who simply plugged in the schematic, pressed the run button, then presented the results without some sort of check against classical methods.

One thing to keep in mind is that PAC will give you the small-signal gain. Stability will depend on the large signal characteristics. The large-signal characteristics may be the same as, or at least very similar to, the small-signal characteristics, but I don't know. Like Frank, I have never tried these simulations.

-Ken