Eugene,

I think without using large cap/ind to block the ac signal, the method is to use the stb (based on middlebrook's double null injection theorem) analysis in spectre. Essentially, if you break the loop at the input of error amp (assume infinite input impedance), you can place a voltage source in between with DC=0, AC=1, run the ac sim and plot the vout/vin to get the gain and phase plot.

I have not read the Ridley's article which talk about small signal modelling of current mode converter. I would be interested to find out how the average model for modelling the two loops. I just find that with the inner loop which has inductor current waveform, to be compared at the comparator, seems to be a large signal behavior, which is not obvious for the model.

Richard

**Eugene wrote on Sep 13**^{th}, 2006, 9:07am:I don't know of anyone who has fully demonstrated that PSS/PAC reliably computes the loop gain of a DC/DC converter. However, if you are using state space averaged models, you can compute the loop gain with AC analysis. If you want to break the current loop of a current mode converter, you will need to correct operating point, which means you will need a closed voltage loop too. If you insert your AC source (to break the loop) just before the DC/DC transformer, you break both loops. However, if you want to look at the current loop with the voltage loop opened in the AC sense, I don't know how to avoid using the larger inductor/capactor trick that everyone says is a bad ideal. I could be wrong, but I think Spectre's AC analysis is designed specifically for single loop systems.

In a multiloop system, it is often helpful to use a "sequential loop closure" procedure to assess stability. In the procedure, you examine the loops one at a time, usually from innermost to outermost. You start with all loops opened in the AC sense. After you assess a loop, you close it before assessing the next one. If you encounter an unstable inner loop, an outer loop can stabilize the system but to assess stability you must read the Nyquist criterion like a lawyer. A better way to assess stability would be to look at the closed loop poles. However, I have yet to see a very reliable pole/zero extraction tool for large circuits. Anyway, getting back to the sequential loop closure procedure, I'd be interested if someone knows of a better way to open the outer loops than to use large LC components.