Eugene

Richard,
I list three references below. I am certain the first is out of print. I pick such books up at garage sales and used book stores. I don't know if the others are in print. All three are fairly involved. I don't think such rigor is necessary if you have a solid understanding of the Nyquist stability criterion (NSC). The NSC states that a SISO system is stable if the Nyquist plot encircles the (1,0) point CCW once for every RHP pole in the open loop system. ("open" refers the SISO loop in question). The trick to assessing a multiloop system as a sequence of single loop systems is to start with a known number of RHP poles. If we start with all loops open, the system is guaranteed to have zero RHP poles. As [3] suggests, it is sometimes most convenient to start with the highest bandwidth loop. For a current mode converter, that would be the innermost loop, the current loop. With all loops open, assess the loop gain of the first loop. It's ok if the first loop is unstable as long as you remember how many times the Nyquist plot encircled the (1,0) point in the CW direction. That number equals the number of RHP poles the system has with just the first loop closed. The NSC does not depend how the plant was constructed or how any RHP poles came about; we can apply the NSC to the second loop with the first loop closed and all the rest open. The same argument applies to the next loop, and the next, and so on. Again, the key is to keep track of the number of RHP poles you add or subtract as you close each loop. By construction, when we close the last loop, we know how many RHP poles the system has with all loops closed.
There are other methods better suited for evaluating relative stability, depending on the system architecture. For example, for the case of one converter driving another, if the source impedance remains far below the load impedance over all frequencies, each loop can be assessed independent of the other. However, I have often gained valuable insight into system stability by checking my alternate methods against the sequential loop closure method.
[1] John Truxal, "Automatic Feedback Control System Synthesis". McGraw HIll. 1955. Pages 147150.
[2] P. K. Sinha, "Multivariable Control, An Introduction". Marcel Dekker Inc. 1984. Pages 584592.
[3] J. M. Maciejowski, "Multivariable Feedback Design". AddisonWesley Publishing Co. 1989. Pages 137142.
