Closed loop impedance of buck converter
In a switching converter, the input voltage and the output current are considered perturbations. The temperature or the air pressure could also be considered perturbations depending on the operating environment. The system strives to reject these disturbances for the purpose of keeping the output variable, \$V_{out}\$ for instance, in regulation. In order to reduce these perturbations efficiently, the system must exhibit gain: no gain, no feedback. In other terms, before the crossover frequency \$f_c\$ the system has gain and can fight perturbations while beyond \$f_c\$, the system operates in ac open-loop and does what it can to resist.

In the below sketch (excerpt from an APEC seminar I taught in 2012), I have represented a simplified view of the path including the output current perturbation. The open-loop output impedance \$Z_{out,OL}\$ is the impedance of the converter when operated without feedback. To determine \$R_0\$, the dc resistance of this open-loop impedance, you can go to the lab and fix a certain output voltage via the selected controller. Measure \$V_{out}\$ at two different close-enough output currents. The ratio of the voltage difference by the current difference gives the small-signal open-loop resistance.



If you determine the output voltage as a function of the reference voltage (which is 0 V in ac analysis) and the output current, you can see that the open-loop output impedance \$Z_{out,OL}\$ is now multiplied by a certain term to become \$Z_{out,CL}\$, the closed-loop output impedance. This term is called the sensitivity function represented by the letter \$S\$: \$S=\frac{1}{1+T(s)}\$. This is the effects of the feedback loop, it reduces the output resistance to almost 0 ohm with an infinite gain provided your compensator features a pole at the origin of course. However, as the equation indicates, when the gain drops as perturbation frequencies approach crossover, the system rejects less and less until it reacts as being in open-loop beyond \$f_c\$. If you add the input voltage \$V_{in}\$ in the right-side circle as a second perturbation and rewrite the expression to unveil the input voltage rejection or audio-susceptibility, you will see that it is also affected by the sensitivity function in the same way that \$Z_{out}\$ was.

You can return to the lab and measure the output resistance with the loop now operating. If the dc gain is not too high, you can measure a new \$R_0\$ which will be the first one determined in open-loop conditions divided by one plus the dc gain of your system. However, it is very unlikely - unless \$T_0\$ is low - that you can measure the voltage drop at two different close-enough output currents. However, this method employed with a network analyzer can let you determine the open-loop transfer function of a converter by measuring the output impedance in open- then closed-loop conditions. Then, using the sensitivity function, you can reconstruct the open-loop gain. It is a convenient method when you deal with black boxes and want to check phase margin and crossover frequency without having access to the internal circuits.