Damping output filter of buck converter?

Question

Does anybody know if we need to damp the output filter of a buck converter? I came across this paper Passive Power Filters written by R. Künzi. In this paper he suggested an approach to damp the LC output filter, but I can not understand why we have to damp the output filter of a DC/DC converter! When we have the feedback loop,

1. It controls output impedance of converter. the open loop output impedance is divided by loop gain and presents closed loop output impedance. So there is no interaction between converters if we want to cascade to DC/DC converters.
2. Output voltage is constant, so we dont have any overshoot So, do we need really to damp the output filter of DC/DC converter?

For input filter, it is necessary, since it can endanger the stability of converter or for 2 stages output filter (LCLC), it is needed since second stage's peak can make the gain margin worst.

Some results: Regarding this article Stabilizing voltage mode converters with ceramic output capacitors, damping the output filter improves the phase margin. It is also useful to help us to prevent conditional stability, since the phase change is reduced by decreasing the quality factor.

Here are also a simulation with and without damping.

Answer 1

No, damping the LC filter of a buck converter should not be done.

A buck converter is an active device, and the paper you refer to is about passive power filters, and indeed when building LC filters for sensitive or noisy components like clock oscillators, the filter is damped by putting a resistor in parallel with the inductor.

Answer 2

The output LC section of a buck converter is often damped, but generally not with a discrete resistor. Instead, two capacitors are used, one (usually ceramic) with a low Equivalent Series Resistance (ESR), and another one in parallel with a higher ESR (usually an aluminum electrolytic cap).

If one neglects the other parasitic impedances except the ESR of the high ESR cap, the output section of such a buck converter looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

Damping in this way is discussed in this EE Times article as well as elsewhere.