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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. enter image description here

Here are also a simulation with and without damping.

enter image description here

enter image description here enter image description here

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  • \$\begingroup\$ By damping, do you mean a resistor over the inductor? \$\endgroup\$ – Justme Jan 31 at 21:13
  • \$\begingroup\$ The damping mostly refers to transient moments, not steady-state. So when switching loads, varying currents, or input voltages, etc come into play, the way the output filter is built matters. You don't want an overshoot to burn your CPU, no (quick example). It's a matter of design, purpose, which means just because someone wrote that the output filters can be damped, it doesn't mean that they should, all the time. For example, LCL filters in inverters do need damping, active or not, since they're 3rd order. \$\endgroup\$ – a concerned citizen Jan 31 at 21:24
  • \$\begingroup\$ yes, or capacitor with resistor in parallel with Cout \$\endgroup\$ – Nakh0d4 Jan 31 at 21:25
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    \$\begingroup\$ @Vinzent only if the converter's compensation doesn't take the filter characteristics into account properly. Just about any circuit done wrong will have problems -- that just means you should do your circuits right. \$\endgroup\$ – TimWescott Jan 31 at 21:57
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    \$\begingroup\$ @Vinzent "properly" means "correctly". If you have more than one LC section in your filter, then you're probably going to need compensation that taps each capacitor voltage, does some filtering on it, and applies it to the feedback. \$\endgroup\$ – TimWescott Jan 31 at 22:06
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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.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Voltage Spike Feb 1 at 12:31

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