L=inductance of choke
RDC=DC resistance of choke
C=capacitance of filter capacitor.
Ideally, the resonance should be critically damped (Q=0.5), which can be achieved by adding external series resistance to the choke. Strictly, the load resistance across the capacitor also damps the resonance, and this may be transformed into a notional extra choke series resistance using:
However, the damping effect of the load resistance is usually negligible. Even worse, a series voltage regulator is a constant current, or infinite, AC load to the smoothing circuitry, so it adds no damping whatsoever.
A typical traditional example: A choke input ...