“Ringing” is a common term referring to the undesired oscillation that occurs when a power semiconductor switch turns on or off in the presence of parasitic inductance and capacitance. Energy stored in the parasitic junction capacitance of the switch is released during the switching transition and rings with parasitic inductance coming from the stray fields of discrete power inductors and the wiring inductance of the PCB traces, component leads, connectors, etc. In real circuits on real circuit boards parasitics are always present, and hence all switching converters produce at least some ringing. This electromagnetic interference (EMI) is typically in the range of 50 to 200 MHz, and at these frequencies PCB traces and the input and output leads act as unwanted antennas, resulting in both conducted and radiated noise.
Most switching converters operate at frequencies of 5 MHz or less, and their switching harmonics are typically very low in power by the time they reach 50 MHz, so ringing shows up in radiated EMI scans as a separate fundamental noise source. Furthermore, while the switching frequency and its harmonics can be filtered with discrete L-C filters, at 50 to 200 MHz many filter inductors are no longer inductive but in fact have become capacitive, and provide little to no attenuation. Likewise the filter capacitors are often inductive in the 50 to 200 MHz range. Ferrite beads are far more effective because they have very low resistance at low frequencies (typically below 10 MHz) but they have high resistive losses from about 10 MHz up to 1 GHz or more, depending upon their design and construction. Ferrites are traditionally employed in series with the input and output connections of switching converters and can also be placed in series with the power switches.
This white paper from our friends at Würth Elektronik gives an excellent guide to using ferrite beads for controlling ringing. You can download the guide below