“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.
Ferrite beads placed in series with the bootstrap pin of buck regulators can make excellent ringing control components, and when applied properly they can be used to treat the source of the high frequency noise without taking up too much PCB area and without large sacrifices in power efficiency. Their benefits over tuned resistors are in ease of selection and reduction in the lab testing time needed to design them in.
Download: ANP025
