Power supplies are often designed for steady state operation, with transient conditions mainly considered as an afterthought. In practice, transient conditions such as startup, shutdown, and load transients are often far more stressful on the components of the power supply than operation in steady state. To suppress high frequency noise, chip bead ferrites are mainly placed at the input and output of power supplies.
There are two good examples of transients that are often overlooked, but merit careful attention. The inrush current occurs when a power supply first starts up or when PWM used for variable loads such as dimming of LED drivers. Chip bead ferrites are often positioned at the inputs and outputs of power supplies where they must endure heavy transient currents, and this creates a need for compact, cost effective devices that are also highly reliable. Such ferrites are placed at the input and output because they are very effective at filtering the high frequency noise in switching regulators. The high frequency noise results from rapid switching currents ringing with parasitic inductance and capacitance.