Most of the problems encountered with SMPS and noise are related to the parasitic components in the design. Parasitic elements are unavoidable. They can come in forms of resistance, inductance, or capacitance. For example, a capacitor has a nominal capacitance but also comes with unwanted electrical elements of equivalent series resistance (ESR) and equivalent series inductance (ESL). In a buck switching regulator (step-down converter), as the high-side and low-side MOSFETs alternate in switching, noise from the input source of the converter and the parasitic inductance in the high di/dt loop appears on the switch node. The noise on the switch node is then be coupled and distributed onto the output voltage through the parasitic capacitance associated with the inductor, package, and PCB layout.

Figure 1-1 Buck Converter With Parasitic Elements