We tend to associate EMI with wireless interference but there is also an interference that travels through a conducted path instead of the air. Conducted EMI is a form of conduction coupling generated via cabling, PCB traces, parasitic elements, or power and ground planes. Radiated EMI is the coupling of unwanted signals from radio transmissions due to the elements of conducting materials. Every conductor has characteristics of an antenna that is capable of both transmitting and receiving signals.

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

The noise elements that are coupled onto the output voltage are high-frequency noise and low-frequency ripple. The low frequency ripple is a byproduct of the inductor ripple current and output capacitor(s) impedance. The high frequency noise is generated by the high di/dt current and any inductance in its path (input capacitor and power switches).

A DC/DC power module integrates the controller, power MOSFETs, inductor and other passive components into a single package. Texas Instrument’s power module family spans isolated and non-isolated architectures as well as step-up, step-down, and inverting topologies. See TI’s DC/DC power modules broad portfolio for more information.