How to Measure High-Frequency Noise in Buck-Regulator Output-Voltage Ripple

In many applications, a switch-mode power supply (SMPS) is the only choice because of its excellent performance, including high efficiency and high power capability. A buck step-down power converter is the most popular topology adopted in a variety of applications. The switching characteristics controlled by pulse-width modulation (PWM) help to regulate power transfer.

At the same time, though, the fast switching transients of power FETs within several nanoseconds or tens of nanoseconds could induce high-frequency noises. For example, in noise-sensitive applications, if the high-frequency voltage spikes caused by the power supply are aligned with the clock signal, it will cause failure or malfunction of the load devices, such as ADCs, ASICs, or FPGAs.

Therefore, it’s important to understand the root causes of high-frequency noises, identify the noise issues, and then find the potential solution to attenuate or eliminate the influence of high-frequency noises. The correct noise/ripple measurement method is critical.

Typical V_OUT Ripple in a Buck Converter

For a typical buck converter, the inductor and the capacitor work as a filter to provide the conductive path for the AC signal generated by the switching action. The inductor current contains the AC component and the DC component. Most of the AC components will flow into the output capacitor, and the DC component will only flow into the load.