This article highlights the essential considerations for effective power rail noise filtering on PCB assembly with ferrite beads.
A ferrite bead is a passive electronic component that filters high frequency noise over a wide band range. Made of magnetic material, it becomes resistive over its intended frequency range and dissipates the noise energy in the form of heat.
Surface-mount (SMT) type ferrite beads are widely used on printed circuit board assembly (PCBA) to filter the power rail noise. They are important to prevent the noise of the switch mode power supply (SMPS) module from propagating to the electronic load. Besides that, ferrite beads are also necessary to prevent the power rail noise from crossing the domain between digital and analog circuitry, as the noise could contribute to severe jitter issue, which in turn could cause the electronic device failure.
The following are essential considerations when using ferrite beads to filter the power rail noise effectively:
Firstly, make sure that the rated current specified in the ferrite bead data sheet is at least 50% larger versus the nominal current loading on the power rail of interest for the designed prototype. For example, if the nominal current on a power rail is 1A, during power cycle, there could be an inrush current on the power rail and the overshot current could be as large as 1.2A, i.e., higher by 20%. Hence, a ferrite bead with 1.5A rated current, i.e., 50% higher, will not be damaged by the inrush current.
Secondly, a ferrite bead with the minimal DC resistance should be selected by the circuit designer. For example, in a power rail with 1V nominal voltage supply and 1A current flow, the voltage drop across the ferrite in series configuration should be less than 0.05V or 5% of the nominal voltage. Hence, according to Eq. (1), DC resistance of the ferrite bead shall be less than 0.05Ω.
V = I x R (Eq. 1)
where V is the voltage drop across ferrite, I is the current flowing through ferrite, and R is the DC resistance of ferrite.
Pie network configuration
Thirdly, the ferrite and shunt capacitors shall be arranged in pie network configuration, depicted in Fig. 1, so that a two-way low pass filter is formed, to suppress the power rail noise generated by either domain A or B.
Frequency response simulation
Fourthly, circuit simulation of the pie network or two-way low pass filter involving the s-parameter (S2P) model of the ferrite bead and the shunt capacitors (Figure 2) shall be carried out to analyze the frequency response S21. The -3dB cut-off frequency of the pie network shall be lower versus the power rail noise frequency. With reference to the simulated S21 plot depicted in Figure 3, the -3dB cut-off frequency of the pie network is less than 450kHz. Assuming the power rail noise at Port 1 has 200mVpp amplitude with switching frequency 500kHz, upon filtering by the pie network, the noise propagating to Port 2 has 15mVpp amplitude, i.e., attenuation as much as 92.5%, as depicted in Figure 4.
By implementing the key points listed in this article, power rail noise on PCBA will be filtered effectively using SMT ferrite beads.
Basics of Noise Countermeasures: Chip ferrite beads, https://article.murata.com/en-sg/article/basics-of-noise-countermeasures-lesson-4.
Chang Fei Yee, a hardware engineer in Keysight, works on digital hardware and signal/power integrity analysis.