4 How to Estimate Inductance of Ferrite Bead for Ripple Reduction

In real implementation of 2^nd stage filter, ferrite beads are often used as the L₂ of 2^nd stage filter to provide nH level inductance. In the specs and data of a ferrite bead, normally an impedance value at a certain frequency at 100MHz and an impedance curve related with frequency are given.

An intuitive approach to estimate the inductance is dividing the given impedance by 2πF (like 2π x 100MHz). But that is not always correct, and we need to read the impedance curve to check if the inductance is dominant factor at given frequency point. Here we use two ferrite beads from Murata as examples for illustration: BLE18PS080SN1 and BLM18SN220TH1.

In the specs of those two ferrite beads, the impedance value at 100MHz are given. But as shown in Figure 4-1 and Figure 4-2, the dominant factors of impedance are different in those two parts at 100MHz frequency.

In Figure 4-1, inductive reactance X determined by L is the dominant part of impedance Z at 100MHz for BLE18PS080SN1. So, if assuming the inductance has small variation with frequency, it makes sense to estimate the L value by using impedance 8.5Ω divide (2π x 100MHz), and we can get inductance 13.5nH at 100MHz.

But in Figure 4-2, the inductive reactance X determined by L is not the dominant part of impedance Z at 100MHz for BLM18SN220TH1. So we cannot use similar method to directly calculate inductance based on the given impedance value at 100MHz. Instead, we can read an inductive reactance value from impedance at lower frequency.

Consider that we use 500kHz switching frequency of TPS62933F, a more accurate value can be read from lowest frequency in the given curve, which is 1MHz point.

Figure 4-3 and Figure 4-4 show above two beads inductance VS frequency curves ^[7]. We can use BLE18PS080SN1 1MHz inductance 15.3nH, BLM18SN220TH1 1MHz inductance 103.4nH during 2^nd LC filter design.