Choosing Ferrite Bead
Points to consider while choosing Ferrite beads for EMI suppression
- Ferrite bead is a frequency dependent resistor and the resistance of ferrite bead varies according to the frequency of signal passing through the ferrite bead
- Often Ferrite beads are the only solution for EMI problems
- Ferrite Beads with Low Q are absorptive beads - very lossy and hence absorbs the high frequency current noise and dissipates as heat
- Things to consider for choosing ferrite bead
- Unwanted frequency range of the signal flowing through the ferrite bead
- EMI source
- Expected amount of attenuation by the ferrite bead
- Electrical condition for the Design - DC Voltage, DC Bias currents, Maximum operating current, filed strengths etc
- Available board space to place the Ferrite bead
- Beads chosen without proper planning may become a source of EMI problems
- Often the impedance of ferrite bead drops to 20% of stated impedance when the current flow through the ferrite is high. The current saturates the ferrite and makes it to lose its inductance property
- AC resistance (Z) must be greater than the inductive resistance (XL) of the ferrite bead. If this is not considered while choosing the ferrite beads, following problem in the figure is very common to occur:
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| Figure: Illustrating the negative effects of choosing ferrite bead without proper input |
- Refer to impedance chart of the ferrite bead. Normally, in the quick specification only at a spot frequency, the impedance value will be mentioned which is quite not a useful information. Referring to figure below, variation of impedance is so high though all spot impedance are the same (120 Ohms) for the 5 different ferrite beads.
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| Figure: 5 Ferrite beads with different impedance curves sharing common spot impedance value |
CISPR 22 Limits for EMI/EMC test reference
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| Table: CISPR limits for reference |
References:
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