AN10912_15 Datasheet, PDF (23/34 Pages) NXP Semiconductors – SMPS EMC and layout guidlines

NXP Semiconductors
AN10912
SMPS EMC and layout guidlines
Rs
Cp
L
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Fig 20. Self resonance frequency of real inductor
Figure 20, shows that this inductor will not be applicable at frequencies above 980 kHz.
As a comparison, in Figure 21 a 68 μH inductor and a 100 E ferrite bead are plotted.
Notice that the ferrite bead displays resistive behavior that increases logarithmically with
frequency. A ferrite bead has a completely different functionality compared to an inductor,
it will dampen high frequency signals. The 68 μH inductor has a self resonance frequency
at 8.35 MHz and can be used to filter frequencies higher than the 4.7 mH inductor can.
The ferrite bead can be used up to 30 MHz, and has little effect at low frequencies such as
10 kHz. It should be stated that the desired impedance is achieved only when the inductor
is operated within the specifications. An inductor that reaches saturation looses its
functionality and will not posses its original value at all. This situation often happens with
inrush currents or repetitive peak currents.
68 μH inductor
100 E ferrite bead
Fig 21. Impedance characteristic of 68 μH inductor and 100 E ferrite bead
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AN10912
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 18 February 2011
© NXP B.V. 2011. All rights reserved.
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