Clean Power for Every IC, Part 3:=20 Understanding Ferrite Beads
Articles That Provide Supporting Information
Previous Articles in This Series
- Clean Power for Every IC, Part 1: Understanding = Bypass=20 Capacitors
- Clean Power for Every IC, Part 2: Choosing and Using = Your=20 Bypass Capacitors
If Capacitance Is Not Enough
We concluded the previous article with a condensed set of = recommendations for=20 properly designing and laying out a power-supply bypass network that = will store=20 sufficient charge for long-term power-supply deviations and present low=20 impedance to dominant noise frequencies. You can be confident that this = approach=20 will be more than adequate for most designs.
Nevertheless, the success of the capacitor-only approach does not = mean that=20 there is no room for improvement, and this is where a lesser-known = decoupling=20 component comes into play. If you feel that any ICs in your circuit are=20 especially noise-sensitive (and therefore need additional filtering) or=20 especially noisy (and therefore need additional decoupling from the = power rail),=20 you should explore the wonderful world of ferrite beads.
What=E2=80=99s in a Bead?
The simplest form of a ferrite bead is a conducting wire inserted = through a=20 hollow piece of ceramic material known as ferrite.
The electromagnetic properties of ferrite allow the material to = influence the=20 current flowing through the conductor. The precise nature of this = influence=20 depends in part on the type of ferrite (e.g., manganese-zinc vs. = nickel-zinc),=20 and the properties of a particular ferrite material can be further = refined via=20 the manufacturing process. In many surface-mount ferrite beads, the = conductor is=20 formed into a coil structure, with individual windings layered between = ferrite=20 sheets. Thus, the electrical characteristics depend also on the details = of the=20 winding construction.
Ferrite beads can be divided into two general categories: high-Q (aka = resonant) beads, and low-Q (aka nonresonant) beads. High-Q beads are = intended=20 for applications that require high levels of resonance, such as = oscillators and=20 specialized filters. In the context of power-supply filtering, however, = we need=20 to minimize resonance (as discussed later in the article), so high-Q = beads are=20 out. For the remainder of this article, you can assume that any mention = of=20 ferrite beads refers to low-Q beads.
It=E2=80=99s Not an Inductor, Not a Capacitor, Not a Resistor. . = .
In our attempt to understand ferrite beads, we can start by = considering a=20 first-order equivalent circuit and then translating this equivalent = circuit into=20 a generic plot of impedance vs. frequency.
The inductor is placed in the center as a reminder that the = predominant=20 response of a ferrite bead is inductive, i.e., impedance increases with=20 frequency. However, at some point (generally somewhere between 30 and = 500 MHz),=20 the parallel capacitance begins to dominate the inductance, and = impedance then=20 decreases with frequency. The relatively small parallel resistance (say = on the=20 order of 100 =E2=84=A6) reduces the resonance associated with the = capacitor and=20 inductor, such that the impedance levels off at the transition point = instead of=20 peaking in typical high-Q fashion. This response is evident in the = following=20 plot showing the measured impedance characteristics of a standard SMD = ferrite=20 bead manufactured by Wurth Electronics:
The black line indicates the overall impedance, which (though not = shown in=20 the plot) starts at Rseries, otherwise known as the = bead=E2=80=99s DC=20 resistance. It then increases linearly during the inductive frequency = range,=20 levels off at 300 MHz, and then begins to decrease before leveling off = at 1.1=20 GHz.
The red and blue dotted lines indicate that the overall impedance is = the=20 result of two distinct elements, namely, inductive reactance (XL) and=20 frequency-dependent resistance (R). This brings up an important point: = the=20 equivalent circuit given above is designed to replicate the frequency = response=20 of the bead=E2=80=94it does not convey the bead=E2=80=99s internal = structure. The equivalent=20 model is helpful for understanding how a ferrite bead=E2=80=99s = impedance changes with=20 frequency and for performing simulations, but it is primarily the = ferrite=20 material itself that determines the component=E2=80=99s impedance = properties. This is=20 important to understand because the equivalent circuit might distract = you from=20 one of the defining characteristics of ferrite beads: they actually=20 dissipate high-frequency energy.
Noise In, Heat Out
Recall that ideal inductors and capacitors do not dissipate any = energy; they=20 merely store energy, either in a magnetic field = (inductors) or an=20 electric field (capacitors). A resistor, on the other hand, takes energy = out of=20 the circuit and dissipates it as heat. Ferrite beads, unlike inductors, = are=20 intentionally resistive at high frequencies. This is why = the above=20 plot has the red dotted line labeled =E2=80=9CR=E2=80=9D=E2=80=94from = about 100 MHz to 1 GHz, the bead=20 exhibits significant resistive impedance, not reactive impedance. = Actually, some=20 ferrite beads and ferrite-core inductors are almost identical in = construction,=20 except that the ferrite bead uses a more =E2=80=9Clossy=E2=80=9D ferrite = material because the=20 manufacturer wants the bead to dissipate rather than store = high-frequency energy.
But why belabor this point? We belabor for two reasons. First, you = cannot=20 truly understand a ferrite bead until you have adequately pondered this=20 fundamental distinction between an inductor and a bead. Second, this = =E2=80=9Clossy=E2=80=9D=20 characteristic makes the ferrite bead especially suitable for noise = suppression.=20 Why? Inductance can lead to resonance and ringing when high-frequency = noise=20 energy stored in the inductor interacts with capacitance elsewhere in = the=20 circuit. As we saw in the previous articles, ringing can become = seriously=20 problematic even when we are dealing only with parasitic inductance. We = don=E2=80=99t=20 want to exacerbate the resonance/ringing situation, and thus we opt for = ferrite=20 beads over inductors.
Choose Carefully
The key to maximizing the noise-suppression benefits of a ferrite = bead is to=20 ensure that the targeted noise frequencies fall within the = bead=E2=80=99s resistive=20 band=E2=80=94i.e., that portion of the frequency response where the = resistive impedance=20 dominates the reactive impedance. This means that you cannot simply look = at the=20 salient specs listed in a catalog or data sheet. For example, = let=E2=80=99s say you=20 expect a noise spike at 100 MHz due to a microprocessor clock signal. = Digi-Key=E2=80=99s=20 description for the Wurth part discussed above is the following:
And among the specifications you see this:
You might assume, based on this information, that the = bead=E2=80=99s resistive=20 band includes 100 MHz. In this case, you=E2=80=99d be right=E2=80=94as = shown in the frequency=20 response plot, this particular bead enters its resistive state at about = 80=20 MHz.
However, there is still significant inductive reactance at 100 MHz, = and=20 clearly the bead would provide optimum performance at noise frequencies = around=20 300 MHz.
Now, let=E2=80=99s say you were thinking about part number = MMZ1608D121CTAH0 from TDK.=20 The description and impedance spec are the following:
If you again assume that the bead is primarily resistive at 100 MHz, = you will=20 find yourself in a bit of trouble:
The plot reveals that 100 MHz is still very much within the inductive = portion=20 of the bead=E2=80=99s impedance curve. Based on the point at which the = curve starts to=20 level out, the resistive band begins at about 500 MHz, and the bead does = not=20 reach its optimum-noise-frequency point until 700 MHz. So if you choose = this=20 bead, not only is the impedance lower at the targeted noise frequency, = but the=20 type of impedance=E2=80=94namely, reactive=E2=80=94could = make your circuit more=20 susceptible to ringing or even severe oscillation on the power line.
Recap
Here we have explored some of the essential physical and electrical=20 characteristics of ferrite beads, and we have seen that these components = can be=20 particularly effective in improving power quality when they are used to = suppress=20 noise frequencies that fall within their band of resistive-dominant = impedance.=20 In the next article, we will discuss specific techniques for = incorporating=20 ferrite beads into power-supply bypass networks.
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You may want to view my video that gives a practical review of = ferrite bead=20 characteristics and applications:
https://www.youtub= e.com/watch?v=3D81C4IfONt3o