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The elegance of ferrite beads as a circuit design and problem-solving component

(Editor's note : this is part of an on-going series of “dialogues” between the authors; there are links to the previous installments at the end, immediately above the “About the Authors” section. Also below are also two “Desert Island Design” articles, by one of the authors.)

[Dave sitting at his desk, web surfing what looks like a jewelry site. Dr. T happens by…]

Dr. T (Tamara Schmitz) : What are you doing?

Dave (Dave Ritter): Oh, hi, Dr. T, I've got some shopping to do. My wife's an avid beader and I've got to sort through all this beading paraphernalia to find the perfect gift.

Dr. T : Beader? You mean a crafting, jewelry making, skilled artisan kind of beader?

Dave :Exactly! She weaves the most amazing things from a pile of beads and some string. I get to thread the needles.

Dr. T : Sounds like the perfect job for you. I've actually been getting into beading myself in the last few days.

Dave : Really? Trying to save money on jewelry, or did you just get a creative craving?

Dr. T : Neither, actually. I was trouble shooting a new board and ran into some noise issues.

Dave : Oh! So we're talking about ferrite beads, the magnetic inductive noise-suppressing kind.

Dr. T : Yes. We're putting together a front end for a light sensing PIN diode that has to operate to a few MHz, and it needs to be sensitive to a few μV.

Dave : And you have to keep the power supply noise to a minimum. Sounds like a job for a ferrite bead!

Dr. T : We often use them, but a lot of my students just put them in a schematic hoping they will help. It sure would be nice to have a little more understanding and maybe even a model to simulate.

Dave : Well, there isn't that much to a ferrite bead. In fact I have some specs from a manufacturer. We should be able to put something together from that.

Dr. T : Sounds good to me. For starters, I know a ferrite bead is something like an inductor.

Dave : Yes, but most inductors are usually designed for highest possible Q, lowest loss.

Dr. T : Which makes them resonate nicely.

Dave :Of course, lots of wiggles. I'll show you more about that in a minute.

Dr. T : But first: do you remember those conversations we've had about using bypass capacitors in parallel with the load. Isn't it common to add an inductor in series before reaching that bypass capacitor?

Dave : Sure. The wire itself provides some inductance, so you end up with a low-pass filter characteristic. If you add a discrete inductor, you can tune the filter characteristic for a specific frequency.

Dr. T : I found this comparison (Figure 1 ) in the pdf library at www.murata.com. I think it highlights the differences between ferrite bead and air-core inductor.



Figure 1: Comparison of ferrite vs. air-core inductor

(Click on image to enlarge)

Dave : Very nice. The ferrite impedance rises with frequency, like an inductor, but it becomes resistive at high frequencies because the magnetic field just can't set up fast enough in the ferrite. It ends up absorbing energy as though there was a resistance. On the other hand, the inductor resonates with its own capacitance (the coils act like the plates of a capacitor) and has a very high resonance peak at a few hundred megahertz.

Dr. T : I hadn't thought of a ferrite bead acting resistive at high frequencies. I've spent more time in RF circuits where I need to tune the signal path. Now that we are focusing on the power, it seems dangerous to add a circuit that likes to resonate (an LC) on a power line, where we most definitely want to keep the voltage level stable.

Dave : If you only need to cancel one frequency, then you could tune an LC, but most of the time there is a whole range of frequencies to block. The wideband nature of the ferrite bead is much better.

Dr. T : That makes perfect sense, just look at Figure 1.

Dave : I can tell you like that plot, you always were a bit of a frequency domain freak. Have you thought about how those effects show up in the time domain (Figure 2 )?



Figure 2: Response of ferrite bead and inductor to 100 mA step.

(Click on image to enlarge)

Dr. T : I don't see that big of a difference, Dave. Just a couple of small glitches. Phooey on you and your time domain.

Dave : Oh yeah? Let's zoom in, Figure 3 . Look again. Now there's a big difference between the ferrite bead and the inductor!



Figure 3: Zoomed response of ferrite bead and inductor to 100 mA step.

(Click on image to enlarge)

Dr. T : Hmm. That ringing could disrupt important RF communications—like radio, cell phone and even my WiFi!

Dave : Notice that the ferrite bead, being resistive at high frequencies, exhibits no resonance. That's why most computer cables have bumps at each end of the cable.

There's ferrite in those sleeves that keeps the switching noise inside the computer from using the cable as an antenna.

Dr. T : There is one of those sleeves on the cable coming out of my keyboard and I never stopped to think about why. Where did you get this data? Did you test this in the lab or find a good model?

Dave : Actually, I've done a lot of RFI protection work in the lab, but this time I borrowed a curve from a ferrite bead data sheet. (Vishay Dale ILB-1206) Then I built a circuit model to match the curve. The model has to be distributed enough (4 stages in this case) to replicate the curve. Take a look at Figure 4 .



Figure 4: Datasheet curve vs. Dave's model

(Click on image to enlarge)

Dr. T : Amazing, but just so I can be even more impressed, can you show me the model, too?

Dave : I got you a bouncing baby ferrite model, Figure 5 .



Figure 5: Dave's circuit model for ferrite bead

(Click on image to enlarge)

Dr. T : I feel like I have a better handle on what ferrite beads do, but when do we use them? Are they only wrapped around my computer cables?

Dave : Oh no. Ferrite beads can be used for big problems. Large ferrite torroids (donuts) are often used to filter common mode noise between a switching supply and a chassis.

Dr. T : Those are pretty big, but I see that they even are available as surface mount devices!

Dave :Absolutely. Intersil's new cable equalizer has over 80 dB of gain at 5 MHz, so we have to keep the power supply pins very clean. We feed the supply through a surface mount bead with about 1k ohm of effective series resistance above 1 MHz.

Dr. T : I've got it. So let's put it to poetry..


Ferrite Beads
Ferrite Beads
Stopping EMI
Stick them on your cables
So voltages comply

Ferrite Beads
Ferrite Beads
Rarely taught in schools
Isolating noisy boards
Beloved system jewels!


Dave : An engineer and a poet, too. Nicely done, Dr. T.

Previous “dialogues” in this series:

Other related articles by Dave Ritter:

About the authors
Dave Ritter grew up outside of Philadelphia in a house that was constantly being embellished with various antennas and random wiring. By the age of 12, his parents refused to enter the basement anymore, for fear of lethal electric shock. He attended Drexel University back when programming required intimate knowledge of keypunch machines. His checkered career wandered through NASA where he developed video-effects machines and real-time disk drives. Finally seeing the light, he entered the semiconductor industry in the early 90's. Dave has about 20 patents, some of which are actually useful. He has found a home at Intersil Corporation as a principal applications engineer. Eternally youthful and bright of spirit, Dave feels privileged to commit his ideas to paper for the entertainment and education of his soon to be massive readership.

Tamara Schmitz grew up in the Midwest, finding her way west with an acceptance letter to Stanford University. After collecting three EE degrees (BS, MS, and PhD), she taught analog circuits and test-development engineering as an assistant professor at San Jose State University. With 8 years of part-time experience in applications engineering, she joined industry full-time at Intersil Corporation as a principal applications engineer. In twenty years, she hopes to be as eternally youthful as Dave. .

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