DC Flter Inductor, measured with Ohms on schematic

[hitech95]

HI,
I was reading into some reference schematic of the chips used inside my chromebook.
So far i've seen a "strange" thing.

I've studied that inductors are measured in Henry, but in this case they used Ohms.

The indictor is marked as 100Ohm@100Mhz, what does this means?


this one is another example:


Have a nice day,
Hitech95

[mikerj]

That component is a ferrite bead, not an inductor.

[MagicSmoker]

More specifically, that is a ferrite bead "noise filter" with a combined resistance and inductive reactance (that is, impedance) of 100R at 100MHz.

[DannyTheGhost]

It's a ferrite bead. For DC current it has almost no resistance, but it's main purpose is impedance. You should read it as "Impedance is 100 Ohms AT 100MHz", which means that it's working like 100Ohm resistor for high frequency currents

[hitech95]

Thanks for the replays. I was expecting that also ferrite beads had the Henry as measure unit. (The fact that it's a first order filter I was expecting it was treated as an inductor.

[Siwastaja]

Saying that ferrite bead is not an inductor is wrong. It is an inductor, and it's doing inductor things, opposing changes in current by storing energy.

It's rated for its impedance at a certain frequency. It could be rated for inductance like any inductor, but the type of rating reflects the supposed application of filtering high-frequency noise, instead of general-purpose use.

It also should be rated for saturation current (instead of max thermal current), but seldom is!

While a ferrite bead is supposed to have almost-zero DC resistance, you often expect the ferrite bead to be a lossy inductor, i.e., include AC loss component, so that while it slows down current changes by storing energy, it internally converts most of this energy to heat: this resistive impedance should be a large part of that 100 ohm rating. Equivalent circuit would be an ideal inductor with a parallel resistor: analogous to car suspension, the ideal inductor being the spring, and the parallel resistor being the shock absorber; and ferrite bead being a spring made of rubber which integrates both the spring thing, and the lossy element.

This expectation of AC loss isn't always right, it's well possible a certain ferrite bead isn't lossy enough but actually capable of releasing most of the stored energy, which, when combined with low-ESR capacitors such as ceramic, may cause amplification of noise, or resonant ringing, at certain frequencies. You need more data to be sure.

Also, if the current exceeds the (often unspecified) saturation current, then the inductance drops and it's just a piece of wire.

If you pick a ferrite bead with the highest impedance (ohms) rating, expecting to get most filtration out of it, it's likely to saturate at very low currents, and do nothing, especially if it's a small package.

So designing with ferrite beads required caution, and testing/deriving unspecified parameters.