Common mode choke specs question - Page 1
Forum section: electronics / beginners

author: ivan747
author_rank: Super Contributor
timestamp: December 10, 2014, 01:24:31 pm
content: Hi guys.I've been looking into common mode chokes and I am a bit confused with what the terms exactly mean. I'm just making sure I'm not gonna do anything stupid here.First there's inductance. The manufacturer will specify an inductance, typically in the order of hundreds of microhenrys. This is the inductance that a common mode signal sees, right?Second, they specify a maximum current. I undestood that this current is differential current, and core saturation will not occur if you go over this, it's just that you will overheat the windings.Third, and this is the one I really want to know about. They will typically have a graph of "impedance vs. frequency" or frequency characteristics. Okay... this is the impedance presented to common mode signals right?I read somewhere that these common mode chokes also have some "differential" parasitic inductance (it looks like two inductors, in series with the choke), that could be used in your favor to make a differential filter, but I haven't found this parasitic inductance specified in any of the datasheets I have searched. So I guess I'll just use a normal inductor.A graph that really got me confused was this one:http://staging.lairdtech.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=4795It has Z, R and Lz. I am guessing R is the parasitic resistance, but... why would it change with frequency? I am guessing Z is the total impedance seen by the common mode signal and Lz is the inductance's contribution to Z.Thanks

author: Paul Price
author_rank: Super Contributor
timestamp: December 10, 2014, 02:32:06 pm
content: I think you've answered your own question correctly.

author: richard.cs
author_rank: Super Contributor
timestamp: December 10, 2014, 03:14:52 pm
content: R is the real part of the impedance and corresponds to lost energy. At low frequencies R is the resistance of the winding, this rises with frequency anyway due to the skin effect limitiing current flow to the surface of the windings, but as the frequency increases further it also begins to include core loss, which electrically looks equivilent to resistance.At very high frequencies all you see is the parasitic capacitance across the winding and it no longer looks lossy at all.

author: ivan747
author_rank: Super Contributor
timestamp: December 10, 2014, 07:39:55 pm
content: Thanks for clearing that up

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2014, 09:35:46 am
content: Lz? I don't see Lz on the datasheet...They give real and imaginary components, which is nice when you want to know these things. For example, it makes a terrible inductor, because although the impedance is useful at most frequencies, the Q factor is not (X is reactance). So it's only a passable inductor below 8MHz or so, and not a good one at that. Which is good, because you generally want to use these sorts of parts to dampen resonances, not make them worse.You don't always see differential mode characteristics; in power line chokes, it usually amounts to a few uH, with dips and swells similar to the common mode case (resonances with the winding geometry and such), but at lower impedances / attenuation.On this part, the (C, N, O) plot shows impedance for three modes: open means one winding only (the other open), which you should expect has about the same result as common mode does, but with differences mostly at higher frequencies (perhaps higher because the core looks relatively larger, or lower because the core is less efficiently utilized). Common mode is both windings in parallel, driven as a single component. Normal is either with one shorted across, or with the two wired series-opposing (shorted at the far end), which gives low impedance at low frequencies (transformer action), peaking to a high impedance (apparently much higher in this case) where the windings act in parallel resonance (between the wire-core-wire capacitance and the wire-to-wire leakage).Chokes for data lines, boasting 100s of MHz to GHz / Gbits of bandwidth, are bifilar wound and, although they'll often provide differential mode impedance curves, you shouldn't read into these too much, because they exhibit very large phase shifts at high frequencies -- you see not just one peak, but several peaks and valleys, corresponding to transmission line stub modes. This is actually a good sign, because it means your data will pass relatively unscathed, even well into that frequency range.Tim