Planes and traces under common-mode choke - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: February 11, 2020, 09:35:39 pm
content: Hi,because of very limited PCB space I might need to place traces under a common mode choke BUT on the opposite side of the PCB. In other words there are the ground and Vdd planes between the CM choke (on the top layer) and the traces (on the bottom layer).The signals that go through the CM choke are 2 signals:- very low currents (just a signal into a instrumentation amplifier)- both in phase- both the same FIXED frequency of 200KHz- only the amplitude is very slightly different between the two (few uV to few mV) and the phase (again very tiny offset)1) Is it ok to place the traces under the CM choke if they are on the opposite (bottom) side of the board - given that there are two planes between them?2) is it better to keep or remove planes under the CM choke?Thank you

author: TimNJ
author_rank: Super Contributor
timestamp: February 12, 2020, 05:24:25 am
content: For my work (switch mode power supplies), we tend to remove planes and components from under CM chokes..but I think this is mostly just a rule of thumb, and may very well be okay for your application.The main issue is capacitance between the choke's windings and the ground plane, which reduces the effective inductance of the choke.I think you're already cognizant of that.The decision here might depend more on what frequencies you need filter out, and if the parasitic capacitance is an issue at those frequencies. If attenuation is only needed for lower frequencies, whether the plane is there or not doesn't matter so much. But, if you're getting up in the 10's of MHz range, maybe consider removing it.At the same time, how does chopping up the ground plane affect your return paths? Does this create return path discontinuities that will ultimately cause other EMI issues?

author: ricko_uk
author_rank: Super Contributor
timestamp: February 12, 2020, 12:33:59 pm
content: Thank you Tim!the ONLY frequency I am interested in passing on both signals is 200KHz and measure their amplitude (it goes into a Instrumentation Amp so currents very tiny). Everything else I want to remove.You mention CM choke for SMPS. Out of curiosity, is that for off-line SMPS or do you use CM chokes also for other topologies/types? If for others too could you please tell me which ones and how as I have never come across it and am interested in learning about it.Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2020, 04:36:59 pm
content: Well, putting a trace under a component, gives about the capacitance between that trace and component. Does the trace carry an interfering signal that would be better not to couple to it? Put a shield between them (e.g. use a multilayer PCB).You will usually find plots of impedance or attenuation in CMC datasheets. These can be turned into an equivalent circuit, modeling the winding and core inductance, and the winding and stray capacitance.Simply put the offending trace capacitance in parallel with, well, somewhere along that network, and you basically have your answer.I would worry more about your description, than the CMC itself -- it's going into an instrumentation amplifier? What is the CMC going to do? It only provides a series impedance. If the amplifier has no impedance to ground then there's no attenuation across the CMC. Surely you mean to put some capacitors or resistors to ground, or something?What interference are you expecting to filter, anyway? Is this part of your 200kHz eddy current system? Why not get a faster in-amp, that offers more CMRR at 200kHz? Is it transients from signal switching? Ambient noise?Tim

author: TimNJ
author_rank: Super Contributor
timestamp: February 12, 2020, 10:02:52 pm
content: Quote from: ricko_uk on February 12, 2020, 12:33:59 pmI guess you should define what "everything else" is. Up to 10MHz? Up to 100MHz Up to 1GHz? It's hard to know whether it's okay without know what you need to filter out first.Common-mode chokes can be used in any power supply type, to improve EMI susceptibility and emissions performance. Common-mode chokes can be used anywhere where you have common-mode noise currents that might want to flow in a large external loop, usually through earth. This can be offline AC-DC or DC-DC, anything with high di/dt switching activity.