# selection of common mode choke

I want to select a common mode choke. This will be placed at the input of a buck regulator. Now, assuming conducted noise or common mode noise, will be at a certain frequency and I will need common mode chokes that have high impedance at that frequency. For eg / I have a buck running at 100kHz. Then I will need a common mode choke tof high impedance at 100kHz and at harmonics of that fundamental frequency. The fundamental is 100kHz. Now, when terming harmonics, what is the max multiple that should be a worry ? Is it the 3rd harmonic ,so 300kHz ?

Also, when it is said we need high impedance at fundamental frequency and its harmonics, what exactly can be termed as high impedance ? WHat is the value and what is the basis for that value ?

• Switched mode converters generate much more harmonics than only the 3rd. I once worked on a product where the 238th (!!) harmonic caused a problem. OK, that was on a sensitive radio but anyway. What problem could arise if you didn't have the choke ? My guess would be that you could get too high EMI emissions and fail an FAA test for example. I would try to see what is "common practice" and select a similar component paying attention to maximum DC current etc. Dec 13, 2015 at 14:52
• Yes precisely my point. But there must be somewhere I must begin. So, assuming I need to say go for a certification like CE or such, what is the harmonic multiple I start with ? In case of more noise I will do the needful then. But, where did you start off with ? The 3rd harmonic ? Dec 13, 2015 at 14:55
• In my opinion it will be almost impossible to predict what you need to pass an EMI test. Since so many variables are involved I would not dare to say what impedance you would need or up to which frequency you need to consider. I think this is very much an experience thing, you try something and see if that is good enough. Dec 13, 2015 at 16:30

One thing to consider is that you will never achieve 100% elimination of conducted and radiated noise. The regulators know that too and take that into account when the standards are written. So it is necessary to just attenuate the conducted or radiated noise down to an acceptable level.

One way to picture things for selection of noise filter chokes is to consider the choke (or filter bead) impedance as part of a impedance divider. The other part of the impedance becomes either the source impedance of inputs or load impedance of outputs as referenced to the GND system. This can even be dealt with on an intuitive basis so that if, for example a power supply, as an input has a source impedance that is very low then the filter choke probably does not need to be one with an extremely high impedance at the frequencies of interest.

Similarly for an output if the signal terminates into higher impedance load then the high frequency impedance of the filter choke/bead needs to be much higher.

It also seems to be that high current filters are typically lower impedance chokes than the ones selected for low current signal lines.

Last comment I can make is that the proper thing to do in a product design is to think of how you want to architect your EMI suppression and the design support for that into the product using the best layout practices that you can apply. The key thing here is to select proper types of components and have footprints for them available in the design. Then when you build up first boards apply experience to populate the filter bead, inductor and capacitor sites with components with the best guess values. This permits you flexibility to swap out component values at the test lab in case problems are seen in particular parts of the product at certain frequencies.

This answer assumes you are connecting your power supply to a household AC source. I have no specific answer for just a DC in and DC out buck regulator because, unless this is say a 48V telecom application there will be the strong possibility of no spec existing. If a spec exists then fine!

Whatever country you are in there are pretty similar standards to be met so concentrate on these standards first and, in particular concentrate on conducted emissions. Understand what levels of emission are allowed to be produced. Not knowing what you aim for gives you zero chance of theoretically selecting an inductor.

So, let's say you have the numbers (limits imposed) and of course they will vary across the full range of conducted emissions so what I'd do next is use a simulator to mimic the switching currents and try out different scenarios with simulated common-mode chokes and capacitors on the AC side.

The inductor/choke and the capacitors form a low pass filter (significantly below the switching frequency) and you can see how much attenuation you get from the simulator.

Sorry, I can't answer this for just a buck regulator unless there are specifications at hand. If there are specs available then follow the same guidelines. This won't guarantee a "pass" but it avoids a miserable and costly "fail".

LTSpice is free!

• Unfortunately, I am connected to a battery source. SO its dc/dc. But thank you for the effort please. Dec 13, 2015 at 15:45
• If there is no standard then try looking at what companies like traco specify for conducted noise on their range of dc to dc converters. Dec 13, 2015 at 16:20