Understanding the Common Mode Choke using LTspice

hello and welcome back so what I want to talk to you about today is a component that's quite common in modern electronics but it's not always that well understood I'm talking about a component that is widely used in power electronics especially in power supply filters but also in digital electronics especially on differential communication lines what I'm talking about of course is the common mode choke and what I want to do today is look at how this thing works what's its purpose in a circuit and then simulate this using ltspice so if you're curious about that and much more thank you watching so to start things off this is a noise filtration component and to understand how it works let's first look at the noise it's supposed to filter so what are the types of noise that we can find commonly in a circuit and for that I created this low simulation here in which I got a generator that contains both differential and common mode noise basically the differential noise is a type of noise that will go between the two or more output terminals so if we consider this to be a power supply like a laptop charger or something the differential noise will be the noise that goes between the plus and the minus terminals of the output now the common mode noise is a bit different this will go between both of the output terminals and the ground now this can be the protected earth ground or some other ground in your circuit but the thing is this common mode noise is a type of noise that goes through both of your wires at the same time so if we run this thing we can see that our common mode noise so noise that is referenced to the ground is present on both pins we can clearly see this using an FFT analysis we can clearly see our two main noise frequencies so my noise generators are the differential mode generator at 500 kilohertz and the common mode generator at 1 megahertz and if we look at these signals from common mode point of view so reference to ground we see both these signals very clearly only B difference being that the differential noise is slightly higher on our plus terminal but the common mode noise is completely identical now if we look at this in a differential way so compare plus in reference to minus let's just delete the extra traces and we look at its FFT content we see that only our differential noise clearly stands out so the common mode noise is very very small when we look at it from differential point of view now the common thing you would do to filter off noise in a circuit is to simply make an LC filter or a PI filter or something like that so what I did here was create a simple LC filter between my generator and my load and let's just see what this does so if we now look at the differential noise or the noise on the two terminals of our load but in reference to ground and we analyze it from an FFT point of view we see that nothing really changed so we see a ton of noise both the common mode noise and differential noise are clearly visible so this filter did not help with the common mode noise now if we look from a differential point of view I mean even without making any sort of FFT analysis if we simply compare it to our previous result we can see that the differential noise has basically vanished so before we had this almost 2 volt peak-to-peak noise whereas now we can barely see anything so our LC filter is doing a great job but only on the differential noise it hasn't done anything with the common mode noise so let's try and improve this filter a bit now the problem with the simple LC filter is that the common mode noise even though it's sort of blocked from coming in through the series inductor we've placed there's nothing preventing it from coming in through the other terminal borders no filtering component so if we put two series chokes so create this sort of arrangement in which we have two series inductors and then the capacitor and we look at what we get on the output so let's just start off with differential noise we see that we still have no more differential noise so it's still working as an LC filter but if we look at the common mode noise so the noise directly in reference to ground and let's just compare to our previous measurements I mean it's not very obvious but it's starting to get reduced let's just see this from an FFT point of view so if we look at our common mode noise we clearly see that the noise in our circuit where we had D double inductors so ground LLC is almost 10 decibels lower than the case in which we didn't have this I mean the filter is working it's not perfect but that has to do with the values of the components but you can clearly see that this is a good way of filtering common mode noise it's just that you need couple of inductors now the final thing I want to show you is an actual common mode choke in action so the difference between two individual inductors and a common mode choke is that the two inductors on the common mode choke are arranged as a transformer and to show that in ltspice I took the two inductors and also added the case statement so that these two are two coupled inductors this time and if you look here just on the ground point we can clearly see that the circuit with a common mode choke has far smaller noise than the one with the two individual inductors so I took the same exact inductances just put them on a transformer and it's already working much better but what the Como joke does differently then to individual inductors is if we look at the differential noise let's just delete everything that we have here and compared to our previous circuit so I'm just inverting it we got our differential noise back so the common mode choke is a component that's especially used for for common mode noise it's not useful for differential noise now what on earth would that be good for I mean why would you want something like this well as I told you this is good for power supplies but also for digital differential signal lines thing is for a power supply you don't want common mode noise but you don't want differential noise either whereas in a signal line you still want to keep your signal so the differential content you still want to keep and you only want to remove the common mode content now this seems a bit weird how can the inductance disappear how can it no longer filter when it's coupled like this let's have a look at a choke and actually measure it a bit so what I got here is common mode choke that I have recycled from an old power supply and an inductance meter and what I did was just interconnect two of the coils so on one side they're interconnected on the other they're left free now if you measure the inductance let's say one of the coils we see that we have around 48 milli Henry so this is probably a 50 milli Henry common mode choke if we look at the other inductor again 48.1 so bit of Tolerance now if I measure the two inductors in series so basically this is what we would see if it would have the inductor used in differential mode we see that we only have around 500 micro Henry so around 100 less than the inductance of one of the coils the two coils together have around 100 milli Henry and we're only seeing 0.5 mini Henry and this basically happens because of the way the magnetic field around the inductors is being formed so when you have current going one way for the coil it generates a magnetic field in one direction if the current is going the other way the magnetic field is generated in the opposite direction so when you have differential current going through the common mode transformer the currents are going in opposite ways and the two magnetic fields cancel each other out so you don't see the inductance in a differential set up in common mode set up both currents go the same way for the inductor therefore the two fields add up so you're seeing the inductors actual inductance now one of the benefits of the differential mode currents cancelling out is that the core material so the ferromagnetic material in the core only sees the common mode currents now the common mode currents are usually much much smaller than the differential mode currents so you can have a few amps of differential current going through this thing but only a few milliamps of common mode current that means that you can get very high inductance values without saturating the core so if you want to make a good filter that filters both common mode and differential mode noise you need to combine the two effects let's have a look at this research paper so what I got here is the paper about making line filters or switching power supplies and they end up with this sort of design in which they have both common mode chokes and uncoupled inductors and in the next section they also put some values to the schematic and you can clearly see that the common mode choke is in the order of million Reed so high inductance but this schematic also includes some uncoupled inductors that only have hundreds of micro henries so more than an order of magnitude difference between the two and by combining the two effects you can get a filter that is both with that common mode rejection and differential mode rejection now you can look at all the formulas in here and measurements and so on so there's quite a lot of things in this research paper but now coming back to the common mode choke to see more clearly the difference between its common mode and differential mode behavior we can have a look at its impedance in these two modes and for that let's have a look at a datasheet so what I got here is a generic data sheet from borns that lists an entire series of common mode inductors and if we scroll through it we see these nice little curves basically what they're showing us is what the common mode impedance is and what the differential mode I mean they're calling it normal mode but it's a differential mode impedance and we can see that throughout almost the entire frequency range that is covered you always have at least a factor 10 of impedance difference between the two connection methods and this is important since you can have useful differential signal going through the common mode choke and that the same frequency have common mode noise and the filter will only filter out the common mode component the differential component will be unaffected so all in all this common mode choke is very important component in modern electronics and to correctly be able to use it you need to understand it so how it works how it actually filters something and that's about it from my site for today so hope you got some useful information after this leave your thoughts in the comments thank you for watching make sure to subscribe to be updated with all my latest videos and see you next time bye bye

Channel: FesZ Electronics

Views: 57,752

Keywords: fesz, electronics, diy, hobby, electronics tutorial, tutorial, ltspice, common mode choke, emi, common mode, differential mode, noise, simulation, electromagnetic compatibility

Id: wakrdus_0qs

Length: 13min 8sec (788 seconds)

Published: Thu Feb 13 2020