How to design an EMI filter

The vast majority of devices and equipment that are connected to the electrical network generate interference, which is why electromagnetic compatibility problems, also known as EMC, frequently arise. To mitigate these problems there are regulations that set limits to the amplitude of the interference that a device can emit. And in order to comply, these devices must include an EMI filter.

Today, at EMZER, we want to tell you what it is and how to design an EMI filter so that your electronic designs comply with EMC regulations and can be marketed.

<H2>What is an EMI filter?

An EMI filter, sometimes known as a mains filter or EMC filter, is a device made up of capacitors, coils and resistors, which is placed between the electronic device and the mains cable. The best place to place the filter is right where the network terminals of the electrical line enter the equipment, since this prevents the coupling of interfering electromagnetic fields to the already filtered electrical line. If possible, a metal box helps block any capacitive coupling between the filter input cable and the power line.

<H2>What is an EMI filter for?

The purpose of an EMI filter is to suppress electromagnetic interference generated in electronic equipment and propagated towards the electrical network through the power cable. Similarly, it also helps to mitigate interference coming from the electrical network that could affect the operation of the device.

<H2>Types of interference in EMI filters

There are different types of classifications for interference (depending on whether the noise is impulsive or continuous, broadband or narrowband, etc.), but the most interesting classification from the point of view of electromagnetic compatibility (and more specifically, conducted emissions) is one based on its modal nature. To explain this concept, let us consider the three conductors of the electrical network, line (L), neutral (N) and Earth (G).

Differential mode interference is that interference which flows in opposite directions at the line and neutral terminals, while common mode interference flows in the same direction at the same terminals and is generally generated due to a current to ground through of a parasitic capacitance, known as leakage current.

<H2>EMI filter structure

The simplest structure of an EMI filter contains one or two capacitors (type X) between line and neutral, to mitigate the differential mode, and a common mode choke and two capacitors (type Y) from line to ground and neutral to ground to mitigate common mode. A resistor (R) is usually added to discharge the capacitors when power is removed. Other more complex architectures are simply based on replicating these components.

<H2>How to design an EMI filter step by step

To design an EMI filter, it is necessary to measure the modal nature of the electronic device’s conducted emissions. In this way, according to its nature, the necessary components will be added to the EMI filter.

For example, if the dominant mode of the interference is the differential mode, the mains filter must contain at least one type X capacitor between line and neutral. On the other hand, if the dominant mode of the interference is common mode, the EMI filter should contain a common mode choke (if the interference is low frequency), and/or Y-type capacitors (if the interference is at the upper part of the spectrum).

In general, real electronic devices emit both common-mode and differential-mode interference simultaneously, and the dominant mode can change over the spectrum.

After a first design of the EMI filter based on the first measurement of conducted emissions, it will be iterated between measurement and adjustment until the desired network filter is achieved. If at any frequency the dominant mode is near or above the limit, the value of the type X capacitor will be increased (or an additional type X capacitor will be added). And, analogously, choke or Y-type capacitors will be tweaked if the dominant mode is common mode. That is why it is vital to be able to obtain complete and rapid information on the nature of the emissions.

<H2>How is common mode and differential mode measured?

The problem is that conventional instruments that measure conducted emissions (EMI receivers and spectrum analyzers) are not designed to measure modal interference, since they can only measure one of the lines at a time (line or neutral), so it is impossible to know the modal nature of the interference. And, without this information, we are blind when it comes to designing the EMI filter.

In order to separate the modal components using a conventional instrument it is necessary to add a device known as a noise separator between the analyzer and the electronic device. However, this measurement system has some disadvantages:

  • First, that common mode and differential mode cannot be measured simultaneously with a single instrument. This makes it difficult to identify the predominant mode, especially when measuring pulsed interference.
  • Intermodal coupling (common mode coupled to differential port and vice versa) is technology dependent, can vary with frequency, and greatly affects the modal measurement.
  • Special care must be taken to avoid impedance mismatches (reflections due to impedance mismatches can amplify intermodal coupling).

All this is solved if instead of using a chain of several devices for the modal measurement of emissions, a single instrument is used, specially designed for the measurement of the common mode and the differential mode.

From EMZER we recommend the EMSCOPE. It is a fully integrated system that allows the simultaneous measurement of conducted emissions present on line and neutral up to 110MHz. This feature allows the simultaneous measurement of the two modes, which facilitates the identification of the predominant mode. In addition, because it is an FFT-based instrument, it can display the entire measurement spectrum instantly, drastically cutting measurement time compared to conventional EMC instruments.

The best thing about this device is that it offers fast and accurate EMI measurements, both normative (between line-earth and neutral-earth pairs) and modal (common and differential mode), essential to be able to design an EMI filter quickly and efficiently.


EMZER: Modal measurements and EMSCOPE demonstration

EMZER is a company based in Barcelona, Spain which designs and develops innovative EMI receiver with focus on conducted emissions.

<H2> About modal measurements – news approach for conducted emissions

First of all, let’s do a quick reminder on how typical EMC measurements are done. The standard approach is that you measure only one channel at a time whether it’s line or neutral. If it’s three-phase you measure line one, line two, line three and then neutral.

If you use an analog receiver it takes a life to make the measurements, but most of all, it’s not possible to determine the source of the noise. This means that you can measure the total emission of one channel, which can either pass or not. But this emission is composed of common and differential mode and if you don’t have the information on how much of this noise is common and which part differential and in which frequency ranges, it is not possible to correctly design the power line filter.

That’s because the power line filter is composed of components, mainly chokes and capacitors whose duty is to reduce a particular modal emission.

All of this, of course, makes it cost consuming and this is why we proposed a new approach, which is design your filter by analyzing the modal measurements.

<H3> Why Modal Measurements are important

Modal measurements are composed of common and differential mode, if you look at the following two images, you will identify the differential mode as the noise that goes through one conductor and comes back in the other conductor in the opposite direction, while the common mode goes in the same direction for both conductors and it recloses through ground or through stray capacitances.

This is very important for filter design, because each component of the filter acts on each mode and by exactly knowing in which frequency ranges there is the noise, you can pick your components.

Of course, one option is to use the big filters, for example the box filters, but the problem is that, for instance the box might be too big or too expensive or simply doesn’t work so you will need to increase the size of your EUT or take something out to reduce performances somehow.

<H3> EMSCOPE Modal Measurements Capabilities

Now, let’s do a quick reminder on the effect of each component on the modal noises: the common mode choke acts on the common mode noise while the capacitors act more on differential mode, especially the CX capacitor placed between line and neutral.

Our EMSCOPE is composed of two receivers with three detector each which always measures simultaneously the two lines.  This is why it is possible to decompose the emission into modal noises.

<H2> EMSCOPE Demos Set-Up

Now, we will present the demo set-up, in the left part, we have our EUT which is a switching mode power supply. This EUT goes through a filter selection box in which we have four positions.

The first one (nº4) goes directly to the receiver, so we measure the emission of the EUT without anything else connected.

Then we have three positions where we placed three different filters.  We have a really  big filter in the box, a smaller filter, and a PCB where we designed our own filter, thanks to the information that the EMSCOPE can give us.

<H2> EMSCOPE’s Software

This software is embedded inside the instrument. All updates are free, this rarely happens in EMC, you usually have to pay a yearly fee maintenance, but this doesn’t happen with the software inside the EMSCOPE.

As any other software, you can go in device configuration and set all your parameters or update advice for free.

On the left you see a menu, top to bottom, you can use it to configure all the measurement that you have to do. For instance, the frequency band it’s already here so we can just select it and modify still the start and stop frequency.

So, according to the CISPR, we don’t even have to know that the resolution band is 9kH, the software does it for use. We can then set the dual time and sweep mode.

In “amplitude”, you can take into consideration external losses and compensate them, since we are using the internal LISN, we are already compensating it.

Now, let’s see how this trace configuration works. As you can see, if you look at the clear/write option, it has a different color, it means that it is selected.

When we go down, you can see we are also selecting EMI measurement; in this case, we are measuring the line with respect to the ground with the quasi-peak detector.

All this information can be found in the tab’s top left corner, in this case, T1 L-G QPK CLR means: Trace one, Line, quasi-peak, and we are clearing. The blue graphic is our measurement.

Now, next to the top of the tab, we have a small plus sign. When you click on it, it will add another tab, just like would happen with Google. The difference is that you will still see, in the same page, the measurement that you were doing before. This is used both for comparison or for multiple measurements at the same time.

For instance, you would see that here, the software is opening another tab, and in red color we are measuring the line with the peak detector, and we are measuring. Of course, clear means that every second (we have set the dual time at one second) it’s measuring. It’s an effective receiver so the measure it’s really fast. In one second, we can measure from 150kHz to 30 MHz.

Keep in mind that, if you have an analog receiver, you have to do the measurement with peak detector, scan all the band, and then go back with the quasi-peak detector. This doesn’t have to happen with EMSCOPE.

Now, we have trace one and trace two with the two colors, but remember that we can still open other tabs because, as you can see in the following picture, the receiver has two channels, which means that we are measuring the line, the neutral and we are also measuring them simultaneously and with three detectors each.

For instance, we could open another tab, click the neutral and put the quasi-peak detector.

We can even open another tab, add the neutral and the average detector, so in one second, we have the measure on the line neutral, with peak, with quasi-peak and average detector.

Now, going back to the first two tabs, as you can see, we are clearly above the limit. We can also check it with our list of maximum emissions. It will mark all of the emissions, all the points, but we can clearly see that we are failing the EMC test.

At this point, the common thing to do is to use a power line filter. To do so, the first thing we are going to do is save the measurements we have gathered in case we would want to see them later.

Now, we are going to show you what happens when you do what is commonly done in the wrong way in EMC: put a big filter and see if it works.

So, when we do put the big filter, we can see that we pass the EMC test, which means that the product is good to go. But it also may happen that the filter is too big and cannot be used or it’s too expensive or whatever problem you may have.

So, what is the correct way to pass conducted emissions tests?

First of all, we are going to go back to the product without filter and, as we were explaining before, perform modal measurements. To do so, we are just going to click in modal, and then click common mode.

So now we have that the trace one is measuring common mode with a quasi-peak detector.

Now, we are going to trace 2, and measure the differential mode with the quasi-peak detector.

As you can see, our emission is composed of both modes. Sometimes it can happen that we have one mode at different frequencies, so we would need different components in the filter. But for now, let’s say that at lower frequency we have a really high peak of common mode, and then differential mode.

Now that we have seen how to correctly test EMC, we will show you another thing the program allows us to do which is the freeze option. To do so, we will freeze both trace 1 and 2.

Next, we will open another tab to measure the differential mode with the quasi-peak detector, and, in another tab, we are going to measure the common mode with the quasi-peak detector.

To sum up, we have frozen two traces, of what the EUT is emitting, and then, we have opened two traces which are continuously measuring.

So, right now, if we’d change the filter and put the big filter, we would see it’s action on the both modes.

Now, if we went back to position one, what we’d see is difficult to understand, so we are using another EMSOCPE function, which allows us to hide traces.

So, for instance, we will hide the differential mode. You will see now why at the beginning we changed the colors to make the common and the differential mode different. As you can see in the image, the blue line is the frozen mode, so EUT without filter. And common mode, represented in red, means WUT with filter number one, the big filter.

We can see that we have a good reduction at high frequencies of common mode, but at lower frequencies not so much. We do have an important reduction but we always have to make sure that this measurement is done in a shielded environment to prevent unwanted couplings, for example.

We are not far from the limit, and measurements of common mode especially are highly affected by the setup and ground connection, so we will do some more adjustments.

To start, we will hide the common mode and show the differential mode.

Here, we have the differential mode in red, the one that is not filtered and, in blue, the filtered one. As you can see, this are really good performances of the filter for differential mode. It’s a really good capacitor.

Now, we are going to hide everything and put the second filter, the smaller one, on. It’s like the metallic box, only smaller, which means less space, and probably less expensive. To check it, we will still use the function to hide the noise.

Let’s compare common modes, so let’s hide the differential modes, and see the difference between the common modes with and without the filter.

We still see a better reduction than with the first filter. As you can see, it has reduced 15 dB at the frequency of 170 kHz which was the point that was giving us problems.

But, if we check the differential mode, we will see something unexpected. The differential mode is higher than the one before, when it was really low, but by changing the filter, it has increased again.

The conclusion, is that these two filters are basically giving the same attenuation but we are not really improving anything. If we were to do a measurement of EMI, (selecting clear/write and putting the line with the QPK detector), we could say that it passes. Remember that this is with the small filter.

Now, if we freeze this measurement, open another tab, and measure the same with the quasi-peak and the big filter, we can see the difference in the overall emissions that these two filters give.

So, we see, apart than the peak at 200 kHz, it’s practically the same, the same happens at 400 700 kHz where it’s giving more attenuation the red one but still, not so relevant.

What we should do here is design a filter using the Wurth filter kit and really solve the problem.

So, for instance, we go in clear/ write and we do the modal measurements with the common mode.

Then, we open another tab for the differential mode with the quasi-peak, and we get the filter out of the configuration again, so that we have the total noise.

Next, we freeze the common mode again, and open two more tabs to measure the same, and we put on the 3rd filter, the PCB, our designed filter.

What we see is that, if we hide and compare one to one, we have a huge reduction in common mode at the lower frequency, we could say a 60 dB reduction.

And also, in differential mode we have a huge reduction too.

We have designed a smaller filter with less components, but with better performances. Let’s check it by measuring it again with the EMI measurements, the quasi-peak detector, and in another tab, we are going to measure it with the average.

As you can see, the results are okay; we have a 40 dB difference, and we are sure that if we were to go to an EMC lab for the final CE stamp on the product it would pass with no surprises. Our filter is perfect for this power supply.

As you have seen, we have designed the most optimized filter both for size, and also for price.

For instance, if your goal is to use the smallest filter in size you can use the modal decomposition to achieve this result.

On the other hand, if your goal is to minimize price, you could settle for components that aren’t as good in performances, but that are cheaper.

The important part is that, when you go to the EMC lab for the final stamp you get a pass. Failing this test means a great loss of time and money and, of course, more time to solve the problem you have. Because if you try to solve an EMC problem without knowing common and differential mode and do a real time comparison, you will enter in a very long loop of trial and error.

Finally, another thing that this software allows you to do is a pdf report which proves your work with your optimized filter and with the results.

Of course, with this software you can do many more things, such as waterfall function. With it, in one click you see the waterfall function and the time frequency diagram, but still, the really major point of EMSCOPE and the advantages it brings, are focused on model noises which are the only components that you need to look at when you design a filter and especially if we wanted to make it small optimized and with good performances.


About EMC: conducted emissions

Conducted emissions are those defects in the quality of electrical energy that occur due to electrical and magnetic coupling. Therefore, conducted emissions are part of electromagnetic compatibility (EMC) problems in electrical engineering.

At Emzer we are experts in the design and development of EMC measurement devices that help detect any electromagnetic interference. For this reason, in this article we will analyze in depth what conducted emissions are and their effect on EMC between electrical devices.

What is EMC?

EMC, also known as electromagnetic compatibility, is the ability of an electronic device system to function correctly without altering in any way the operation of other systems.

Therefore, EMC comprises the rules that prevent electrical and electronic devices (EEs) from interfering with each other, as well as from interfering with their surroundings (electromagnetic pollution).

There are many reasons why EMC is an increasingly important concept, here are some of the most relevant:

  • Increasingly large and highly complex equipment
  • Increase in the amount of equipment and electronic devices in multiple fields (industry, medical, at home…)
  • Increase in the telecommunications systems

What are conducted emissions?

As we have mentioned, within the EMC it is worth highlighting the role of conducted emissions

Conducted emissions are part of circuits electromagnetic interferences. This generates problems in the quality of the electrical energy supplied. The main reason this happens is interference caused by linear and non-linear loadsthat exist in the electrical system, due to other consumer electronic devices.

Therefore, conducted emissions decrease the quality of the electrical energy that is supplied from the main electrical system, thus affecting the performance of the appliances that depend on it.

On a technical level, conducted emissions could be described as electrical tension or noise in the current. This should not be confused with noise in the signal, which differs from emissions because they exist in a finite power signal, while noise exists in a signal of finite energy. 

The emission must be filtered into the device being tested, as it can exist from the receiver and to the sourcegoing through the entire circuit where there is electron flow. To achieve this, devices must be tested at the factory, following the common conducted emission standards specified in the EMC test list.

The importance of EMC measures

It is essential that all electronic and electrical devices comply with the limitation of interference that they can emit, specified in the EMC standards.

The process to reduce such interference can be difficult since it is possible that the device being tested has different modes of operation. From this point, the next step is to discover which mode has the worst case and determine if it can make the test fail.

If proper testing equipment is not available, this process can be long and tedious, but it is imperative to do so, since complying with EMC standards and reducing conducted emissions and all kinds of interference is essential to avoid serious consequences of the use of electrical and electronic devices.

EMC: common and differential mode conducted emissions

As we have seen, one of the main problems within EMC is conducted emissions. It is clear that to measure electromagnetic compatibility we must design filters to detect and reduce these conducted emissions.

To do so, we must take into account which part of the interference is common mode and which part is differential mode. The latter are understood as the unwanted potential difference between the conductors that carry any type of current. This type has a small amplitude and low frequency, while the common mode type has a large amplitude and high frequency.

There are several causes that can cause common mode interference in EMC. Next, we tell you a few: 

  • Interference by the device’s internal wiring to the power cable
  • Interference from nearby radio stations or lightning, for example
  • Potential difference (difference in ground voltage)

Emzer: EMC experts 

At Emzer we are experts in EMC measurement, therefore we offer the best and most innovative equipment on the market to detect interference of all kinds.

Do you have doubts about how to do an EMC analysis and measurement on your devices? Our team is formed by researchers and engineerspassionate about technology and eager to continue innovating and evolving in the field of EMCContact us and we will help you with anything you need.


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