Fundamentals in EMC (Electromagnetic Compatibility): Common Mode and Differential Noise

Common Mode and Differential Noise

As a quick reminder, different currents flow through the source and return in opposite directions, while common mode current flows through the source and return in the same direction, completing the circuit through the path to ground.

Figure 1. Differential-mode and common-mode noise paths
How do you know if you are dealing with common-mode noise or differential noise? This is a common question that needs to be answered.
Here's a quick trick to get you going in the right direction. While it's not 100% accurate, it helps to start the process.
Imagine a doctor prescribing antibiotics without knowing whether you have a bacterial infection or a virus. He knew that if you had a bacterial infection, the medication would work and the problem would be resolved. If the medicine doesn't work, at least he knows he's dealing with a virus and will treat you accordingly.
So in our case, you can simply clip the ferrite to the cable, remembering the two wires in the cable (Vcc and ground). If noise is reduced (or immunity is increased), then this is a common mode problem. If it doesn't work, it's a poor mode problem.
So, at the board level, if it is a common mode problem, a common mode choke can be used. If the problem is differential mode, a chip bead ferrite can be used.

Material property

Clamping ferrites are typically made of two different materials: manganese zinc (MnZn) and nickel zinc (NiZn).
Zinc nickel can be used in the case of high frequency conducted or radiated noise. Manganese-zinc is primarily used for low-frequency conductive noise because its higher permeability provides more impedance.
This image provides a guideline on which materials to use based on your situation. Of course, there are exceptions, but this is the typical example we found in

Figure 2. Clamped ferrite materials, where conducted and radiated immunity are tested and differential and common mode noise occurs in common frequency ranges

Reduce common mode noise

Here we have a visual representation of how common pattern chokes work.

Figure 3. Common Mode and Differential Signals
The red arrows indicate the incoming differential signal. This is a useful signal. According to the right-hand law, it generates a magnetic field inside the core that goes in one direction.
The differential signal then returns to the signal source, creating another magnetic field according to the right-hand law. These two fields will cancel each other out in the core.
Common mode noise also creates magnetic flux inside the core, but this time the two noise signals are in the same direction, as shown by the black arrows in the figure, causing the magnetic fields to add up. The core will respond to unwanted noise with high impedance.

when the signal fades

One thing to keep in mind when using a common mode filter is that there will be a differential impedance that will attenuate the useful signal. As shown in Figure 6, the blue line represents the common-mode impedance, and the red dashed line represents the differential impedance.

Image 6. The common-mode and differential impedances of a typical common-mode inductor
This means that if the signal is at 100 MHz, making

With this common mode solution, there will be some unintentional attenuation of the signal from the differential impedance.

Best way to filter out noise close to the signal frequency

Below is a concrete example. The blue line represents the common-mode impedance, and the red dashed line represents the differential-mode impedance. The useful signal is shown with a thick black line at 4mhz.

Figure 7. Avoiding the effect on the wanted signal.
This makes good use of the common mode impedance, high common mode impedance at just 4 MHz and low differential mode impedance.
Therefore, the influence on the noise is large and the influence on the useful signal is kept to a minimum. When reading the datasheet, check the common mode impedance and the differential mode impedance of the section.

Segmented and bifilar windings

Common mode chokes have two types of windings: segmented and two-wire.



Figure 8. Segmented vs. Bifilar Winding
Segmented winding components are most useful in power applications where high voltage wires must be separated by a minimum distance. They can also be used as inductors to attenuate differential mode noise. Segmented elements attenuate differential mode noise as well as common mode noise due to their high leakage inductance.
Dual wire wound sections are often used for low voltage data signal lines, since we don't want to attenuate differential mode signals (useful signals). Therefore, you have a lower leakage inductance, which is used to filter noise in the signal.
To sum up, the methods for attenuating common-mode noise for any part are very similar. However, there is a big difference in differential mode noise attenuation between segment winding and bifilar winding.
You can see these differences shown by the red and black lines in Figure 8. Note that the common-mode impedances (solid lines) are very similar, while the differential-mode impedances (dashed lines) are very different.

Select CMC via REDEXPERT

Würth Elektronik's online tool REDEXPERT can help you find the right components for noise reduction. Use REDEXPERT's EMI Filter Designer to help design and visualize the results. Just enter the details and noise of the system you are looking for attenuation to find the proper components.
By understanding the difference between common-mode and differential-mode noise, and how to use common-mode blocking, you will be able to reduce unwanted noise in your design. Our next article will have more information on differential noise and chip beaded ferrites, so keep an eye out for improving your EMC knowledge!

• Mr. Bai Jilong has been engaged in the electronics industry for 15 years. He has developed more than 100 products so far, and most of them have been mass-produced.

• It took 5 years since 2018 to record thousands of practical-level electronic engineer series courses, from components to core modules to complete products

• Lao Bai's original intention is "May the world's engineers not take detours" Among them, there are courses explaining MOS tubes and IGBTs in detail