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.
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.
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.
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 interference that 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.
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, 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.
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 main filter must contain at least one type X capacitor between the 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 the 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 the common mode. That is why it is vital to be able to obtain complete and rapid information on the nature of the emissions.
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, the 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 online 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.
