Introduction
A few basic system components are frequently used to mitigate or suppress electromagnetic interference (EMI) in devices. As engineers and technicians involved in compliance engineering, it is important to know what these components are, what they do, how they’re most effective, and when they’re ineffective. You will often be involved in reducing EMI or Electrostatic Discharge (ESD) to pass compliance tests or simply need to make your product more robust to EMI/ESD. So, it would be wise to know a little about the systems components used to reduce EMI. In alphabetical order, some of these components are:
- Capacitors
- Common Mode Chokes
- Diodes
- EMI Filters
- Ferrites
- Inductors
- Resistors
The following will briefly describe the characteristics of each one and how they are used to reduce EMI/ESD.
Capacitors
Capacitors are comprised of a dielectric material sandwiched between the conductive top and bottom plates. Capacitance (measured in Farads: pF, nF, µF, etc.) is the charge needed to create a certain potential between the plates. Larger capacitors can store more charge than smaller ones, and the dielectric quality is a determining factor for capacitance.
The material used for the dielectric can be air, paper, fiber, glass, PP, PET, Mica, Aluminum Oxide, Tantalum Oxide, Titanium Oxide, and Barium Titanate, all producing various dielectric constants (ε).
Capacitors that filter out system noise are called bypass or decoupling capacitors. Bypass capacitors create a shunt path to ground to remove system noise and prevent it from getting to susceptible areas. Decoupling capacitors are installed near digital devices to provide a localized DC power source. Ceramic capacitors are usually selected for decoupling.
Each type of capacitor (with its varying dielectric types) comes with pros and cons, the length of which is too vast to cover here. We’ll cover more on this in November 2023’s Product Insights article covering “Types of Capacitors Used in Filtering.”
Common Mode Chokes
Common mode chokes are special inductors used to attenuate unwanted common mode signals. Often seen at the front end of switch-mode power supplies, these devices come in various types that offer varying degrees of attenuation of various lines that require noise suppression.
Diodes
Diodes are analogous to one-way check valves, frequently used in plumbing. As such, in switching applications involving large inductive loads, diodes are often placed across plus/minus control terminals and as close to the transient noise source as possible to act as a transient voltage suppressor/clamp.
There are several different types of diodes used in EMI suppression. These include:
- Rectifier
- Schottky
- Transient voltage suppressor (TVS)
- Varistor
- Metal Oxide Varistor (MOV)
- Voltage-dependent resistor (VDR)
- Zener
Each type of diode listed above has various characteristics that work best to reduce specific types of EMI. This is a vast topic not covered here.
EMI Filters
EMI filters are low-pass filters used in a wide variety of situations. They are dual purpose in that they not only stop noise from entering a system but also stop noise in the system from leaving it.
There are various types of EMI filter configurations. The so-called feed-through capacitor is the simplest, consisting of only one component. Another simple form is called the L-circuit filter. More complex forms, comprised of both L & C components, is the PI-circuit or T-circuit filter. The latter two types are determined by the impedance (Z) of the circuit node that requires filtering. A high-Z noisy node needs a low-Z path (capacitor) to provide a diversion path for the noise current to flow. This situation requires a PI-circuit filter (capacitor-inductor-capacitor). A low-Z noisy node needs a high-Z filter element (inductor) to suppress the noise. This situation requires a T-circuit filter (inductor-capacitor-inductor).
See Reference 2, “What Every Electronics Engineer Needs to Know About: Filters”, for more information about EMI Filters.
Ferrites
Components like ferrite beads and inductors that are made from ferromagnetic materials are widely used for EMI noise suppression. They have three important properties to consider when applying them to various EMC applications. These properties are saturation, frequency response, and the ability to concentrate magnetic flux. In addition, these materials have large relative permeabilities (µr) that decrease when the current passing through them increases (saturation). Inductance also decreases with increased current. The high permeability of these materials creates the ability to concentrate the magnetic flux created in them. Nickel-Zinc (NiZn) material tends to have lower relative permeabilities than that of Manganese-Zinc (MnZn) materials (about half) but at the benefit of a flatter frequency response.
The authors of reference 3 provide the most practical explanation of how a ferrite works I have ever read. They state, “Ferrites are your friends for ESD. They’re like penicillin for ESD problems. The materials are very lossy in the 100- to 500-Mhz range, so they act like resistors against ESD. (Small beads look like 50 to 100Ω at 100 MHz).”
Inductors
Inductors are electrical elements used to introduce a desired amount of inductance into a circuit. Inductors are typically used in conjunction with capacitors to form low-pass filters.
Resistors
Resistors are typically used for Z-matching in signal integrity applications, current limiting, and in conjunction with capacitors to form RC low-pass filters, where sometimes the resistor used by itself will help suppress undesired EMI. The carbon film type is the most widely used resistor type. Other popular types include carbon composition, metal film, metal oxide, foil, and wire wound. Due to undesired inductive effects, wire-wound versions should be avoided for EMI critical applications.
Parasitic Behavior
When using any of the above components to reduce EMI, it’s important to consider their non-ideal, parasitic behavior. You may experience this effect when first trying to use a component to suppress an unwanted signal, only to find out it doesn’t work. This could be because of the component’s non-ideal, parasitic behavior. This is where capacitors no longer act like capacitors above their self-resonant frequency and act more like inductors! Similarly, inductors may act like capacitors above their self-resonant frequency. In your study of system components used for EMI, it is important to understand how each behaves above its self-resonant frequencies and no longer acts like a real capacitor, inductor, resistor, etc.
References and Further Reading
- Hu, R., PCB Design and Fundamentals for EMC, RANDSpace Technology LLC, 2019.
- MacArthur, D., “What Every Electronics Engineer Needs to Know About Filters,” In Compliance Magazine, November 2018.
- Paul, C.R., Scully, R.C., Steffka, M.A, Introduction to Electromagnetic Compatibility, 3rd Edition, John Wiley & Sons, 2023.
- Kimmel Gerke Associates, Ltd., EDN Designer’s Guide to Electromagnetic Compatibility, 2005.
