Figure: Understanding Electromagnetic interference (EMI from a computer disturbs the TV)

Electromagnetic interference (EMI) refers to disturbances in the operation of electrical and electronic equipment. 

EMI is caused by unwanted electromagnetic signals (i.e., noise signals) emitted from man-made sources, such as the operation of electrical and electronic devices (e.g., switching operations in power electronic converters), as well as natural phenomena like electrical storms, lightning, or cosmic noise. EMI can sometimes even damage the equipment. 

 When such interference occurs in the radio frequency range, it is referred to as radio frequency interference (RFI), which can affect radio services and equipment. The figure shows EMI generated from a computer interfering with a nearby TV.  

Note: EMI generated by a device can sometimes disturb its own operation.

Electromagnetic Interference (EMI) types:

Classification based on the type of interference source:

Man-made EMI: This type of EMI is caused by man-made equipment, such as electrical & electronic devices, communication systems, or industrial machinery, which can disrupt the operation of nearby devices. For example, a drilling machine used nearby room may disturb the operation of other nearby electronic devices (e.g., TV) in the surrounding area.

Natural EMI: This type of EMI is caused by natural phenomena such as electrical storms, lightning, or cosmic noise, which can disrupt the operation of electrical and electronic devices in the surrounding area.

Classification based on duration:

Continuous EMI: This EMI is emitted from a source continuously and keeps disturbing the nearby device. The continuous EMI can be caused by man-made or natural sources.

Impulse EMI: This type of EMI is a pulsed electromagnetic signal that occurs either within a very short time period or sporadically. It can be caused by man-made or natural sources, such as current switching systems, electrostatic discharges, lightning, and other similar events.

Classification of EMI based on bandwidth:

Narrowband EMI: This type of EMI occurs over a discrete frequency (often comprising a single frequency) and can be caused by oscillators, communication transmitters, and test equipment. It mainly affects communication devices such as radios, televisions, and smartphones.

Narrowband EMI is less likely to cause permanent damage to equipment or systems and is easier to mitigate than broadband EMI.

Broadband EMI: This type of EMI occurs over a broad frequency spectrum or comprises multiple frequencies. Broadband EMI can permanently damage the equipment or system and can be caused by both natural and manmade sources. Examples of manmade sources include computers, switching circuits, arc welding, SMPS, defective fluorescent lights, communication transmitters, equipment malfunction, and more. An example of a natural source is solar radiation, which can disrupt the satellite signals.

EMI coupling methods:

Electromagnetic interference (EMI) signal generated from a source may reach or interfere with the nearby victim through four coupling methods: conductive coupling, radiated coupling, inductive coupling, and capacitive coupling.

 For example, the EMI signal emitted from a computer may travel via one or more of these coupling paths/mechanisms to reach or interfere with the nearby victim device (e.g., TV). Let's understand these EMI coupling methods in more detail.

Figure: EMI coupling methods

Conductive coupling:

Conducted coupling occurs when an electrical path (i.e., supply lines or signal wires) exists between the EMI source and the victim. In this type of coupling, the high-frequency noise currents are generated from an EMI source and travel through connected lines (e.g., power supply lines or signal lines) and will disrupt the operation of other connected equipment on the same lines. This phenomenon is known as conducted EMI, which occurs in the low-frequency range (150 kHz to 30 MHz). Since the electromagnetic noise signal is transmitted along physical conductors, it is referred to as conductive coupling.

 A common example of conductive coupling is turning on the mixer in a kitchen, which may cause a disturbance on the TV screen connected to the same electrical lines.

Conductive coupling can occur in two forms: common-mode coupling and differential-mode coupling. In common-mode coupling, the noise currents flow in the same direction on both the conductors (e.g., live and neutral wires) with equal magnitude and phase relative to the reference ground and complete the circuit through the parasitic capacitances and ground path. In differential-mode coupling, the noise current flows through the conductors with equal magnitude and opposite directions (a 180° phase difference) to each other.

Inductive Coupling:

Inductive coupling occurs when an EMI signal from a source couples to a nearby victim (e.g., a nearby conductor or circuit) through a time-varying magnetic field.

 In this type of coupling, changes in current in the source circuit (e.g., conductor) create a time-varying magnetic field around the source circuit that will induce a noise voltage in the nearby circuit or conductor by the principle of electromagnetic induction. This induced noise voltage can interfere with the operation of connected devices, a phenomenon referred to as inductive-coupled EMI.

The intensity of the inductive coupling depends on the rate of current variation in the source circuit and the mutual inductance value between the source and the victim.

Inductive coupling is most likely to occur between conductors or circuits that are located very near to each other, typically within a distance of less than one wavelength.

Capacitive coupling:

Capacitive coupling occurs when an EMI signal from a source couples to a nearby victim (e.g., a conductor or circuit) through a time-varying electric field from the source. In this type of coupling, changes in voltage in the source circuit (e.g., conductor) induce a noise current in the nearby circuit or conductor. This induced noise current can interfere with the operation of connected devices, a phenomenon referred to as capacitive-coupled EMI. This type of coupling is most likely to occur between conductors or circuits that are located very near to each other.

The capacitive coupling depends on several factors, including:

• The distance between the source and the victim.
• Insulation medium between the conductors.
• Height of the conductors above the ground plane.

Note: Both inductive and capacitive coupling mechanisms can contribute to the conducted EMI.

Radiated coupling:

Radiated coupling occurs when the source and victim are physically located at a large distance and act like radio antennas. This type of coupling typically occurs at higher frequencies, where an EMI signal emitted by the source travels through air or space in the form of an electromagnetic wave and may disrupt the other nearby devices or circuits operation. For example, radiated emissions from power electronic converters (e.g., SMPS) may disrupt the operation of nearby devices, such as TVs, in the same environment.

A good understanding of EMI coupling methods is important for designing equipment with efficient and effective EMI mitigation solutions, such as EMI filters for mitigating conducted EMI and EMI shielding for reducing radiated EMI.