EMI: what it is and how to keep it in check

Electromagnetic Interference (EMI) is essentially invisible noise or pollution. It is unwanted electromagnetic energy generated by one electronic device that disrupts the normal operation of another nearby device. This FAQ will explain what an EMI is and what are the ways to prevent it.

This interference does not just travel in one way. Figure 1 illustrates the four primary methods by which EMI can travel from its source to a victim device:

Figure 1. Different sources of EMI, i.e., conducted, radiated, inductive, and capacitive, traveling from a source device to the test device. (Image: HardwareBee)

Conducted EMI, as the name suggests, is interference that travels along physical conductors. Figure 1 shows this as noise moving along the wire connecting the two devices. This is common in power cords and data cables, where the noise from one device is directly fed into another.
Radiated EMI is interference that travels through the air (or space) as electromagnetic waves, much like radio or Wi-Fi signals. The “Source Device” acts as a tiny, unintentional radio transmitter, broadcasting noise that the “Device Under Test” picks up like an antenna.
Inductive EMI is a near-field effect also known as magnetic coupling. As shown by the transformer symbol, a changing electric current in the source creates a changing magnetic field, which in turn induces an unwanted current in the nearby circuits of the test device.
Capacitive EMI is the other near-field effect, also known as electric field coupling. As represented by the capacitor symbol, a changing voltage (an electric field) on the source device can couple with the test device, inducing an unwanted voltage and disrupting its signals.

Understanding these four pathways is the first step in achieving Electromagnetic Compatibility (EMC), the goal of designing electronics that can function correctly without either causing or being affected by EMI.

How can EMI be prevented?

Preventing EMI is a core part of the entire electronics design process and involves three main strategies: reducing the noise at its source, blocking its transmission path, and making the receiving device less sensitive.

Reducing the source is the best approach to create less noise from the start. This can be done by choosing electronic topologies (circuit designs) that are inherently quieter, such as quasi-resonant flyback converters. Another method is softer switching, which involves slowing down the fast voltage and current changes that are the root cause of EMI.

This concept is visually represented in Figure 2. The “Hard switching” (red line) path shows high voltage and current at the same time, creating a large, abrupt energy spike that generates significant EMI. In contrast, “Soft switching” (blue line) ensures that either voltage or current is near zero during the switch, resulting in a smooth transition that drastically reduces the source noise.

Figure 2. Impact of soft switching and hard switching on the voltage vs current curve of a power switch. (Image: MDPI)

Block the path—if noise is generated, the next step is to stop it from traveling. This can be achieved in the following ways:
- For conducted EMI, the solution is an EMI Filter, which is illustrated on the left side of Figure 3. The filter is placed on the power line, taking in the signal + noise and diverting the unwanted noise to an electrical ground, allowing only the clean signal to pass.
- The right side of Figure 3 shows what this looks like on a real circuit board. The copper-wound coils (chokes) and gray blocks (capacitors) are the physical components of a two-stage filter designed specifically to block this conducted interference.
- For radiated EMI, the paths are blocked using physical layout and shielding. This includes making high-current loops on the circuit board as small as possible to reduce magnetic (inductive) coupling. It also involves placing components at curved angles or 45° bends to each other and using metal shields to block electromagnetic waves, just like a Faraday cage.

Figure 3. (Left) Illustration of how a filter helps in reducing EMI, (right) an actual implementation of filters in an electronic circuit. (Images: KEB Automation KG, EMC FastPass)

Hardening the receiver is the final strategy to make the “Device Under Test” immune to any noise that still gets through. This is achieved by designing sensitive circuits with low impedance (making them harder to disrupt) or by using differential signaling. Differential signaling works by detecting the voltage difference between two complementary signals, effectively rejecting noise that affects both signals equally.

Summary

EMI is an unavoidable side-effect of modern electronics, traveling as unwanted noise through conductive, radiative, inductive, and capacitive paths. Effectively managing it requires a comprehensive approach: designing quieter circuits from the start, using filters and shielding to block transmission, and building robust devices that can ignore interference.