Design Techniques For Reducing EMI In SMPS Circuits

It is important to understand what EMI is and how it can be reduced in Switch Mode Power Supply or SMPS circuits

We live in the age of the technological revolution. The creation and implementation of innovative solutions to contemporary electronic problems continues at an increasing pace and in a very fast time. Communication is one area that has developed rapidly over time, with many systems now preferring wireless transmission, thus relying on radio and electromagnetic waves to transmit signals.

While there are many benefits as a result of this change, it doesn’t come without downsides. One of the main challenges this technology has to face is electromagnetic interference. The inability to measure and respond to such intrusions can result in the production of equipment that simply falls short of acceptable standards for certification. Therefore, it is important to understand what EMI is and how it can be reduced in Switch Mode Power Supply or SMPS circuits.

What is Electromagnetic Interference (EMI)?

Any device with electronic circuits that is subject to electromagnetic or electromagnetic interference, including SMPS circuits. As the EMC Handbook explains, when an electromagnetic field or electromagnetic signals reach where they are not needed, they can cause significant damage. These are essentially “noise signals” that interfere with communication signals sent by a particular system, resulting in inconsistencies such as distortions, distortions, and even causing some information to be lost in the transmission process.

In addition, these signals also have the potential to damage the SMPS circuit itself by causing electromagnetic induction or electrostatic coupling due to an additional unwanted electromagnetic field. Electromagnetic interference is a phenomenon that must be considered in any and all fields that use electronic components, including but not limited to telecommunications, military, aerospace and other electronic devices.

EMI also has a relationship with electromagnetic susceptibility (EMS) and electromagnetic compatibility (EMC). EMS refers to the sensitivity of electronic equipment to electromagnetic interference while electromagnetic compatibility does not require dangerous interference sources and equipment within electronic equipment such as power units, or the power system has good anti-interference ability.

Types of electromagnetic interference

Electromagnetic interference can arise from many sources that can be natural (eg electrical storms) or man-made (eg other nearby electronic devices). They can be broadly classified as:

1. Intentional EMI

Intentional electromagnetic interference, also known as functional electromagnetic interference, is intentionally created by equipment designed to emit electromagnetic energy, such as radars and radio transmitters. This type of interference is often used in jammers and other forms of electronic warfare.

2. Unintended EMI

Unintended or non-functional electromagnetic interference is generated in systems that do not need it or in systems that are not designed to emit electromagnetic emissions. You may find unintended EMI in devices such as motors, computers, power lines, electrical control units, etc.

3. Intrasystem EMI

As understood from the name, the interference that results in the system and causes unwanted coupling or damage within that system is within the system EMI. For example, voltage spikes on power cables can have a self-distorting effect which is an undesirable consequence of unwanted electromagnetic interference.

4. Intersystem EMI

Intersystem EMI is interference from one system that causes damage to another separate system, usually operating within a wide frequency range (from 50 Hz to several GHz).

Main source of EMI coupling in SMPS circuits

EMI coupling in SMPS circuits basically refers to the mechanism by which electromagnetic interference reaches its destination, the affected electronic circuits or devices, from its source. In order to effectively confront the problem, it is important to understand how it occurred in the first place. There are three main ways EMI coupling occurs in SMPS circuits:

1. EMI radiated

The most common type of EMI coupling mechanism is the emission that reaches the receiver via radiation. Such interference is able to travel through space from the source to the circuit. SMPS circuits are particularly vulnerable to radioactive electromagnetic radiation that arises due to alternating currents with high di/diostic and is propagated by poor layouts that encourage formation of current loops.

2. Conducted EMI

As the name suggests, the EMI made is the emissions that are transmitted along the conductors. These are usually wires, cables, and traces of copper connecting the affected SMPS circuit receiver to the interference source. Conducted electromagnetic interference is also categorized as common-mode conductive emissions and differential-mode conductive emissions based on whether the interference appears in phase or out of phase on the conductors respectively.

How to reduce EMI in SMPS circuits

In order to ensure that electronic devices and equipment are not excessively vulnerable to the effects of EMI, certain regulatory standards have been established for these devices to meet before they are approved for use and distribution. There are several ways to reduce EMI in SMPS circuits, the most effective of which include the following:

1. Go linear

While linear power supply designs tend to be bulkier, they certainly produce much less interference. Therefore, if your device is able to operate efficiently without SMPS circuits, consider using a linear power supply to reduce the EMI effect.

2. Use power units

Power modules are often used as quick and easy solutions to reduce electromagnetic interference. These are prepackaged units designed to offer a good combination of efficient power supply and good EMI performance. The latter is made by ensuring that there is an insignificant loop area within the SMPS circuit. Other units may supplement this with additional shielding of the inductors and switch node. The goal is to reduce radiated electromagnetic interference which is often the main instigator of damage in these circuits.

3. Use a suitable ground system

Signals and return currents use terrestrial systems. Proper grounding depends on several factors, such as frequencies, impedances, and length of cables required.

The most suitable type of ground for low frequency applications is the single point ground. Return currents from the three circuits will flow through the common ground impedances connecting the circuits. Hence, series or cascade connection should be avoided when sensitive circuit or cables are used.

On the other hand, parallel connection to grounding is preferred to avoid common impedance coupling. It is more complex and more expensive to implement due to the amount of wire required.

In general, most systems use a combination of both topologies.

4. Electromagnetic shielding

Electromagnetic shielding is one of the most common methods used to reduce electromagnetic interference in SMPS circuits. Circuit Digest describes it as using a “metal casing or shield” to enclose the electronic circuit. This creates a physical barrier between the source and destination of the interference, weakening or attenuating the electromagnetic interference signals. The shield is made of strands of braided copper (or similar metal), helical copper tape, or an additional conductive polymer. The effectiveness of shielding depends on the three factors; Reflections, absorption and multiple reflections.

There are two types of cables for protection against electromagnetic interference:

  • Cables with insulated conductors are called shielded cables. Shielded cables are regularly thicker and tougher than unshielded cables.
  • Unshielded cables do not have an internal shielding to reduce electromagnetic interference. Under this condition, uninsulated cables cancel out EMI by using a twisted pair of wires. These lightweight and thin cables make them best for indoor applications with LAN or similar network cabling systems in an office environment.

5. Filter design

As for the noise being made, common mode (CM) and differential mode (DM) sounds are caused by fast switching and voltage or alternating current. The combination with an inductor and a capacitor (LC filter) can effectively reduce noise disturbing the transmission line.

Common mode noise solution:

In solution, the normal throttle and Y cap are the main components. The common style choke consists of two inductors of the same steering polarity on the core. The Y cap is the capacitor from the positive or negative line connected to ground.

The path from the positive line to ground or the negative line to ground is a high-pass filter. The filter dampens high-frequency noise between the positive and negative lines to the ground state.

Differential mode noise solution:

The combination of a differential mode throttle and two capacitors is a Pi filter (low pass filter), which reduces noise between the positive and negative lines.

6. Optimization Planning

Bad design is often the main problem with most SMPS circuits that experience EMI-related damage. Here are some basic ideas to use to improve SMPS circuit layout to reduce EMI generation:

⦁ Place irritating knots as far away from noise-sensitive knots as possible.

⦁ Keep the distance as short as possible by moving the components.

⦁ Route the wire loop paths parallel to the return paths to reduce over-inductance.

⦁ Let the uninterrupted ground level lie below the EMI source.

⦁ Place the wires bearing the switching waveforms close to the loop area to reduce the radiated electromagnetic interference.

Employing the use of EMI filtering to counteract the electromagnetic interference that was made.


EMI mitigation is sometimes referred to as the “dark art” due to how difficult switching power supplies can sometimes be designed. However, the more you get used to these techniques, the more natural the process becomes for you.

The article was submitted by MORNSUN, a national high-tech enterprise based in China. MORNSUN specializes in magnetic insulation technology and product research and application, and manufactures high quality products including AC-DC Converter, DC-DC Converter, AC-DC Closed Switching Power Supply, Transceiver Module, Signal Conditioning Module, IGBT Driver, LED Driver, EMC assistant device, etc.

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