Design Techniques For Reducing EMI In SMPS Circuits

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

We live in the age of technological revolution. Innovative solutions to contemporary electronic problems continue to be created and implemented with increasing frequency and in a very quick time. Communication is one of the fields that has evolved rapidly with passing time, with many systems now preferring to go wireless, thus relying on radio and electromagnetic waves for signal transmission. 

While there are many benefits as a result of this change, it does not come without its downsides. One of the major challenges that such technology has to face is electromagnetic interference. Being unable to measure and counter such interferences can lead to the production of equipment that is simply not up to the acceptable standards for certification. Therefore, it is important to understand what EMI is and how it can be reduced in Switch Mode Power Supplies or SMPS circuits.

What is Electromagnetic Interference (EMI)?

Any device having electronic circuitry is susceptible to electromagnetic interference or EMI, including SMPS circuits. As explained by the Electromagnetic Compatibility Handbook, when an electromagnetic field or electromagnetic signals arrive where they are not required, they can cause significant damage. These are essentially ‘noise signals’ that interfere with the communication signals being transmitted by a given system, thus producing incoherencies such as distortions, crackles, and even causing some information to be lost in the transmission process. 

In addition to that, such signals also have the potential to damage the SMPS circuit itself by causing electromagnetic induction or electrostatic coupling due to the presence of an additional, unwanted electromagnetic field. EMI is a phenomenon that has to be accounted for in any and all domains that utilise electronic components, including but not limited to communication, military, aerospace, and other electronic appliances.

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 EMC requires no serious interference sources and equipment inside electronic equipment such as power modules, or the power system has good anti-interference ability.

Types of Electromagnetic Interference

Electromagnetic interference can arise from many sources which can be either natural (such as electrical storms) or man-made (such as other electronic devices in the proximity). They can be broadly categorised as: 

1. Intentional EMI

Intentional EMI, also known as functional EMI, is generated intentionally by equipment that are designed to emit electromagnetic energy, such as radars and radio transmitters. This kind of interference is often employed in jamming devices and other forms of electronic warfare.

2. Unintentional EMI

Unintentional or non-functional EMI is interference generated in systems where it is not needed or in systems that are not designed for electromagnetic emission. You may find unintentional EMI in devices such as motors, computers, power lines, electrical controllers, etc.

3. Intrasystem EMI

As understood by the name, interference that is produced in a system and causing unwanted coupling or damage within that system is intrasystem EMI. For example, voltage spikes on power cables can have a self-jamming effect which is an undesirable result of unwanted EMI.

4. Intersystem EMI

Intersystem EMI is interference from one system causing damage in another discrete system, typically one that is operating within a wide frequency range (from 50 Hz to up to several GHz).

Major Source of EMI Coupling in SMPS Circuits

Coupling of EMI in SMPS circuits essentially refers to the mechanism by which the electromagnetic interference arrived at its destination, which are the affected circuits or electronic devices, from its source. In order to effectively counter the problem, it is important to understand how it is occurring in the first place. There are three main ways EMI coupling occurs in SMPS circuits:

1. Radiated EMI

The most common type of EMI coupling mechanism is emissions that reach the receiver via radiation. Such interference is able to travel through space from the source to the circuit. SMPS circuits are particularly susceptible to radiated EMI that originates because of switched currents with high di/dt and is propagated by poor layouts that encourage the formation of current loops.

2. Conducted EMI

As suggested by the name, conducted EMI are emissions that are transmitted along conductors. These are typically wires, cables, and copper traces that connect the receiver of the affected SMPS circuit with the source of the interference. Conducted EMI is further categorised as Common Mode Conducted Emissions and Differential Mode Conducted Emissions based on if the interference appears in phase or out of phase on the two conductors respectively.

How to Reduce EMI in SMPS Circuits

In order to ensure that appliances and electronic equipment are not overly susceptible to the effects of EMI, certain regulatory standards have been put into place for such 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 a significantly smaller amount of interference. Therefore, if your device can work efficiently without SMPS circuits, consider using a linear power supply to minimise the effect of EMI.

2. Use Power Modules

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

3. Use proper ground system

The signals and return currents use ground systems. The proper grounding depends on several factors, such as the frequencies, impedances, and length of cabling required. 

The most suitable type of ground for low-frequency applications is the single-point ground. The return currents from the three circuits will flow through the common ground impedances linking the circuits. Hence, the series connection or daisy chain should be avoided when sensitive circuitry or cabling is used.

On the other side, a parallel connection is preferred for grounding to avoid common-impedance coupling. It is more complicated and more expensive to implement because of the amount of wiring required. 

In general, most systems utilise a blend of both topologies.

4. Electromagnetic Shielding

Electromagnetic shielding is one of the most common methods used to reduce EMI in SMPS circuits. Circuit Digest describes it as using a ‘metallic enclosure or shield’ to enclose the electronic circuit. This creates a physical barrier between the source and the destination of the interference, resulting in the weakening or attenuation of the EMI signals. The shield consists of strands of braided copper (or a similar metal), a spiral copper tape, or some additional conducting polymer. The effectiveness of shielding depends on the three factors; reflections, absorption, and multiple reflections.

There are two types of cables for EMI shielding:

  • Cables with insulated conductors are called shielded cables. Shielded cables are regularly thicker and more rigorous than unshielded cables. 
  • Unshielded cables do not have internal shielding to reduce EMI. Under the condition, unshielded cables cancel out EMI by employing a twisted pair of wires. These lightweight and thin cables make them best for indoor applications with LAN or similar network cable systems in an office setting.

5. Filter design

For conducted noise, both Common Mode (CM) and Differential Mode (DM) noises result from fast switching and alternating voltage or current. The combination with an inductor and capacitor (LC filter) can effectively mitigate the noise disturbing the transmission line.

Common mode noise solution:

In the solution, the common mode choke and Y cap are the two key components. Common mode choke consists of two inductors with the same polarity routing on a core. The Y cap is the capacitor from the positive or negative line connecting to the ground. 

The path from the positive line to the ground or the negative line to the ground is regarded as a high-pass filter. The filter mitigates the high-frequency noise between positive and negative lines to case ground.

Differential mode noise solution:

The combination of the differential mode choke and two capacitors is the Pi filter (low-pass filter), which lessens the noise between positive and Negative lines.

6. Layout Optimisation

A poor layout is often the main issue in most SMPS circuits that experience EMI related damage. Here are some basic ideas to use to optimise the SMPS circuit layout to minimise EMI generation:

⦁ Position noisy nodes as far away from noise-sensitive nodes as possible.

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

⦁ Route the tracks of wire loops parallel to the return paths to reduce excess inductance.

⦁ Let the unbroken ground plane locate below the EMI Source.

⦁ Place wires carrying switching waveforms close to reduce the loop area for radiated EMI.

⦁ Employ the use of EMI filtering to counter conducted EMI.

Conclusion

EMI mitigation is sometimes referred to as a “dark art” for how challenging it can be at times to design switching power supplies. However, the more you get used to these techniques, the more natural the process becomes to you.  


The article has been presented by MORNSUN, which is a national high-tech enterprise headquartered in China. MORNSUN specialises in magnetic isolation technology and product research and application, and manufactures high-quality products including AC-DC converter, DC-DC converter, AC-DC enclosed switching power supply, transceiver module, signal conditioning module, IGBT driver, LED driver, EMC auxiliary device, etc. 

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