Correct Selection and Application of Electromagnetic Compatibility Components

In a complex electromagnetic environment, every electronic and electrical product must withstand a certain level of external electromagnetic interference (EMI) to function properly. At the same time, it must not generate electromagnetic interference that exceeds the tolerance of other nearby products. This requires meeting both electromagnetic sensitivity limits and electromagnetic emission limits as outlined by relevant standards. Achieving this balance is crucial for ensuring electromagnetic compatibility (EMC) and obtaining certification for electronic and electrical products.

Many engineers face challenges when selecting and using EMC components during product design. It is important to understand the components' characteristics in order to design electronic and electrical products that meet standards while optimizing performance and cost. Below, we explore common EMC components and circuit design techniques used to reduce or suppress electromagnetic interference.

Components Group

There are two primary groups of electronic components: leaded and leadless components. Leaded components have parasitic effects, especially at high frequencies. Each pin introduces a small inductance (~1nH/mm/pin) and a capacitive effect (~4pF). Therefore, minimizing the length of the pins is important for reducing these parasitic effects.

In contrast, leadless and surface-mount components exhibit smaller parasitic effects, with typical values around 0.5nH for parasitic inductance and ~0.3pF for terminal capacitance. For electromagnetic compatibility purposes, surface-mount components are the most effective, followed by radial lead components, and finally, axial parallel lead components.

1. Capacitors in EMC Components

Capacitors are among the most commonly used components in EMC design. They are primarily used to create low-pass filters, decouple circuits, and bypass signals. Proper selection and use of capacitors can effectively solve many EMI issues while offering a low-cost and efficient solution. However, improper selection may exacerbate EMI problems.

In theory, larger capacitance values offer better filtering, but capacitors with larger capacitance also tend to have larger parasitic inductances and lower self-resonant frequencies. For example, a typical ceramic capacitor with a capacitance of 0.1 μF has a self-resonant frequency of around 5 MHz, while a 0.01 μF capacitor resonates at around 15 MHz. Capacitors with these characteristics can perform poorly in decoupling high-frequency noise, especially above 10 MHz.

To address these challenges, capacitors with special structures, such as chip capacitors, are preferred for RF applications. These capacitors have negligible parasitic inductance and a much higher self-resonant frequency, making them ideal for high-frequency applications (up to 200 MHz or more).

For VHF and higher frequency ranges, feed-through capacitors are used to suppress noise and protect shields from penetration.

2. Inductors in EMC Components

Inductors are components that can create a magnetic field, making them sensitive to EMI. Like capacitors, they can be used to address various EMC challenges. There are two main types of inductors: open-loop and closed-loop. In an open-loop design, the magnetic field is controlled by air, which can cause radiation and EMI issues. In contrast, closed-loop inductors completely control the magnetic field within a core, which helps minimize EMI radiation.

Inductors are typically made from two core materials: iron and ferrite. Ferrite core inductors are used for high-frequency applications (up to MHz), making them particularly well-suited for EMC applications.

In EMC applications, special inductors such as ferrite beads and ferrite clips are commonly used. Ferrite core inductors are most effective for high-frequency noise suppression.

3. The Choice of Filter Structure

Filters in EMC design are usually low-pass filters composed of inductors and capacitors (L-C filters). The filter's effectiveness depends not only on its structure but also on the impedance of the connected network. For example, a filter with a single capacitor works well in high-impedance circuits but performs poorly in low-impedance circuits.

Filter Classification

Filters can be classified based on either function or structure:

• Function-based classification: filters designed to block certain frequencies or reduce EMI.
• Structure-based classification: different arrangements of capacitors, inductors, and resistors to achieve optimal filtering performance.

Filter Selection

When selecting filters for EMC design, consider the frequency range, impedance, and placement within the circuit. Proper selection ensures that the filter meets the system's needs and effectively mitigates EMI without introducing additional noise.