Introduction to EMC Filtering

 Introduction to EMC Filtering

Filtering is essential in PCB design. On one hand, other methods cannot completely suppress conducted noise entering and exiting equipment; effective filtering is necessary when electrical signals enter and exit the equipment. On the other hand, changes in the output state of integrated circuits or other factors can generate noise in the chip's power supply, affecting the normal operation of the chip itself or other chips. Filtering measures are indispensable and the most commonly used countermeasures. Many filtering measures are often considered in schematic design, such as decoupling capacitors, three-terminal capacitors, ferrite beads, power supply filtering, and interface filtering. However, if filtering components are not placed properly during PCB design, the filtering effect will be greatly reduced, or even rendered ineffective.

The general principle for installing filtering components is proximity. For example:

Decoupling capacitors should be placed as close as possible to the IC's power pins;

Power supply filters should be placed as close as possible to the power input or output;

Filters for local functional modules should be placed close to the module's input;

Filters for external interfaces (such as ferrite beads) should be placed as close as possible to connectors.

There are many commonly used filtering components, including resistors, inductors, capacitors, and ferrite beads.

I. Resistors

[Image] Resistors cannot be used alone for filtering; they are generally combined with capacitors to form an RC filter.

The characteristics of a resistor are shown in the figure below. Due to the presence of lead inductance (ESL) and parasitic capacitance, the high-frequency and low-frequency characteristics of a resistor differ significantly. This should be considered when designing a filter.

II. Inductors

The high-frequency and low-frequency characteristics of an inductor are shown in the figure above. Due to the presence of lead resistance (ESR) and parasitic capacitance, an inductor has a self-resonant frequency f. Below f, the inductor exhibits inductive characteristics, but above f, it exhibits capacitive characteristics. This is an important point to consider when calculating the insertion loss of a filter.

III. Ferrite Beads

[Image] Ferrite beads are also commonly used filtering components. Ferrite, used for electromagnetic noise suppression, is a magnetic material composed of iron, nickel, and zinc oxides. It possesses high resistivity and high permeability (approximately 100-1500). Ferrite beads are connected in series in signal or power paths to suppress differential-mode noise. When current flows through a ferrite bead, low-frequency currents can flow with almost no attenuation, but high-frequency currents suffer significant losses, resulting in heat dissipation. A ferrite bead can be equivalent to a resistor and inductor in series, but both resistance and inductance values ​​vary with frequency. A typical frequency characteristic of a ferrite bead is shown in the figure.

Ferrite beads offer better high-frequency filtering characteristics compared to ordinary inductors. Ferrite exhibits resistivity at high frequencies, equivalent to an inductor with a very low quality factor, thus maintaining high impedance over a relatively wide frequency range, thereby improving high-frequency filtering efficiency.

IV. Common-Mode Inductors

By inserting them into the transmission wire pair, common-mode high-frequency noise from each wire to ground can be suppressed simultaneously. The common practice is to wind two identical coils onto the same ferrite ring. Ferrite has low magnetic loss, and the winding method allows the magnetic flux in the ring to superimpose when common-mode current flows through the two coils, resulting in a significant inductance that suppresses common-mode current. However, when differential-mode current flows through the two coils, the magnetic flux in the ring cancels out, resulting in almost no inductance, allowing the differential-mode current to pass through without attenuation.

V. Capacitors

Capacitors are the most widely used components in PCB EMC design. Capacitors can be categorized into three types based on their function:

Decoupling: Breaks the coupling between ports of a system or circuit to ensure normal operation.

Bypass: Provides a low-impedance path to ground at the point where transient energy is generated. This is an essential condition for good decoupling.

Bulk: Bulk capacitors ensure that the voltage does not drop when the load rapidly changes to its heaviest state.

For EMI filters, the requirements for the dielectric material of the capacitor are not high; usually, the absolute capacitance value, temperature coefficient, and voltage coefficient of the capacitor are not important. In EMC design, the primary function of filtering is to attenuate high-frequency noise, so filters are typically designed as low-pass filters.