Filter Components and Filter Circuit Design in PCB EMC Engineering

Filtering is a fundamental part of PCB design. It includes both dedicated signal-filter design and the widespread use of power-supply filter capacitors. Filtering is necessary for two reasons. First, other design measures cannot fully suppress conducted noise that enters or leaves an equipment. When electrical signals cross the equipment boundary, effective filtering is required. Second, changes in the output state of an integrated circuit, and other switching events, can create noise on the chip supply rail. That noise can disturb the operation of the same chip and of other chips that share the rail.

This article explains the role of filtering in power delivery, reviews the high-frequency behavior of resistors, inductors, capacitors, ferrite beads, and common-mode chokes, and describes the low-pass filter topologies used in EMC design. It also covers the layout and routing rules that determine whether a filter performs as intended. The emphasis is on the physical behavior of components at high frequency, because parasitic inductance, parasitic capacitance, equivalent series resistance, and self-resonance often decide filter performance more than nominal component values.

Highlights:

Power-supply filter capacitors supply transient current locally, preventing the equivalent inductance of the supply path from producing voltage fluctuation and radiated noise when an IC changes state.
Resistors, inductors, capacitors, and ferrite beads all have frequency-dependent parasitics; inductor self-resonance and capacitor ESL/ESR must be included in filter design and insertion-loss calculations.
Low-pass filter topology must be selected according to high-frequency source and load impedances, and the layout must provide a low-impedance ground, isolate input from output, shorten signal paths, and place interface filters near connectors.

Filtering in PCB Design

Filtering is essential because other methods cannot fully suppress conducted noise entering or leaving equipment. When electrical signals enter or leave an equipment, effective filtering is required. Separately, changes in the output state of an integrated circuit, and other causes, can generate noise on the chip supply and affect the normal operation of that chip or other chips. The following example shows the function of a power-supply filter capacitor.

In a practical supply path, there is always some inductance. Figure 3-1 illustrates this condition: the power trace that feeds IC1 has an equivalent inductance L. When the output of IC1 changes from 0 to 1, the supply VCC must charge the load capacitance C with a pulse of current, dI. Because the current changes, a voltage ΔV appears across the equivalent inductance L.

Figure 3-1 | There is a certain amount of inductance on the power trace.

This ΔV can cause the circuit to fail functionally. It is also a major radiation source, so it increases the radiation from the board. Figure 3-2 shows how the output-state change of IC1 produces fluctuation on the supply.

Figure 3-2 | Changes in the output state of IC1 cause fluctuations in the power supply.

A filter capacitor reduces the problem. The improved circuit is shown in Figure 3-3. When the output of IC1 changes from 0 to 1, the required instantaneous current is no longer supplied by VCC through the inductive path. Instead, the filter capacitor C2 discharges and supplies the transient current needed for the logic transition. The current loop through the supply-line inductance is thereby avoided, along with the supply noise that the inductance would otherwise produce.

Figure 3-3 | Application of power-supply filtering capacitors.

Filter Components

Common filter components include resistors, inductors, capacitors, and ferrite beads. Each has a different high-frequency equivalent circuit, and that difference determines where it can be used.

Resistors

Resistors are not used alone for filtering. They are normally combined with capacitors to form an RC filter network. The high-frequency and low-frequency behavior of a resistor differs significantly because of lead inductance (ESL) and parasitic capacitance. This difference must be considered when designing a filter. The same figure also compares the low-frequency and high-frequency behavior of a wire, which further demonstrates why filtering is necessary.

Figure 3-4 | High-frequency and low-frequency characteristics of wires, resistors, inductors, and capacitors. Figure 3-4 also shows the difference between the low-frequency and high-frequency characteristics of wires, further illustrating t

Inductors

An inductor also has equivalent series resistance (ESR) and parasitic capacitance. These parasitics create a self-resonant frequency f_c. Below f_c, the component behaves as an inductor. Above f_c, it behaves as a capacitor. This change in behavior is particularly important when calculating the insertion loss of a filter. A design that treats the inductor as an ideal element over the full frequency range can therefore give a misleading result.

Capacitors

Capacitors are the most frequently used devices in filter circuits. Their detailed characteristics are addressed in the subsequent discussion of capacitors.

Ferrite Beads

Ferrite beads are another common filtering component. The ferrite used for electromagnetic noise suppression is a magnetic material made by mixing iron, nickel, and zinc oxides. It has high resistivity and relatively high permeability, typically about 100 to 1500. A ferrite bead is connected in series with a signal or power path to suppress differential-mode noise. When current flows through the ferrite, low-frequency current passes with almost no attenuation, while high-frequency current is subject to large losses and is converted into heat.

A ferrite bead can be modeled as a resistor and an inductor in series, but both the resistance and the inductance vary with frequency. Figure 3-5 shows a typical frequency characteristic.

Figure 3-5 | Typical frequency characteristics of a ferrite bead (Source: muRata).

Compared with an ordinary inductor, a ferrite bead provides better high-frequency filtering. At high frequency, the ferrite becomes resistive, behaving like an inductor with a very low quality factor. It can therefore maintain high impedance over a wide frequency range and improve high-frequency filtering effectiveness. This material behavior is one reason ferrite beads are useful in noise suppression, where a conventional inductor would become capacitive above its self-resonant frequency.

Common-Mode Chokes

A common-mode choke is inserted into a pair of transmission conductors. It suppresses common-mode high-frequency noise on each conductor with respect to ground. The usual construction places two identical windings on the same ferrite ring. The ferrite has low magnetic loss, and the winding arrangement is made so that common-mode currents produce magnetic flux that adds in the core. The resulting inductance is large, so common-mode current is suppressed. When differential-mode current flows through the two windings, the magnetic fluxes cancel, so the inductance is almost zero and the differential current can pass without attenuation.

Figure 3-6 shows a typical frequency characteristic of a common-mode choke.

Figure 3-6 | Typical frequency characteristics of a common-mode inductor (Source: muRata).

Filter Circuits

In EMC design, the purpose of filtering is mainly to attenuate high-frequency noise, so filters are usually designed as low-pass filters. Figure 3-7 shows the typical structures.

Figure 3-7 | Structural forms of low-pass filters.

The choice of structure depends on the high-frequency source and load impedances:

Figure 3-7(a) is an inductor filter. It is suitable when both the source impedance and the load impedance are small at high frequency.
Figure 3-7(b) is a capacitor filter. It is suitable when both the source impedance and the load impedance are large at high frequency.
Figure 3-7(c) and Figure 3-7(d) are Γ-shaped filters. The former is suitable when the source impedance is small and the load impedance is large at high frequency; the latter is suitable when the source impedance is large and the load impedance is small.
Figure 3-7(e) is a Π-shaped filter. It is suitable when both the source impedance and the load impedance are large at high frequency.
Figure 3-7(f) is a T-shaped filter. It is suitable when both the source impedance and the load impedance are relatively small at high frequency.

Another frequently used filter is the power-supply EMI filter. Its structure is shown in Figure 3-8.

Figure 3-8 | Power-supply EMI filter.

Layout and Routing of Filter Circuits

Layout and routing of a filter circuit require strict attention. First, the ground of the filter circuit should be a low-impedance ground, and different functional circuits must not share a common ground impedance. A shared ground impedance allows noise currents to develop voltage drops that can couple into other circuits, so the filter return path must remain dedicated and low impedance. Second, the input and output of the filter circuit must not cross each other; they should be isolated. Third, the signal path should be as short and as simple as possible, and the equivalent series inductance and equivalent series resistance of the filter capacitor should be minimized. Fourth, an interface filter circuit should be placed as close as possible to the connector.

Related Reading: