When to Use Chokes or EMI Filters in High-Speed PCBs

Solving EMI challenges can be difficult, but one course of action that often arises in digital designs is to use EMI filters on lines entering or leaving via a connector. EMI filters can be designed from discrete components, but most commonly they are placed as simple chokes or integrated filter components on data lines. Chokes are also often used on power lines to filter EMI from incoming power, both for AC and DC inputs.

The use of chokes and EMI filters on power lines, both incoming and outgoing, is well understood. What about the use of EMI filters on data lines? Should this be done, and is it a better option than addressing the root cause of an EMI problem? Let's take a look at the use of these parts at low speeds and high speeds.

What Are EMI Chokes and EMI Filters?

These two types of components provide a simple function, namely to remove conducted EMI on a PCB trace. However, they do this in different ways and have different electrical characteristics, so they are typically used in different applications. Chokes are a type of EMI filter: all chokes are EMI filters, but not all EMI filters are chokes.

The main differences between these components are as follows:

• The components have different impedance characteristics as a function of frequency
• Due to the previous point, the components are qualified in different ways
• These components have different levels of power handling
• Chokes are purely magnetic components, while EMI filters have inductance and capacitance
• Both types of components aim to function as low-pass filters
• Both types of components can be single-ended components or can be used on coupled lines
• Both types of components are intended for suppressing common-mode or differential-mode noise

EMI Chokes

An EMI choke is essentially an inductor or a pair of coupled inductors, but they are used for a different purpose than typical inductors. For example, an inductor may need to function at a high frequency and have high peak current handling without entering saturation at the same time. In contrast, a choke may need to operate at lower frequency, such as an AC input or a DC input, and high-frequency magnetic saturation is not a concern.

There's also a difference in function: inductors are used to store and release energy, including at high frequencies, while chokes are meant to provide high impedance. It is sometimes said: all chokes are inductors, but not all inductors are chokes.

To better understand an EMI choke, let's look at an impedance curve for a typical component. An impedance curve for a common-mode choke (part number B82722A2302N001) is shown below.

In the graph, we can see that the impedance rises linearly to a high value over frequency and eventually reaches a peak. Then, the parasitic capacitance in the component takes over and the impedance starts to decrease, to show how chokes can be effective for suppressing low-frequency noise.

Chokes also have a typical structure for an inductor, which means they can have large windings with high current rating. A current rating of multiple amps is common, which is why these components are typically used on power lines.

Finally, we have chokes which can be used for common-mode noise suppression or differential-mode noise suppression. Most common usage is a common-mode choke, which would be used on incoming positive and negative power lines. Common-mode chokes also suppress a bit of differential-mode signal, as seen from the graphs below for a set of common-mode choke part numbers from Pulse Electronics.

EMI Filter ICs and Modules

EMI filter ICs are generally low-pass filters with much larger bandwidth than a choke. They are available for differential-mode and common-mode noise suppression, and they have lower power handling capability. For these reasons, EMI filter ICs are normally used for data lines rather than power. However, there are some packaged EMI filter modules which can be used for mid-range power levels.

Here are two good examples of these EMI filter components:

• Part number: PCMF2USB30Z - 6 GHz bandwidth, -15 dB common-mode noise suppression at 1 GHz, integrated ESD protection, BGA package
• Part number: BNX026H01L - Common-mode and differential-mode Insertion loss of 60 dB at 600 kHz, 20 A current limit, 12.1 mm x 9.1 mm SMD package

The first packaged EMI filter IC is a good example of a data line filter for high-speed differential channels, while the second packaged module is a good example of an EMI filter for power lines. In particular, the power filter circuit is a fourth-order filter that provides very strong roll-off at low frequencies while still providing decent power handling.

Data Line EMI Filter ICs

As I mentioned above, these designs can primarily target common-mode or differential-mode noise on data lines. Of course, the most common is the common-mode EMI filter version as this type of component is normally placed on data lines leaving a PCB through a connector.

Typically, these are differential lines, and although the differential interface is tolerant to common-mode noise, the common-mode currents on a cable will radiate strongly and can easily produce an emissions failure. For example, under FCC Class A and Class B, typical maximum common-mode current values are on the order of 10 microamps.

Next, one should note that there are EMI filter circuits which are rated for specific differential lines which require routing over a cable in order to reach a peripheral, such as HDMI, USB, or DisplayPort. These EMI filter ICs have been tested against the relevant standards by the manufacturer, so these should be selected when working with a standardized computing interface.

Just how much noise can you expect to suppress with a typical common-mode EMI filter IC? Take a look at the example below. The graphs below show the common-mode insertion loss and differential-mode insertion loss for part number ECMF02-2AMX6, which is rated for USB 2.0 and MIPI D-PHY/MDDI interfaces.

Here we can see that the filter provides significant insertion loss for common-mode noise, even well below the bandwidth limit.

The trade-off is some differential-mode insertion loss within the required range. However, the level of differential-mode loss is significantly lower than the amount of common-mode loss, so the majority of the differential signal still passes through the filter. This additional loss will reduce the signal level seen at the receiver, so the total channel length will be somewhat limited. Make sure to include this differential loss in your loss budget calculations when designing the channel.

Comparison with Discrete Filter Circuits

The EMI filter ICs listed above are very useful for addressing certain high-speed digital channels. However, it is worth asking, can we achieve the same performance in an EMI filter circuit design with discrete components?

First, it's worth looking at the circuit topology for the EMI filter IC listed above and a typical packaged EMI filter module.

Technically, you could build the same circuit topologies from discrete components. For the low-frequency filter case, you could match the same specs as the packaged module. However, for the data line EMI filter IC, you're unlikely to match their performance unless special considerations are taken.

• High-frequency rated components: you would need to use components that are rated up to the bandwidth range you are trying to reach and at your target impedance
• Traces and pads: these PCB elements add capacitance and inductance which will distort the inductance and capacitance encountered by a signal traversing the filter circuit

This is one of the reasons that high-bandwidth data line filters are used as packaged components rather than designing one on your own. For a much lower bandwidth interface, such as CAN bus, you could design a similar circuit topology from discrete components.

Input Power Filters With Discrete Components

A much more common instance of EMI filtering, both for differential-mode and common-mode noise, is for filtering input power coming into a system. This is most commonly used on input AC power which includes an earth connection, and it involves the use of chokes and capacitors for the construction of differential-mode and common-mode noise suppression.

An example circuit is shown below. This circuit would be placed on the AC input into an isolated DC power supply. The chokes shown in this example address common-mode noise and differential-mode noise about a virtual ground between the line and neutral connections. The earth ground is also used as a dump for high-frequency noise which couples through the safety capacitors.

• Learn more about EMI filters with discrete components

If you compare this to the packaged EMI filter module, you will see essentially the same single-ended circuit topology, but duplicated across the earth connection for the line and neutral inputs. Here the capacitors labeled “C” are Y-type capacitors and the other line-to-line capacitors are X-type capacitors.

The low-pass filtering limits for the circuit are given by the L and C values of the filter; this can also be simulated in SPICE. Additional circuitry is often needed for circuit protection or generally, such as surge protection, power factor correction, or a parallel MOV across the input.

PCB Layout for an EMI Filter Circuit

There are some simple layout guidelines for filter circuits, whether they are filter ICs or chokes. A common question I've seen about PCB layout for EMI filters is the placement of ground below the components. In general, if you are placing the components over a ground plane, do not place a cutout below the component unless the component rating listed in the datasheet is given based on the presence of a cutout.

Packaged EMI filter modules also have some important routing guidelines that must be maintained. This is particularly true of power, where you typically have a connection to a ground plane.

• Place the filter module close to the connector where power or signal enters the system
• Know where your ground connection is; do not connect a ground plane to both low-side pins on a module

For EMI filter ICs for differential lines, the most important guideline is to ensure there is no skew between the differential signal on the input ports of the filter. The filter specifications (for differential signals) are only valid when the signals are synchronized in time. If there is mode conversion on the line before the filter, some of the common-mode noise you want to eliminate will have been converted to differential-mode noise, and it will experience much lower attenuation.

Whether you need to build reliable power electronics or advanced digital systems, use Altium’s complete set of PCB design features and world-class CAD tools. Altium provides the world’s premier electronic product development platform, complete with the industry’s best PCB design tools and cross-disciplinary collaboration features for advanced design teams. Contact an expert at Altium today!