Design and Selection of Magnetic ICM Modules to Ease Ethernet EMI/EMC Challenges

One critical challenge related to developing local area networks (LANs) lies in signal integrity and the ability to demonstrate compliance with electromagnetic interference (EMI) and/or electromagnetic compatibility (EMC) requirements found in IEEE 802.3 standards for Ethernet. 

Working through Ethernet EMI/EMC challenges can be a difficult process; however, magnetic ICMs, such as those shown in Figure 1, could help simplify the problem. 

 

Figure 1. These RJ45 connectors have integrated Ethernet magnetics to aid signal integrity and reduce the effects of EMI/EMC. Image used courtesy of Bel

 

These ICMs, when correctly selected, can be a potential approach to minimizing the effects of EMI/EMC on Ethernet signal integrity. However, choosing the right magnetic ICM for your system requires a basic knowledge of how magnetic ICMs are structured and which magnetic solutions work best.

In this article, we’ll explore some of the high-level basics of a magnetic ICM, some of the tasks they are used to accomplish, and look at an example product from Bel Fuse.

 

The Basics of Magnetic ICMs—Uses and Applications 

Controlling the effects of EMI and EMC can be critical to the reliable performance of all electronic devices, including LANs. Magnetics are highly effective at ensuring that signal integrity, even with PoE (power over ethernet) applications, meets applicable EMI/EMC standards.

Magnetic ICMs integrate EMI/EMC filtering circuits into the actual RJ45 connectors instead of applying them discretely to a PCB. This integration within the jack frees up much-needed PCB space while also achieving EMC/EMI compliance with IEEE 802.3 standards.  

Magnetic ICMs are rated for a wide range of ethernet speeds and PoE power levels:

• 10/100 BT AutoMDIX
• 1 GBT (10/100/1000 BT)
• 2.5 GBT, 5 GBT,and 10 GBT
• 30 W, 60 W, and 100 W PoE

As for Ethernet applications that already benefit from magnetic ICM solutions, these include: 

• Wireless access points
• 5G cellular equipment and base stations
• Medical imaging
• Security systems
• Industrial controls
• Video display systems

 

Magnetic ICM Tasks—Isolation Transformers, CMCs, and PHY Transceivers

Three primary tasks of magnetic ICMs, such as those whose schematics are shown in Figure 2, are to provide:

• CM (common mode) noise reduction, signal shaping, and conditioning
• Impedance matching to minimize signal reflection
• High-voltage isolation to, among other things, suppress the effects of CM disturbances

 

Figure 2. Typical schematic of ICM modules. Image used courtesy of Bel

 

These tasks are most effectively accomplished through the careful design of PHY transceiver architectures, isolation transformers, and CMCs (Common Mode Chokes), all of which are critical elements to a successful magnetic ICM. 

 

PHY Transceiver

The Ethernet PHY transceiver operates at the physical layer of the OSI (Open Systems Interconnection) network model and its task is to provide the analog signal access to a physical medium. 

There are two categories of PHY transceiver architectures: 

• Voltage mode line drivers (which have issues with differential mode signals)
• Current mode line drivers (which have issues with CM signals)

Magnetics can mitigate the noise issues in both types of PHY architecture, but a different approach is required depending on the architecture involved—and some solutions create more problems. For example, many current mode line drivers utilize conventional two-wire CMC winding through the toroid core, which leads to a lack of flux cancellation in the core and signal distortion.

 

Isolation Transformers

Isolation transformers are used to meet IEEE 802.3 isolation requirements by providing voltage isolation between the input and output of a LAN device and suppressing the effects of any CM disturbances. There is no metal-to-metal contact in an isolation transformer, but transmits signals via inductive coupling using magnetic field flux. However, isolation transformers will have a small level of capacitance that results in a low-impedance path for unwanted CM currents to transmit across the transformer and leads to electrostatic coupling with other circuits that impact EMI performance.

 

Common Mode Chokes (CMCs)

CMCs, which are usually placed in the series with the isolation transformer, are critical to meeting EMC requirements when high frequencies are involved. In addition, CMCs are necessary because they (a) choke high impedance against problematic CM noise while (b) displaying low impedance against DM noise. 

Critical aspects of CMC design include positioning of the input and output windings to minimize stray capacitance. And keep in mind that the CMC impedance is determined not only by the size and number of turns but by the core material, too. All of these are ingredients for an effective CMC.

 

Example ICM—BEL MagJack ICMs

One potential solution of ICMs that help fulfill IEEE compliance and Ethernet standards is the BEL MagJack ICMs, shown in Figure 3. These devices are Ethernet connector modules with integrated EMI magnetics that support IEEE 802.3 compliance. This product line covers a wide variety of Ethernet applications, including the latest technology for 5G cellular networks. 

 

Figure 3. Bel MagJack ICMs come in various configurations, allowing them to be used in every Ethernet application. Image used courtesy of Bel

 

There are several unique ways that the BEL MagJack ICMs implement the magnetic ICM solutions discussed in the previous section.

For example, CMC exhibits high impedance against CM and low impedance against DM (differential mode). This is achieved by winding wires around the core so that the fluxes due to the two CM currents add together while the fluxes due to the DM currents subtract in the core. In addition, the input and output are positioned to minimize stray capacitance. In PoE applications, the Bel magnetic ICM also includes CMC to suppress the CM noise from the power supply.

For PHY transceiver architectures, engineers at Bel designed a patented, proprietary three-wire winding CMC where transient current flows through the middle winding of the CMC. The current is then equal and out of phase, with the currents flowing through the outer windings of the CMC, which cancels out flux in the core.

Implementing a practical isolation transformer involves a carefully designed internal PCB design combined with meticulous magnetic design and high-quality components to defend the system against CM transients. The isolation transformer designed by BEL also provides a low-impedance path to the ground to divert the high energy from CDE (cable discharge events) and ESD (electrostatic discharge) from the signal path, thus preventing CDE and EDE from coupling into the lines and damaging the PHY.

Industry Articles are a form of content that allows industry partners to share useful news, messages, and technology with All About Circuits readers in a way editorial content is not well suited to. All Industry Articles are subject to strict editorial guidelines with the intention of offering readers useful news, technical expertise, or stories. The viewpoints and opinions expressed in Industry Articles are those of the partner and not necessarily those of All About Circuits or its writers.