Implementing Gigabit Power over Ethernet to achieve EMC compliance – Part 1

In this article Adrian Stirn from Würth Elektronik looks into the intricate details of implementing Gigabit Power over Ethernet (PoE) while addressing the often-overlooked challenges of achieving EMC compliance.

To investigate the EMC properties of the Gigabit Power over Ethernet interface, Würth Elektronik started by developing and implementing its own reference design RD022 [3] (Fig. 1). This “GB PoE+ Ethernet USB adapter” was designed based on reference design RD016 “GB Ethernet USB adapter” [1].

Reference design RD022 "GB PoE+ Ethernet USB adapter" is a key aspect in EMC considerations

The new reference design “GB PoE+ Ethernet USB adapter” has three interfaces:

• a USB Type-C (USB 3.1)
• an RJ45/Ethernet one Gigabit interface with integrated Power over Ethernet supply (PoE+)
• a terminal connection to the DC/DC converter, with an adjustable output voltage of 6–18V and a maximum output power of 25W

The board was developed to familiarise user with PoE technology. As with the design without PoE, the bit error rate and transmission speed can be investigated using a Windows application.

The EMC evaluation of the PoE reference design was performed in two steps: first, the board was tested with the power supply without additional filters. Optimisations were necessary during the tests, which will be explained later. The circuit diagrams are shown in [4]. Reference design RD022 is already based on the optimised design [3].

The EMC behaviour of the Gigabit Ethernet interface was already discussed in detail based on the RD016 reference design description and Application Note ANP116. The insights gained there regarding shield connection, cable shielding, and the performance of the Ethernet front end, also apply for the PoE version. The ideal shield connection determined in ANP116 with two 10nF capacitors and a parallel SMD varistor is also used here.

EMC basics for PoE

PoE applications are usually compact electronic devices whose power supply is combined with data communication via an Ethernet interface. They are usually multimedia devices, which are covered in EMC standardisation CISPR 32 (emission) and CISPR 35 (immunity). The devices tend to be compact and have short lines apart from the Ethernet cable. Typical examples:

• Wi-Fi access points
• DECT stations (Digital Enhanced Cordless Telecommunications)
• IP phones
• Surveillance cameras
• Monitoring of indoor climate in storage and production facilities

The IP phone is an example of a PoE device with comparatively large dimensions, as the phone receiver cable together with the PCB in the phone is a relatively large structure that can act as an antenna. A DECT station is rather compact by comparison. This results in two possible approaches for the reference design during EMC testing:

• A short output line or compact load resistor (point of load) – compact application
• Long output lines at the load output – corresponds to a larger PoE application

The advantage of using long cables and a variable resistor during EMC testing is that the load on the switching regulator can be readjusted to always allow the maximum power to be achieved. When using a compact TO220 resistor, the load is fixed at 10Ω.

EMC test setup and operating parameters

The schematic test setup shown in Fig. 2 is similar to the test setup used for testing the Gigabit Ethernet interface and was only extended to include the PoE switch and the load. The focus during EMC testing is on the Ethernet interface, the PoE reference design board, and the various load configurations.

Fig. 2. Test setup for the Gigabit PoE board during EMC testing

The USB interface is considered as a short line; the focus during testing is on the Ethernet front end. As the USB interface is needed to operate the board, it is also evaluated in many tests. During the interference immunity tests, it was found that a direct connection between the USB cable shield and the board GND layer is required for operation at high levels of interference immunity. Only a direct shield connection ensures that the interface remains in a stable operating mode even at high test levels such as 20V/m above 1GHz.

The load lines are measured with a 2m to 3m cable length for radiated interference emission and interference immunity and should be as short as possible (cable length below 3m) with regard to EMC. Burst or conducted RF coupling is not applied to the voltage output of the board.

The notebooks required to operate the board and the PoE switch are operated inside a shielded box in order to eliminate their influence on the results of EMC testing as far as possible.

Influence of the output voltage on emission

Fig. 3 shows that the interference emission of the board is higher at 12V and 2A output current than at 18V and 1.3A output current. For this reason, some approaches, such as filter design, are carried out at 12V rather than 18V.

Fig. 3. Radiated interference emission from the Gigabit PoE design when operating with long load lines without an external filter and with shielded Ethernet cable (cat5e SF/UTP)

Output filter for long cables

The isolated switching regulator on the board has no output filter. When connecting long lines and a load resistor, interference may be radiated directly from the switching regulator output, so an output filter may be necessary for large designs or for designs with cables.

The filter from Fig. 4 can be used to reduce the radiated interference. This results in an increased margin to the Class B limit of CISPR 32. For larger devices, an output filter with the following components is recommended:

• Ferrite Bead (1812 package) with 780Ohm at 100MHz (742792515)
• MLCC 4.7µF X5R 50 V (885012209048)
• Common-mode choke for signal lines with 17µH (744237152)

Fig. 4. Circuit and photo of the output filter for long load lines

The capacitors around the common-mode choke form a differential filter with its own leakage inductance. Any coupling caused by stray magnetic fields from the board into the filter, which would result in a differential current, are short-circuited in the process. If a load resistor with short leads or a TO220 resistor is used instead of long lines with a filter, the emission changes.

Reducing the package size of the load resistors does not improve the emission in the absence of an output filter. The emission with filtered long load lines is now partly lower than with compact load resistors.

The second part of this article continues with conducted interference emission and immunity testing.

[1] Zenkner, H.: Reference Design Gigabit-Ethernet Front End, RD016, Würth Elektronik: www.we-online.com/RD016   

[2] Stirn, A.: EMC Aspects ofGigabit-Ethernet Interfaces , AppNote ANP116, Würth Elektronik: www.we-online.com/ANP116  

[3] Zenkner, H.: GB-PoE+-Ethernet-USB“-Adapter for industrial use  with an EMC perspective, Reference Design RD022, Würth Elektronik: www.we-online.com/RD022

[4] Stirn, A.: Gigabit PoE Interface from an EMC perspective, AppNote ANP122, Würth Elektronik: www.we-online.de/ANP122