Single Pair Power over Ethernet

The System is powered from an external power supply at the Power Sourcing Equipment. To avoid noise coupling to and from the Power Supply, a DC Filter including overvoltage protection is necessary.

A tried and tested filter design for the DC input can be found in our reference design RD041.

In contrast to traditional Ethernet which requires electrical isolation of network segments from each device, PoDL and SPoE devices only need to provide 1MΩ of isolation to all accessible external conductors when measured at 5V. I.e. for most devices, the isolation requirement are met by using a non-conductive chassis.

By definition, the power supply system is not galvanically isolated. If galvanic isolation is necessary due to the application, this must be ensured by the external 24 V supply.

Operating voltage may only be applied to the signal lines, when a Powered Device is present. Therefore, a PSE Power Controller is necessary to operate the dc/dc converter accordingly.

A more in depth assessment can be found in our Reference Design RD041.

This Choke provides a high impedance for the noise from the DC/DC converter and also blocks the differential signal current to the power path.

Since the choke is pre-magnetized by the supply current, a coupled inductor with an appropriate saturation current is suggested.

These line filter chokes are additional inductors to decouple the power supply from the system to increase the signal to noise ratio of the Ethernet signal. They also reduce emissions from the DC/DC-converter to the signal line.

These line filter chokes are additional inductors to decouple the power supply from the system to increase the signal to noise ratio of the Ethernet signal. They also reduce emissions from the DC/DC-converter to the signal line.

Transient protection is important for this interface due to the possible length of the transmission line. This design includes a three-stage limitation of transient overvoltage for effective protection of the semiconductors.

This stage consists of bidirectional TVSP diodes connected to earth to limit common mode voltages at 58 V. Steering Diodes connected in series lower the total capacitive load on the signal.

The CMC reduces noise coming from the cable side (e.g. transients) and reduces any high-frequency interference generated by the PHY and power supply system in the direction to the cable. The choke should have a low leakage inductance so as not to impair the signal too much.

Transient protection is important for this interface due to the possible length of the transmission line. This design includes a three-stage limitation of transient overvoltage for effective protection of the semiconductors.

The second stage is a TVS diode with steering diodes to reduce the total capacitive load. This circuit clamps at approximately 58 V, in case differential mode transient overvoltage occurs between the signal wires.

IEC 63171-6 defines a standard connector for Single pair Ethernet. For lower transmission rates in cost sensitive applications, a 3-pin terminal is also functional.

The Varistor reduces any residual charges in the cable or on the capacitors when the system is not in operation, or the cable is removed from the interface. If the board is installed in a plastic housing, there is no stable ground reference for a low-impedant diverting path, so the Varistor and the neighboring capacitors can be replaced by a simple resistor.

This circuit of capacitors and a resistor ensures termination of the signal line with an impedance of 100 Ω. It also ensures balancing in the event of any imbalance in the signal voltage.

To prevent direct current from the applied supply voltage of +24 V through the transformer,  capacitors between the terminals of the center tap is used as a DC-Block. Connecting two capacitors in parallel upholds that feature while decreasing the influence on the signal.

The transformer must be selected to meet the droop, return loss, and mode conversion specifications according to IEEE 802.3cg. Our WE-STST transformer series combines excellent performance regarding return loss and mode conversion loss with a very compact size.

This Capacitor increases common mode rejection by balancing the center tap of the transformer.

On the PHY-side of the signal transformer as TVS Diode array protects the signal pins of the PHY in the event of transient common mode voltages of > 4.5V.

Keep the XTAL as close as possible to the IC. Where applicable connect the XTAL housing to GND. A via fence reduces coupling to the GND plane. An oscillator can be used alternatively.

For SPoE to work properly special Controllers are needed on either side of the Network. Both Power Sourcing Equipment (PSE) and Powered Device (PD) need a Physical Interface Controller (PHY) and a Power Controller respectively. For the PSE an additional microcontroller is used to coordinate the system. For the PD, the microcontroller can by omitted, depending on the application.

In either case, these integrated circuits require peripheral components such as programming connectors and blocking capacitors. Our reference design RD041 offers a detailed view on the required circuitry