How the PoE interface is protected against differential mode transients

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As Ethernet proliferates in the networking space, the number of systems adopting PoE (Power over Ethernet) on 10/100 and Gb ports is rapidly increasing. The benefits and cost advantages of powering remote devices over Ethernet cables enable many applications, including IP telephony, digital video surveillance, WLAN access points, and other low-voltage network-connected systems.

A typical PoE system uses a power delivery device (PSE) to send a DC voltage to a remote powered device (PD) over an Ethernet twisted pair. Since PoE systems are often threatened by transient voltages, one of the important design considerations is to protect Ethernet physical layer transceivers (PHYs) from overvoltage shocks.

While PoE applications are growing, Ethernet PHY is rapidly shrinking in size. Currently, Ethernet PHYs are mostly manufactured using 90nm technology, but chipmakers are about to launch smaller products made using 65nm process technology. Effective chip-level ESD protection on CMOS has proven to be impractical with these advanced manufacturing processes, as the chip area is too small to provide system-level robustness, and the cost of implementing effective chip-level protection is too high. To meet global standards and ensure system reliability, Ethernet-based system designs increasingly require better off-chip circuit protection.

Transient voltage threat

Ethernet interfaces are vulnerable to a variety of transient overvoltage events, the most common of which are electrostatic discharge (ESD), cable discharge, and lightning surges. In addition, in PoE systems, delivering DC power over twisted pair introduces some characteristic transient faults caused by differential mode connections.

ESD is a very fast transient pulse. According to the model given by the IEC61000-4-2 standard, the rise time of the ESD waveform is 700 picoseconds to 1 nanosecond, and the pulse duration from pulse peak current attenuation to 50% is 60 nanoseconds. Large current spikes and energy contained in transient processes can damage the submicron input structure of silicon chips.

In conventional environments such as frictional live effects or induction, cable discharge (CDE), or cable electrostatic discharge (CESD), occurs when Ethernet cables are live. Plugging a live cable into the system interface is dangerous. It has been shown that discharging a cable through an Ethernet magnetic channel to an Ethernet port creates several different patterns of power surges. Similar to ESD, cable discharge surges have a short rise time (less than 1 nanosecond), but unlike ESD, their secondary waveforms have oscillations that change rapidly in polarity and last for a long time. For Ethernet designers, the energy in the cable discharge waveform poses a more serious problem than human electrostatic discharge.

Lightning surges are a common threat in network connectivity. Lightning strikes can induce high-voltage pulses on Ethernet lines that may be transmitted to the Ethernet PHY. Unlike nanosecond ESD events, lightning surges have a duration of milliseconds. The EMC industry describes this pulse in terms of rise time (in milliseconds), spike current, and fall time. Lightning shocks have orders of magnitude greater energy than ESD-level shocks.

Differential mode transient response in PoE applications

As mentioned earlier, the protection of PoE interfaces can be particularly challenging because, in addition to transient processes caused by ESD and power surges, there are several scenarios that often occur when connected to DC power sources that cause differential surges on Ethernet transmission lines. This naturally causes catastrophic failure or problems for the PHY, and severe shocks can damage the IC.

Most PoE

circuit designers protect PoE circuits with some form of common-mode protection, typically using common-mode capacitors connected to ground or TVS transient voltage suppressors across the power supply, which rely on very fast Schottky diodes to direct current to ground. However, many designers mistakenly overlook differential mode protection. Ethernet differential pairs use transformers or common-mode current suppression to isolate the PHY from the outside environment. Transformers provide a high level of common-mode isolation from external voltages, but do not provide protection against metallic or differential (line-to-line) surges.

As shown in Figure 1, the PoE system has +48V or -48V across the differential pair. In the signal pair, this DC voltage is common, so the differential DC voltage is 0 volts. However, in some cases, the connection may introduce transient processes.