Ethernet provides high speed data communications between two nodes that operate according to the IEEE 802 Ethernet Standard. The communications medium between the two nodes can be twisted pair wires for Ethernet, or other types of communications mediums that are appropriate. PoE systems provide power and data over a common communications link. More specifically, a power source device (PSE) coupled to the physical layer of a first node of the communications link provides direct current (DC) power (for example, 48 volts DC) to a powered device (PD) at a second node of the communications link. The DC power is transmitted simultaneously over the same communications medium with the high speed data from one node to the other node.
Example PDs that utilize PoE include Internet Protocol (IP) phones, and wireless access points, etc. The PSE typically includes a serializer/deserializer (i.e. SERDES) coupled to a transceiver, and/or a physical-layer (PHY) device, to support high speed serial data transport. Herein, data ports and their corresponding links can be interchangeably referred to as data channels, communication links, data links, etc, for ease of discussion.
PoE integrated circuits are sensitive to harmful surge events, such as electrostatic discharge (ESD) and cable discharge events (CDE). The PHY of a PoE system is particularly susceptible to damage due to ESD and CDE. During these surge events, currents can be extremely high (e.g., 100 amps) and it becomes vital to ensure that voltages do not exceed critical breakdown and spark gap limits of PoE systems and their respective circuits.
An ESD event typically occurs when a device becomes charged as a result of mishandling or improper packaging and then discharged by a sudden connection to ground. CDE, on the other hand, can occur when a charge accumulates on a cable, such as a twisted pair cable used in Ethernet networks, and is connected to an Ethernet port of lower potential. The resulting high-energy discharge may damage the device to which the cable is connected. Coupling of external events, like a lightning strike, is yet another example source of CDE.
Some PoE integrated circuits have a conventional protection circuit to combat detrimental ESD and CDE. These conventional protection circuits discharge electrostatic or harmful surge energy using a capacitor and/or a transient-voltage-suppression (TVS) diode. Unfortunately, when managing ESD and CDE events on multiple ports, conventional PoE protection circuits require at least one capacitor per port to provide a low impedance path to ground, and/or at least one TVS diode per port to supply protection from differential and common mode transients. These capacitors are commonly referred to as bulk capacitors and are typically large and consume a significant amount of circuit board space, as well as contribute to additional cost. Similarly, the need for multiple TVS diodes contributes to circuit board space requirements and cost.
Thus, what is needed is a protection circuit for PoE devices that overcomes the shortcomings described above.