Surge Protection Brings Big Benefits to PoE Systems

Mar 23, 2007 3:25 PM
By Jean Picard, Systems Engineer, Power Management Group, Texas Instruments, Dallas

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Electrical overstresses can cause failure, permanent degradation or temporary erratic behavior in electronic devices or systems. The trend to reduce circuit geometries for communication systems and applications results in an increasing sensitivity to electrical transients. Suppressing these transients can challenge designers because the origin and severity of the overvoltage may be unknown.

When designing an electronic circuit or defining a complete system, it is important to identify the sources of those stresses and correctly understand their mechanism to properly define the environment of the operating system. In doing so, you can define simple design rules to adequately protect sensitive electronic systems with cost-effective solutions.
Power over Ethernet (PoE) equipment represents one class of systems in which sensitive power circuitry must be protected. Although PoE specifications provide for overcurrent protection, these systems are still vulnerable to the same electrical transients that can damage other types of power equipment.[1-2]

In PoE equipment, power-sourcing equipment (PSE) delivers power to a powered device (PD) via an Ethernet cable. The power is delivered through a difference between the respective common-mode voltages of the two twisted-wire pairs used for the two data channels in the Ethernet cable, as shown in Fig. 1. Even more power can be supplied with the additional use of the two spare twisted pairs. PoE applications are found in environments ranging from offices to industrial networks. Ethernet cables or equipment installations are usually located indoors but can be outdoors.

Transients in PoE Applications

Many standards have been developed to simulate or represent the transient overvoltage environment in various applications. For example, as per IEC transient immunity standards, transients can be classified into three categories:

• IEC 61000-4-2: Electrostatic Discharge (ESD)
• IEC 61000-4-4: Electrical Fast Transient/Burst (EFT)
• IEC 61000-4-5: Surges.

These IEC standards also define immunity test methods that apply to each transient category, and they also provide manufacturers of transient suppression components some standardized waveforms and overvoltage levels to which their components can be characterized and specified.^[3]

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