Electronic Design

Add Coordinated Overcurrent, Overvoltage Protection To PoE Equipment

POWER OVER ETHERNET (POE) enabled devices and their electronic components are designed for operation within specified current and voltage ratings. If these ratings are exceeded due to short-circuit or voltage transients, components may sustain permanent damage and the equipment may fail. Overcurrent and overvoltage protection devices are used to help protect both power-sourcing equipment (PSE) and powereddevice (PD) equipment.

A growing number of PoE applications—ranging from smart signs, vending machines, building access control, and time and attendance systems to phone and PDA chargers and electronic musical instruments—has created a demand for more reliable and flexible overcurrent- and overvoltage-protection devices. These devices are required in order to:

• protect the PSE from damage caused by shorts in the Ethernet cable or PD
• protect the PD from faults in the PSE
• protect both the PSE and PD from overvoltage shorts/transients

In many cases, single-use fuses help to provide overcurrent protection in PoE applications. Polymeric positive temperature coefficient (PPTC) devices, installed in series with electronic components, also provide a reliable, resettable method of interrupting current flow. Solid-state thyristor overvoltage-protection devices may also be installed in parallel with these components to switch rapidly from a high to a low impedance state in response to an overvoltage surge.

The fuse is generally considered one of the simplest and lowest-cost solutions. However, many equipment manufacturers find it easy to justify the cost of resettable PPTC device protection if it helps protect against overcurrent damage caused by electrical short, overloaded circuit, or customer misuse. PPTC devices don’t generally require replacement after a fault event, and they allow the circuit to return to normal operating conditions after the power is removed and the overcurrent condition is eliminated.

In applications where resettable functionality isn’t desired, high-current, surface-mount fuses that provide clean blow characteristics and physically contain the fusing event within the package can be used to meet the overcurrent-protection requirements of the IEEE 802.3af standard. It’s important to note that single-use fuses must be tolerant of the current spikes and fluctuations associated with PoE applications.

Overvoltages caused by switching or lightning transients can damage PoE-enabled equipment. However, a variety of methods is available for protection. There are two major categories of overvoltage protection devices—clamping devices and foldback, or “crowbar,” devices. Clamping devices, such as metal oxide varistors (MOVs) and diodes, allow voltages up to a specified clamping level to pass through to the load during operation. Foldback devices, such as gas discharge tubes and thyristor surge suppressors, operate as shunt devices in response to a surge that exceeds the breakover voltage.

Foldback devices hold an advantage over clamping devices. In the foldback state, very little voltage appears across the load while it conducts harmful surges away from the load, whereas clamping devices remain at the clamping voltage. Therefore, the power dissipated in the foldback device is much lower than in a clamping device.

For many PoE applications, the thyristor surge suppressor is the preferred solution. The thyristor “folds back” the overvoltage transient to a lower voltage level than the transient voltage suppressor (TVS) diode and has lower peak and average voltage let-through values than the TVS diode. This results in less overvoltage and power stresses passed through to the PoE equipment.

In addition, the thyristor’s low on-state voltage allows for smaller form-factor devices—as compared with a TVS diode of comparable energy-handling capability—conserving valuable printed-circuit-board real estate. The relatively low capacitance of the thyristor also allows its use on high-data-rate circuits.

See Associated Figure

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