Protecting PoE equipment

Fuses, MOVs, PPTCs, and thyristors can be used to prevent overvoltage and overcurrent damage

BY MATT WILLIAMS and THERESA LAGOS
Tyco Electronics
Menlo Park, CA
http://www.circuitprotection.com

The evolution of Power over Ethernet (PoE) continues to expand the functionality of Ethernet technology by supplying reliable dc power over the same cables that currently carry Ethernet data. PoE, modeled after the technology used by the telecommunications industry, enables lifeline quality power for IP telephones (VoIP) as well as many other low-power Ethernet network devices, such as wireless access points (WAP) and IP security cameras.

Surge-suppression devices such as thyristors are one way of protecting sensitive datacom and telcom equipment.

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 circuits 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 powered-device (PD) equipment.

Power-sourcing options

The IEEE 802.3af standard defines two types of power source equipment: end-span and mid-span. End-span PSE integrates the power-sourcing functionality with a network switch.

End-spans look and function the same as any Ethernet switch, except they can deliver data and power over the same wiring pairs. Since Ethernet data pairs use transformers coupled at each end of the link, dc power can easily be added to the center tap of the transformer without disrupting the data. In this mode of operation, an end-span injects both power and data on pin-pairs 3 and 6 and pin-pairs 1 and 2.

Mid-span PSE devices resemble patch panels and typically have between 6 and 24 channels. They are placed between older legacy switches and the powered devices.

Each of the mid-span ports has an RJ-45 data input and data/power RJ-45 output connector. Mid-span devices tap pin-pairs 4 and 5 and pin-pairs 7 and 8 to carry power, while data runs on the other wire pairs. It is important to note that although the PSE can only use pin-pairs assigned from an end-span or a mid-span, the PD must be able to accept power from both.

Power requirements

The 802.3af standard defines power requirements up to 15 W. Typically defined at roughly 330 mA at 48 V, Ethernet ports may supply a nominal 48 Vdc power on the data wire pairs or on the “spare” wire pairs, but not both. And, the PSE must never send power to a device that does not expect it.

For higher power requirements, IEEE802.3af sets the output voltage range for PSE devices to 50 to 57 V. This voltage range is an increase from the 44 to 57 V specified in the IEEE802.3af standard. The PD voltage will remain the same as the IEEE802.3af standard at 36 to 57 V.

Improving safety and reliability

A growing number of PoE applications—ranging from smart signs and vending machines to phone and PDA chargers and electronic musical instruments—have 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 PDProtect the PD from faults in the PSE Protect both the PSE and PD from overvoltage shorts/transients

Single-use fuses are often used to help 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-low impedance state in response to an overvoltage surge.

Overcurrent protection

PPTC devices are commonly used to help provide overcurrent protection on both PSE and PD equipment. The resettable functionality of the device allows for placement in inaccessible locations, and a wide range of electrical and physical sizes facilitates precise design solutions.

Although the fuse is generally considered one of the simplest and lowest-cost solutions, 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 do not generally require replacement after a fault event, and allow the circuit to return to the normal operating condition after the power has been removed and/or the overcurrent condition is eliminated.

In applications where resettable functionality is not 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 is important to note that single-use fuses must tolerate the current spikes and fluctuations associated with PoE applications.

Overvoltage protection

A variety of methods can be used to help protect PoE-enabled equipment from overvoltages caused by switching or lightning transients. There are two major categories of overvoltage protection devices: clamping devices, and foldback or “crowbar” devices.

Clamping devices, such as metal oxide varistors 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 have an advantage over clamping devices because 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.

Fig. 1. Thyristors can offer performance advantages over traditional TVS diodes.

For many PoE applications, the thyristor surge suppressor is the preferred solution. The results of recent testing by Tyco Electronics comparing the behavior of a TVS diode with the company’s SiBar thyristor are shown in Fig. 1 . The SiBar “folds back” the overvoltage transient to a lower voltage level than the TVS diode and has lower peak and average voltage let-through values, resulting in less overvoltage and power stresses passed through to the PoE equipment.

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

For more on fuses, MOVs, PPTCs, and thyristors, visit http://electronicproducts-com-develop.go-vip.net/passives.asp.