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Posted on 01 March 2019

Protecting PoE Equipment from Overvoltage and Overcurrent Damage

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Thyristor surge suppression devices meet the immunity and test requirements of IEEE 802.3AF standard

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, which is 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.

By Matt Williams, Applications Engineering Manager and Theresa Lagos, Overvoltage Product Manager, Tyco Electronics´ RaychemTM Circuit Protection Products

 

The IEEE (Institute of Electrical and Electronics Engineers) 802.3af standard addresses the requirement for interoperability among a growing number of proprietary methods of distributing DC power to network devices. It has facilitated the development of technology that allows a broad range of devices to supply or draw power over the network without modification to existing infrastructure and provides these advantages:

• Eliminates the need to run A/C power wires and permits use of existing IT infrastructure
• Permits the most efficient and convenient installation, regardless of where AC outlets are located
• Allows for the use of a centralized UPS to provide power to the appliance even during mains power failure
• Improves safety by eliminating presence of mains voltage
• Permits remote monitoring and control of devices on the network

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 Powered Device (PD) equipment.

Power Sourcing Options

The IEEE 802.3af standard defines two types of power source equipment: end-span and mid-span. An 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 six 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. Midspan 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 endspan or a mid-span, the PD must be able to accept power from both.

Power sourcing options per IEEE 802.3af standard

Power Requirements

The 802.3af standard defines power requirements up to 15 watts. Typically defined at ~330mA@48V, Ethernet ports may supply a nominal 48V DC 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 for PSE devices to 50V to 57V. This voltage range is an increase from the 44V to 57V specified in the IEEE802.3af standard. The PD voltage will remain the same as the IEEE802.3af standard at 36V to 57V.

IEEE 802.3af PSE and PD Power Classifications

Improving Safety and Reliability of PoE 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

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 a low impedance state in response to an overvoltage surge.

Overcurrent Protection Options

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 they allow the cir- cuit to return to the normal operating condition after the power has been removed and 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 be tolerant of the current spikes and fluctuations associated with PoE applications.

Figure 2 illustrates how either the PolySwitch™ decaSMD device or the Tyco Electronics slow blow chip fuse can be used to help protect PoE equipment from overcurrent damage.

Typical circuit diagram using a SiBarTM thyristor for overvoltage protection with a PolySwitchTM device

Overvoltage Protection Considerations

A variety of methods can be used to help protect PoE-enabled equipment from damage caused by overvoltage events, such as 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 (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 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.

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 that of a SiBar™ thyristor are shown in Figure 3a and 3b. The SiBar thyristor “folds back” the overvoltage transient to a lower voltage level than the TVS diode and has lower peak and average voltage let-through values than the TVS diode – resulting in less overvoltage and power stresses passed through to the PoE equipment.

Performance of TVS diode.

Performance SiBar thyristor

Additionally, 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 PC board real estate. The relatively low capacitance of the thyristor also allows its use on high data-rate- circuits.

Table

Summary

The low resistance, fast time-to-trip, low profile, and resettable functionality of PPTC devices helps circuit designers provide a safe and dependable product and comply with regulatory agency requirements. In applications where resettable functionality is not desired, Surface-mount fuses with slow-blow characteristics can help manufacturers meet the overcurrent protection requirements of the IEEE 802.3AF standard.

Thyristor surge suppression devices help meet the immunity and test requirements for PoE equipment, providing lower peak and average voltage let-through values during an overvoltage transient, and their low on-state voltage allows for smaller form factor devices – as compared with clamping devices of comparable energy-handling capability. The relatively low capacitance of thyristors also makes them useful in high data rate circuits.

For more information, please contact Matt Williams at MattWilliams@tycoelectronics.com

 

 

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