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currents generated by electrical transients to ground without damaging the TVS diode. MOVs are also current/voltage clamping devices. But the devices differ in operation from clamping diodes by using a variable resistance instead of a p-n junction as the control device. The MOV's resistance decreases rapidly above the predetermined clamping voltage so that when fitted across power lines or between the power line and ground, the device will act as a short circuit directing the electrical transient current/ voltage away from the sensitive electronics. In addition to generic non-linear current limiting devices, semiconductor vendors also offer proprietary devices that integrate matched, low resistance, bidirectional, high-speed transient current suppressors. Such devices limit the maximum current to which sensitive electronics is exposed and are typically used in conjunction with low capacitance TVS diodes to form a low bypass "energy barrier" for the excessive voltages and currents that occur during electrical transients. Isolation Standard Ethernet isolation transformers can be repurposed as lightning isolation transformers to limit both the peak current and duration of an electrical transient caused by a lightning strike (Figure 3). The performance of solutions from different manufacturers can vary, but for a 100A, 20µs electrical transient induced in the PoE system by a nearby lightning strike, a transformer placed near an Ethernet cable connector should limit the peak current to 25A with the current dropping back to zero in less than 10µs. Isolation transformers can be used alone for relatively inexpensive protection against lightning- induced electrical transients or in combination with blocking and suppression components for more comprehensive protection. Building protected PoE systems To some extent, the choice of secondary circuit protection employed for PoE installation will be influenced by the application. For example, a security camera installation set on the outside of a building is more prone to electrical transients from lightning than one situated indoors. Moreover, cost and space constraints might limit the type and number of protection devices that can be employed. A further complication is that PoE comes in various types. There are currently three versions: PoE, PoE+, and High-Power PoE. In turn, PoE and PoE+ can use two modes of operation: "Alternative A" applies a common mode voltage on the data conductors of the cable to power PDs; "Alternative B" instead uses the two spare twisted pairs in the CAT5 cable. High-Power PoE uses all four twisted pairs. While transients can affect any part of the PoE installation, a common source of damage occurs because the 48V (or greater) power supply powering the PSE features some common mode capacitance to ground in the form of discrete capacitors or built- in capacitance to the PCB. The capacitance prevents the PSE from floating independently and dictates that, without protection, a voltage transient incident on a data pin can damage PSE components and in particular the PSE power port switch. AC power supply electrical transients generally affect power lines, but ESD and electrical transients caused by external phenomena such as lightning can affect both power and data lines. It is, therefore, good design practice to include as much circuit protection as feasible within the constraints of the budget. Application example | 7 | Figure 3: Standard Ethernet isolation transformers can be repurposed as lightning isolation transformers to limit peak current and duration of an electrical transient. (Source: Bourns.) Digi ® XBee ™ Cellular LTE Cat 1 SM13100EL 10/100 BASE-T TRANSFORMERS LEARN MORE 4 • Includes common mode chokes for noise rejection in Ethernet applications • Offers a 4000Vrms isolation voltage and a 250Vrms working voltage • Ideal for industrial Ethernet, Ethernet home appliances, and other market segments

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