Supplier eBooks

Unlike the other common mode electrical transients to which a PoE system could be subject, these large, induced surges could be either common or differential mode. One twisted pair in the PoE cable forms the positive side while a different (and well separated) pair carries the negative side of the PoE power supply. Such an arrangement introduces the probability of a differential mode transient between the positive and negative sides. Circuit protection design Protection against all three types of electrical transient comes in two forms: primary and secondary. Primary protection derives from circuit design techniques that limit the incidence or impact of electrical transients. Secondary protection attempts to mitigate the impact of electrical transients by techniques such as blocking, suppressing or isolating with components designed for the purpose. PCB shielding and grounding Good PCB design offers primary PoE circuit protection from transient voltages on data or power lines in the Ethernet cable. A key technique is to divert the transient voltage to provide a path to ground so that the electrical transient can easily bypass sensitive components and safely dissipate. Otherwise, the electrical transient will find its own way to ground which could include passing through a vital component such as the power port switch. This safe path should include multiple solid, low impedance earth ground connections where the designer intends the current to flow and single-point grounds where it's not intended to flow. Other guidance includes ensuring any loops that might be subject to fast voltage/current transients are contained to a small area creating a physical separation between any I/O connectors and sensitive circuitry. Where possible, the designer should place all the connectors on one edge of the circuit, with the sensitive circuitry at the center of the PCB. Further, it is good practice to route signals in parallel with their return signals in order to prevent unwanted transformer effects. Secondary protection design guidelines There will be times when even the best PCB layout fails to divert electrical transients safely. To ensure against these eventualities, secondary protection should be implemented by using components designed to mitigate the effects of electrical transients. It's important that adding protection components has no impact on the performance of the circuit's PoE characteristics during normal operation; for example, the PoE specified amount of power for a given implementation should always be delivered to PDs on the network and not attenuated or interrupted by the protection circuitry. Secondary protection can be implemented by the following methods: • Blocking using fuses or circuit breakers; • Suppressing using non-linear devices such as clamping diodes and varistors; • Isolation using transformers. Blocking Electrical transients in the AC supply can be prevented from reaching sensitive components by blocking the conductive path and the simplest and lowest cost method to do this is to employ surface mount (0603 or 1206) chip fuses at the Power Sourcing Equipment (PSE) and PD on any twisted pairs that carry power. The rated voltage and current of the chip fuse will be determined by the degree of immunity required (according to the IEC standard) for the specific PoE implementation. An alternative is to employ resettable fuses which can be set to trip at a relatively low current level in order to not impact any AC power fault protection. Suppression While carefully selected chip fuses and resettable fuses are inexpensive and simple to integrate into a design, once triggered they prevent further operation of the system. Maintaining PoE operation and some signal integrity during and after electrical transients demands a combination of suppression components such as transient voltage supression (TVS) diodes (and TVS diode arrays), metal oxide varistors (MOVs) and proprietary transient current suppression devices. TVS diodes respond to electrical transient events faster than most other types of circuit protection. They function by clamping the transient overvoltage to a predetermined level using a p-n junction of much greater cross-sectional area than that of a normal diode. This large junction allows conduction of the large | 6 | Digi ® XBee ™ Cellular LTE Cat 1 SM91074AL AEC-Q200 COMPLIANT LAN 10/100 BASE-T TRANSFORMERS LEARN MORE 4 • Transformer modules with common mode chokes for noise rejection in Ethernet applications • Offers 1500Vrms isolation voltage and is RoHS compliant • Extended operating temperature range of -40°C to +125°C

• Cover