Ethernet Protection ------ A Whole Solution Han Zou, ProTek Devices Introduction: As Ethernet applications progress from 10BaseT to 10Gigabit and beyond, IC components are becoming more complicated with different kinds of Very Large Scale Integrated (VLSI) techniques. As a result, the circuits are more than before vulnerable to the damaging effects of lightning, Electrostatic Discharge and Switching Transients. Protecting equipment such as switches, routers, and network card while maintaining performance presents a challenge to design engineers. In addition, high frequencies, low power consumption and a reduction of board space play a role in the difficulty of choosing the right type of transient protection device. This application note discusses protection solutions for the high-speed Ethernet network equipment and identifies specific devices for circuit protection in terms of lightning and Electrostatic Discharge. A typical surge protection solution is described in Figure 1. To users ESD threat Lightning Threat PCB board Primary Protection Cat3- Cat5-, twisted pairs Building Interfaces Twisted pairs (1~10 meters) Inside Building Routers, Switches Secondary Protection Figure 1. A picture of possible transients to an Ethernet system Protection Technologies: There are several types of components currently available for surge protection. These devices are classified into two categories - Parallel components and Series components. 1
Parallel components include Gas Discharge Tubes (GDT), Thyristor Surge Suppressors (TSS) and Avalanche Breakdown Diodes (ABD), which are often used for transient voltage protection (overvoltage protection). An additional parallel component is a pair of surge rated forward diodes called Steering Diodes. Series components include Positive Temperature Coefficient (PTC) resistors and fuses, which are used for overcurrent limiting. Gas Discharge Tube (GDT) Gas Discharge Tubes are glass or ceramic packages, filled with inert gases (some GDTs are not gas filled) being ionized by means of electron collision. This causes the substance to conduct large amount of currents. When a transient voltage exceeds the DC breakdown rating of the device, the electrodes of the gas tube will be triggered. The GDT will go into breakdown mode and change from a high resistance state to a virtual short circuit with a low arc voltage. During this state, the GDT will divert a high level of transient current to protect the equipment. When the transient drops below the DC holdover voltage and current, the gas tube will return to the off-state. GDTs can conduct very high surge currents (up to several kiloamps) and having capacitance as low as 1 pf. The disadvantages of GDTs are a high firing voltage when circuit dv/dt is large, and performance degradation after each firing. Typically, Gas Discharge Tubes are used in combination with semiconductor protection devices. Thyristor Surge Suppressor(TSS or TVS Thyristor) Thyristor Surge Suppressors are solid-state components containing three P-N junctions. They are crowbar devices that switch to a low on-state voltage when triggered. Because the on-state voltage is low, a TSS is able to conduct very high currents by a transient event. Compared with a GDT, a TSS does not wear out nor exhibits a large overshoot voltage. The disadvantage of this device is that its critical rate of rise of both current and voltage (di/dt, dv/dt). Avalanche Breakdown Diode (ABD or TVS Diode) Avalanche Breakdown Diodes are solid-state p-n junction components. A large cross sectional area is employed for the ABD junction allowing it to conduct high transient currents. ABDs are clamping devices. When the transient voltage exceeds the breakdown voltage of the ABD device, the diode becomes low impedance and clamps the voltage down to a defined level. ABDs are available in a wide range of operating voltages and have a fast response time (Although considering the lead inductance and its own capacitance, they do have short time delay). A low capacitance forward diode is usually connected in series with the TVS diode to reduce the total capacitance (as low as several pf), thus reducing the insertion loss of the protection device. ABDs are effective for board level protection due to their small sizes and low clamping voltages. ABDs are widely used when protecting sensitive components from the Electrostatic Discharge, Electrical Fast Transients, and secondary lightning. Steering Diode 2
Steering Diodes are surge rated diode arrays designed to protect high-speed data interfaces. During the transient condition, the steering diode directs the transient to either the positive or negative side of the power supply lines or to ground. Overcurrent Protection Components (PTC Resistors or SMD Fuses) Positive Temperature Coefficient (PTC) resistors are overcurrent protection devices, which function as current limiting components. When an AC power line crosses a data line, as in a power line fault situation, a large amount of current flows through the PTC onto the line. As a result, the PTC will heat up, causing its resistance to rise. The resistance of the PTC will go back to normal when the circuit fault is cleared. Due to the effects of insertion loss from the PTC to the data signal, the combination of surge rated resistors and fuses are often used to replace PTC resistors Solutions for Lightning and ESD Protection Lightning Protection Lightning is the most severe transient treat to electronic systems, such as the Ethernet equipment connected to the external world. It is an atmospheric discharge of electricity caused by the accumulation of static charges. A lightning stroke has a typical peak current of 20 ka, and produces intense electric and magnetic fields, which can couple into nearby power lines, data lines, and circuit wiring causing catastrophic or latent damage to Ethernet network equipment. A two-stage network - primary and secondary protection, provides a whole solution for lightning protection. Figure 1 shows such a network setup. In the exposed environment such as building interfaces or close to the building, primary protection is used by applying damping resistors and high current-rated crowbar devices like Gas Tubes to shunt the bulk of the surge current. Inside the building, near the equipment side, secondary protection is applied to suppress the rest of lightning transient and switching transients. At this stage, Avalanche Breakdown Diodes (ABDs) are often used to divert the transient current, clamp the voltage, and keep the internal circuit safe from lightning. The fast response time and low clamping voltage of the ABDs also compensates for the high firing voltage of GDTs. 3
Line Side Figure 2. Primary lightning protection close to the building interface Building entrance protectors should be able to withstand overvoltage ratings that exceed 5000V and surge currents up to 250A for TSS or 10~20kA for GDT. Some regulatory requirements are described in GR974-CORE, ITU K.28, and UL497A. Figure 2 shows the protection topology for Ethernet external I/O port protection. R1 and R2 represent the resistors used to limit the large current; Z1 and Z2 can be a Gas Discharge Tube (GDT) or a Thyristor Surge Suppressor (TSS). They are both crowbar devices used for commonmode protection. In this application, both lines of the twisted-pair are protected to the ground. Secondary transient protection is often recommended close to the Ethernet circuits. The protector should be able to withstand overvoltage that exceeds 1,500V and surge currents up to 100A (8/20µs, 10/1000µs or 10/700µs surge waveforms). The regulatory requirements often referred to are IEC 61000-4-5 (EN 61000-4-5), GR 1089-Core (Bellcore 1089), FCC Part 68, UL497B, and ITU-T K20 and K21. Data line protection elements are normally required on both the line side and chip side of the line transformers. Line side suppression elements divert the bulk of the transient currents, thus protecting the transceiver as well as the transformers. Chip side protection is required to protect the transceiver IC from fast transient events that are coupled through the transformer due to parasitic winding capacitance. 4
Figure 3. Secondary lightning protection at the equipment (a) Figure 4. Secondary lightning protection at the equipment (b) There are several solutions available for secondary lightning protection. As shown in Figure 3, the circuit is protected by using Avalanche Breakdown Diodes, represented by Z1, Z2, and Z3. Protection is imposed on the both sides of the magnetic transformer. On the line side of the isolation transformer, during the metallic (differential) lightning surges, a low-capacitance ABD (Z1) provides voltage and current limiting to differentialmode surges. On the chip side, two low-capacitance ABDs are shunted to ground to 5
protect the internal IC from any residual lightning energy that is coupled through the transformer. There are different choices for ABDs such as the GBLCxxC and the SLVDA2.8LC, depending on how many lines are to be protected. Another solution, as shown in Figure 4, is to use a steering diode array (such as SRV05-4, SR2.8, PSR05) at the chip side. On one hand, a regular TVS diode (separately or together with in the steering diode array like SRV05-4) provides the protection to the power line surges. On the other hand, the steering diodes direct the transient from the data line to either power supply line or to the ground. Therefore prevent the transreceiver IC of the Ethernet equipment from latch-up and the other possible damages. Steering diodes also have very low junction capacitance (3 pf per line), which reduces insertion loss of the signals for high-speed Ethernet applications. To minimize the path length between the protection devices, and the lines, place the TVS diodes close to the RJ45 connector and the terminal of the isolation transformer to restrict the transient coupling to nearby traces. The total voltage seen from the IC chips in the equipment side will be the combination of the TVS clamping voltage and the overshoot voltage due to the wire inductance. So it is important to improve board layout to make the wire trace inductance as small as possible. Grounding is another concern. Lightning, as a common-mode transient threat, is referenced to a low impedance ground such as a chassis or a PCB ground plane. It is important to design a low impedance grounding (less than 0.5 Ohm) system with minimum discontinuities. ESD Protection Electrostatic Discharge (ESD) is another significant transient threat to Ethernet IC components connected to I/O ports. Electrostatic charge occurs when two dissimilar materials come together, transfer charge, and move apart, producing a voltage between them. Typical sources of ESD include Human Body Model (HBM), Machine Model (MM), and Charge Device Model (CDM). The differences between these sources exist in their transient voltage (kilovolts) and peak current (10s of amperes) levels. For instance, a Human Body Model ESD transient can have a charge greater than 40 kilovolts with a current over 80 amperes. Compared to lightning surges, ESD has a shorter time duration (ns) but a higher transient voltage level. Usually ESD transients have less energy, therefore protection devices can be smaller devices. Application of these devices also minimizes the trace and lead inductance, which can cause large-voltage overshoots on the data lines. Protection Strategies to Electrostatic Discharge For low speed I/O (10BaseT) or power supply, flipchip devices are recommended due to their low cost and small size. For high-speed applications such as 1000BaseT, low- 6
capacitance devices like the SR2.8, SLVU2.8-8, GBLCxxC, PLC496, SRV05-4 should be chosen to minimize insertion loss. These protection devices can be put close to the RJ45 jack or close to the transformer but on the chip side. For instance, Figure 5 shows a protection circuit for Ethernet ESD protection. A TVS and steering diodes combo, SR2.8, is put on the line side to suppress line-line ESD transients. In addition, the protection chip can be put at the chip side, similar to secondary lightning chip side protection, as shown in Figure 4. Figure 6 shows another solution for ESD protection, especially for long haul Gigabit Ethernet protection with ultra low insertion loss. Figure 5. ESD Protection for Ethernet data lines (a) 7
Figure 6. ESD Protection of Ethernet data lines (b) ESD Test Standards There are several standards that define human body model ESD protection and test methods, such as IEC 61000-4-2 (EN61000-4-2), EN50022, MIL-STD-883 Method 3015.7 etc., each with a different emphasis. For example, IEC 61000-4-2 requires the use of an ESD "gun," which allows testing with either contact discharge or air discharge. Contact discharge requires physical contact between the gun and the I/O pin before test voltage is applied by a switch internal to the gun. Air discharge requires the gun to be charged with test voltage before it contacts the I/O pin (from the perpendicular, and as quickly as possible). The second technique produces a spark at some critical distance from the test unit. ESD produced by air discharge resembles real ESD events. However, like real ESD, the air-discharge variety is not readily duplicated. It depends on many variables that are not easily controlled. Therefore, attesting to the general importance of repeatability in testing, contact discharge of IEC 61000-4-2 is recommended. In either case, the test procedure calls for at least 10 discharges at each test level. According to IEC 61000-4-2, the severity levels range from 2kV to 15kV (air discharge), depending on the environment. For contact discharge the highest level is 8kV. Deficiencies of On-chip Protection 8
In the ESD protection, some concepts should be clarified. On-chip protection functions are now available for most of interface IC chips such as the transceivers. With no doubt, on-chip protection can save the circuit space previously used by the external protection devices. However, the on-chip protection structures usually provide less protection ability than the external devices due to the limits in the process technology and the area available on the chip. Furthermore, a poor on-chip protection design could lead to some unexpected, latent defects within the IC chip. Summary Lightning and Electrostatic Discharge are threats to the Ethernet applications that should not be ignored. A variety of protection strategies are available to counter these transient threats. Transient Voltage Suppression Diodes in combination with other devices are recommended to provide a whole protection solution. As with any design solution, protection strategies will depend upon the threat source, immunity requirements and circuits being protected. For specific applications and solutions, please contact ProTek Devices. Email: (han@protekdevices.com). 9