Electronic Design

Prevent ADSL Modems From Getting Zapped

Familiarity with the standards, guidelines, and devices strengthens your ability to protect an ADSL modem's telephone and data lines from overvoltage surges.

Full article begins on Page 2

Major regulation authorities have set specific standards or test recommendations for qualifying telecom equipment, including central-office and customer-premises ADSL modems. Such standards depend on the type of equipment, its location in the network, and the network's geographical region. Yet a parallel element, particularly one with "crowbar" characteristics, supplies the necessary protection against lightning strikes. A crowbar device can short-circuit the twin-pair telephone lines to dissipate the overvoltage with minimal heat.

Protection standards also address overvoltages, which occur when an ac power line makes electrical contact with the telephone lines. To block such overcurrents, the protection circuit typically combines the parallel crowbar device with a voltage-clamping device, and possibly a positive-temperature-coefficient (PTC) resistor, in series with each telephone line.

This article details the various standards with which designers must become familiar. The more familiar they are, the better armed they are to protect an ADSL modem's telephone and data lines from overvoltage surges. Four tables list the main protection standards associated with specific telecom equipment, both in the U.S. and in Europe and Asia.

Also described are the various devices used in protection schemes, such as the aforementioned crowbar and clamping devices that help suppress damaging overvoltages.

HIGHLIGHTS:
Protecting The Line Interface To cover the entire ADSL modem system, protective circuitry starts at the line interface and is usually placed between the telephone lines and splitter. There are three key parameters when selecting the right protection components: standoff voltage, surge current, and capacitance.
Data-Line Interface When it comes to the data line, the three most common interfaces are the Universal Serial Bus (USB), Ethernet, and the Peripheral Component Interface (PCI). USB and Ethernet connect to external equipment; PCI connects to internal plug-in boards.
Sidebar: The ABCs Of ADSL Modems By using advanced modulation techniques, ADSL service can move digital data at up to 8 Mbits/s continuously over conventional phone lines while allowing voice communication.
Tables: Protection Standards The main protection standards used in the U.S. and Europe/Asia are broken down for telecom in general, and specifically for central-office and customer-premises equipment, and Ethernet. Lightning and power contact are highlighted.

Because they connect to ordinary phone lines, asymmetrical-digital-subscriber-line (ADSL) modems are subject to electrical hazards like lightning, power-line crossings, and electrostatic discharge (ESD). As a result, they must be adequately protected against damaging overvoltages and overcurrents.


Full article begins on Page 2

Familiarity with the standards, guidelines, and devices strengthens your ability to protect an ADSL modem's telephone and data lines from overvoltage surges. Because they connect to ordinary phone lines, asymmetrical-digital-subscriber-line (ADSL) modems are subject to electrical hazards like lightning, power-line crossings, and ESD. As result, they must be adequately protected against damaging overvoltages and overcurrents. Major regulation authorities, specifically the International Telecommunication Union's Telecommunication Standardization Sector (ITU-T), Core (formerly Bellcore), and the Federal Communications Commission (FCC), have all set specific standards or test recommendations for qualifying telecom equipment. These include central office and customer-premises ADSL modems (see "The ABCs Of ADSL Modems").

As shown in Table 1, the required standards depend on the type of equipment, its location in the network, and the network's geographical region. However, a parallel element, particularly one with "crowbar" characteristics, supplies the necessary protection against lightning strikes. A crowbar device offers the advantage of effectively short-circuiting the twin-pair telephone lines to dissipate the overvoltage with minimal heat. At normal operating voltages, the crowbar device acts like an open circuit.

Protection standards also address overvoltages caused by power crossings—that is, when an ac power line makes electrical contact with the telephone lines. To block overcurrents in this case, the protection circuit typically combines the parallel crowbar device with a voltage-clamping device and possibly a positive-temperature-coefficient (PTC) resistor in series with each telephone line. In addition, some standards call for ESD protection, which is an increasingly important concern as devices get faster and smaller. This requires specific protection devices, often diode arrays.

Typically, designers use parallel-wired protection devices to suppress high-current, short-duration stresses, which represent most overvoltage effects. Both crowbar and clamping devices protect against such stresses via different mechanisms.

In the crowbar situation, it switches on—in effect, short-circuiting the signal lines—when the line voltage exceeds the device's breakover voltage (±VBO). This voltage can be a fixed value or programmed by the device's gate. The crowbar remains in the on state until the current falls below the specified holding value, IH.

In contrast, the clamping device suppresses overvoltages by holding the signal lines to the device's breakdown voltage (VBR). Clamping devices come in both unidirectional and bidirectional versions. The unidirectional versions clamp voltages in only one direction and act like a rectifier for reverse voltages. Yet when using clamping devices, designers must consider that current flows through the protection device only during the clamping phase, which takes less time that the overvoltage surge.

For both crowbar and clamping devices, power dissipation is a function of the voltage across the device and the current through it. Thus, for a given type of package, the current-handling capability of the clamping device depends on its breakdown voltage. Because the crowbar presents a virtual short circuit, it tends to carry a higher power-dissipation rating.

PROTECTING THE LINE INTERFACE
To cover the entire system, protective circuitry starts at the line interface and is usually placed between the telephone lines and splitter (Fig. 1). To select the right protection components, one must consider at least three key parameters: standoff voltage, surge current, and capacitance.

First, the component's standoff—the maximum voltage for which the device remains off—must equal the maximum normal operating voltage, which typically occurs during ringing. Next, the component's surge current must meet the requirements of the corresponding lightning protection standard. Finally, due to the ADSL's high baud rate, the protection device's capacitance must be minimized to avoid increasing the bit-error rate.

The exact line-interface requirements for protecting central office equipment differ from those for customer-premises gear. Central office equipment in the U.S. must comply with Core standard GR-1089, while equipment destined for Europe or Asia must comply with ITU-T K20/K21. Both standards call for protection from lightning surges and power-line crossings (Table 2).

Accordingly, protection devices, like those in the SMP100LC series of the Trisil crowbar components, have been developed to meet both sets of standards. The SMP100LC family comprises bidirectional crowbars available with breakover voltages of 8 to 262 V. Capacitance is low, normally from 30 to 45 pF at 50 V, as is the 2-µA maximum leakage current. Other key specs include a 150-mA (minimum) holding current and 100-A repetitive peak (10/1000 µs) pulse current.

Two such devices provide adequate line-interface protection. One is between the tip line and ground, and the other is between the ring line and ground. Each has a 270-V breakover voltage. In addition, because the crowbar devices turn on quickly, they can generate spikes on the secondary side of the line-interface transformer.

In those cases, a low-power clamping device like the SMAJ15CA-TR can suppress the spikes. A subset of the TRANSIL family of clamping devices, SMAJ15CA-TR devices have breakdown voltages of 6.4 to 209 V and corresponding standoff voltages (normal operating voltages) from 5 to 188 V. Their peak (10/1000 µs) pulse power is rated at 400 W. Available in unidirectional and bidirectional versions, this family features a fast response time suited for protecting MOS and low-voltage ICs.

For customer premises equipment, the line-interface protection requirements for the U.S., Europe, and Asia are less severe (Table 3). Consequently, they demand less from the protection devices. Here, members of the SMP30 and SMP50 Trisil bidirectional crowbar families offer the necessary lightning protection while keeping down parasitic capacitance.

For example, SMP30 devices, which carry a 30-A peak pulse current rating, exhibit typical capacitances from 12 to 20 pF at 50 V; SMP50 devices are rated for 50 A with 15 to 30 pF at 50 V. Members of both families have clamping voltages ranging from 62 to 270 V, 50-mA minimum holding current, and 2-µA maximum leakage current.

As with central office equipment, an SMAJ15CA-TR voltage clamp can abate spikes spawned by the crowbar's fast turn-on. Plus, protection from power crossings is secured by PTC resistors on each line.

DATA-LINE INTERFACE
On the data-line side, the three most common interfaces are the Universal Serial Bus (USB), Ethernet, and the Peripheral Component Interface (PCI). USB and Ethernet typically connect to external equipment, which might include an ADSL modem, while the PCI bus generally connects to internal plug-in boards. Because PCI is internal to the end-user equipment, it's less susceptible to environmentally generated electrical stresses. For this reason, only the protection of USB and Ethernet lines is considered here.

USB, arguably the most popular interface for home computer equipment, moves data at rates of 1.5 Mbits/s or 12 Mbits/s. To ensure that EMI and RFI are suppressed at USB's high data rate, U.S. equipment must meet FCC part 15 requirements and, internationally, International Special Committee on Radio Interference (CISPR) publication 22. These requirements are in addition to meeting ESD standards like IEC61000-4-2 (8-kV contact, 15-kV air discharge) and Mil-Std 883E, Method 3015-7 (25 kV).

Fortunately, as with the line interface, protection components that meet all of these requirements are commercially available. One example is the USBUF01W6, an integrated passive and active device comprising ESD protection, as well as EMI filters and line terminations for upstream USB ports. The USBUF01W6, which integrates bypass capacitors and series-termination (pull-up) resistors, accommodates both high and low baud rates. Specifically, the data rate is selected by connecting a 1.5-kΩ resistor to data line D+ or D− (Fig. 2).

Ethernet local-area networks, used largely for communications within a building, transmit data over two twisted-wire pairs at data rates of 10 Mbits/s (10BaseT) or 100 Mbits/s (100BaseT). In either case, one wire pair carries received data while the other carries transmitted data. Both wire pairs connect to an Ethernet transceiver IC through a line transformer that serves to isolate the end equipment.

Generally, the surge protection required for Ethernet lines is similar to that for standard telephone lines (Table 4). Consequently, the protection devices are also similar. Specifically, a crowbar device is placed across each wire pair, before the line transformer. Beyond protecting equipment in accordance with required specifications, a key requirement for the crowbar device is a low capacitance to accommodate Ethernet's high data rates. Also, the device should work at Ethernet's low-signal voltages, maintaining open-circuit characteristics in the absence of a surge.

Primary protection is commonly achieved with a tripolar crowbar device like the TPN3021, which contains three crowbar devices in one package. Presenting a typical capacitance of only 16 pF, the device handles surge currents from 30 A (10/1000 µs) to 200 A (2/10 µs). Minimum holding current is a low 30 mA. Alternatively, protection can be provided by two separate crowbar devices, like the SMP100LC (Fig. 3).

In both cases, the need for low breakover voltage applies. Moreover, multiple-diode packages like the DALC208SC6 afford secondary-side spike protection. That device is a diode array that features rail-to-rail clamping and a capacitance of less than 5 pF per diode. Protecting up to four lines, the array carries leakage current and peak-reverse-voltage ratings of less than 1 µA and 9 V per diode, respectively. Housed in a space-saving six-lead SOT-23, the DALC208SC6 integrates the eight diodes necessary to connect all four wires to both the power-supply rail and ground.

Philippe Rabier is a product marketing engineer for the telecommunications sector at STMicroelectronics (www.st.com), Tours, France. He has an engineering degree from the Conservatoire National des Arts et Metiers, Tours. Rabier can be reached via e-mail at [email protected]

Stephane Serrier is a strategic marketing engineer for the telecommunications sector at STMicroelectronics. He has an engineering degree in electronics from the Ecole Superieur d'Electronique de l'Ouest. Serrier can be reached via e-mail at [email protected]

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