Modern IC transceivers can be damaged during power-off. During power-on, they can also go into latch-up due to voltage or current surges caused by lightning, nearby power-line disturbances, or electrostatic discharge. This article provides specific schemes designed to protect T1/E1 line interfaces on twisted-pair cable systems.

To ensure the protection of humans and equipment, standards and regulations (including FCC Part 68, UL 1950, Bellcore GR-1089, and ITU K-20 and K-21) require the line-interface portion of telecommunications systems to pass various surge tests in different application modes. These tests generally simulate two types of hazardous conditions: lightning surge and ac power cross. The application modes include longitudinal (hazardous voltage appears on both tip and ring terminals simultaneously) and metallic (hazardous voltage appears across tip and ring).

Lightning surge tests are of short duration, but they produce large peak voltages and large-peak short-circuit currents. On the other hand, ac power-cross tests are lower in voltage, but can be very long. Specific details of test setups and criteria are provided in Bellcore GR-1089, UL-1950, and similar standards.

Protection elements are necessary components usually located next to the transformer or common-mode choke. The protection from hazardous voltages provides operator safety and ensures correct and reliable operation of system equipment. Such protection circuitry is designed to suppress excessive transient voltages and to limit the current flow to the equipment side of the interface.

There are two classes of protection: primary protection and secondary protection. Primary-protection elements are usually located at the building entrance of telecommunications terminals. This is the first set of devices in the signal path to see the overvoltage or overcurrent surges. Secondary-protection elements are usually located on the line-interface card.

Secondary-protection circuitry can provide two different functions: protect the transformer (e.g., line-side protectors) and protect the transceiver (e.g., IC-side protectors). Figure 1 depicts a typical block diagram of a T1/E1 line-interface unit, with all protection circuitry in place.

Figure 1. Front-end interface block diagram.

Line-Interface Components

Three components are essential in any T1/E1 line interface: the IC transceiver; the magnetics, which contain at least a line transformer; and the protection circuitry (see Figure 1).

The IC transceiver converts the data stream into clean pulses for transmission over the cable. It also detects them during reception.

The transformer serves several purposes. First, it provides the isolation between the equipment and the cable. In the event of an equipment malfunction at one end of the cable, the isolation ensures that no excessive power is drawn from or to the other end. Second, the isolation barrier on the transformer helps protect the equipment from longitudinal surges (overvoltage surges that appear simultaneously on tip and ring terminals of twisted-pair wires). All T1/E1 transformers can withstand 1500 V rms across the primary and secondary windings for 1 minute. Finally, the transformer also helps improve the differential signal so that electromagnetic radiation from the two conductors cancel each other out, enabling the system to comply with electromagnetic radiation regulatory requirements such as FCC Part 15, VDE, and CISPR 22.

Unfortunately, surges that travel past the transformer can damage sensitive line-card components. Metallic (or differential-mode) surges can be coupled through the mutual inductance between windings. Longitudinal (or common-mode) surges can be coupled through the interwinding capacitance of the transformer.

Located between the signal connector and the transformer, secondary line-side protectors are called high-voltage protectors. This protection circuitry usually consists of fuses and crowbar voltage clamping devices.

The fuses are used to prevent constant, excessive current, such as in the case of an ac power-cross condition, when the power lines may be in contact with the twisted-pair cable carrying the telecommunication signals. These telecom fuses, which are a slow-blow type, are designed to withstand high-amplitude, short-duration current surges without opening. This eliminates the need to replace a fuse every time such surges occur. In addition, certain safety standards require a system to remain operational after such short-duration surges.

Crowbar voltage-clamping devices are power semiconductor devices (transient voltage suppressor thyristors). These devices are designed to clamp the voltage to a relatively low, preset crowbar level. In addition to absorbing both short- and long-duration excess voltages, they can also absorb up to hundreds of amperes of short-duration current, depending on the type of suppressor used. Because these devices are semiconductor-based, performance does not degrade over time.

A combination of a series fuse and parallel voltage-clamping device provides the necessary protection for a transformer and choke against overcurrent and overvoltage.

With line-side protection devices in place, high-voltage surges can still be coupled to the IC transceiver side through mutual inductance of the transformer. Even though these surges are generally smaller in amplitude than those on the line side, they can still cause permanent damage or temporary latch-up to the transceiver, making the system inoperable. Most transceivers cannot withstand a few volts outside of their power supply rails (below GND and above VCC levels).

To protect the transceiver from the damaging effects of latch-up fast transients, line-card designers must use special-protection components to shunt these transients to manageable levels. A common implementation technique is to use diode arrays to limit the voltage on the transceiver input and output pins to just a few tenths of a volt (diode drop VD) above VCC or below GND levels.

Another protection method is to use a voltage-clamping device similar to those used on the line-side protection circuitry, but with much lower clamping voltage. The use of such device limits the differential voltage across the transceiver input and output pins to a minimal voltage (in the range of VCC to GND volts), which the transceiver pins can handle.

Depending on the type of protection required and the desired level of integration allowed by available printed-circuit-board space, designers could choose different front-end interface modules, which include a variety of magnetics and protection circuitry. Four examples of the Pulse (San Diego, CA) integrated modules include T9021 (four-port, IC-side protection devices and transformers), T9030 (four-port, line-side protection devices, transformers, and chokes), T9050 (dual-port, line- and chip-side protectors, transformers, and chokes), and J1501F21 (single-port, line-side protection and a built-in RJ45 connector).

Figure 2 shows a typical T1/E1 front-end line-interface card with all magnetics and protection components in place. F1–F4 represent fuses that limit long-duration current, such as the current caused by ac power-cross condition. S1–S4 represent voltage-clamping devices that help protect the transformers and the EMI choke from both longitudinal and metallic surges. The diode array shown on the left side of the transformers helps the voltage at the transceiver input/output pins (T1, R1, T2, R2) stay within a volt of the power supply rails.

Figure 2. Schematic of a fully protected line-interface card.

Conclusion

To ensure compliance with safety regulatory requirements such as those specified by Bellcore GR-1089, UL 1950, and FCC Part 68, telecommunications line-card designers must include line-side protection circuitry in their front-end interface circuitry. Although IC-side protection is not required by any regulation, such protection may be necessary to ensure reliable system operation.