Electronic Design

Gas Discharge Tubes Help Protect VDSL Equipment And xDSL Splitters

Telecommunications equipment must be able to survive surges and power faults as defined in the relevant standards. Survivability is achievable by providing protection remotely, at the terminals of the equipment, or both. In addition, or alternatively, we can provide reliable protection by making the equipment more robust.

When designing a circuit-protection strategy, it is important to consider the complete system. To reduce cost, we can diminish the capabilities of the protection scheme somewhat, though the other components must be more robust to compensate. In such a case, the cost of enhancing the reliability of the downstream components may exceed the cost savings of a less robust protector. A good design will optimize the tradeoffs.

VDSL CIRCUIT PROTECTION CONSIDERATIONS

Very high-speed digital subscriber line (VDSL) technology facilitates the delivery of information at speeds of up to 52 Mbits/s. Standard VDSL deployment uses a frequency spectrum up to 12 MHz, whereas VDSL2 allows up to 30 MHz as an option.

The capabilities of VDSL depend on the distance between the operator and end-customer equipment as well as the condition of the existing copper plant and copper infrastructure outside the plant. Depending on loop conditions, VDSL can support varying bit rates and high-bandwidth services such as a channel of HDTV programming over telephone copper pairs.

Since VDSL equipment connects to the copper infrastructure of the public switched telephone network (PSTN), the equipment runs the risk of exposure to overcurrent and overvoltage hazards from ac-power-cross, power-induction, and lightning surges. One possible solution deploys resettable polymeric positive temperature coefficient (PPTC) overcurrent protection devices in a coordinated protection scheme with overvoltage devices such as gas discharge tubes (GDTs) and thyristor surge suppression devices.

REDUCING INSERTION AND RETURN LOSS

Because signal spectrum is increasing from 10 MHz to 30 MHz, VDSL system designers face a number of new challenges. The most important issue is reducing insertion and return loss and the effect on reach and rates in high-speed applications.

The capacitance of overvoltage protection devices becomes a concern in the upper range of the VDSL frequency spectrum, as the devices used to protect the system may increase system insertion loss. Tests demonstrate that low-capacitance thyristors and GDTs are suitable for use in high-data-rate circuits including VDSL applications.

The test results in Figure 1 illustrate the effects of capacitance on insertion loss in several overvoltage protection configurations. The results show that low-capacitance GDTs, in the realm of 1 pF, have the lowest insertion loss with standard 50-A thyristors (15 pF at 50-Vdc bias) and 100-A micro-capacitance thyristor devices (20 pF at 50-Vdc bias) having slightly greater insertion loss.

Regarding test procedures, the inset modules in the test diagram consist of either a three-pole, 230-V GDT or two 270-V in-series thyristors interfacing with two 0.3-m pieces of CAT5e cable. An Agilent 8753ES Vector Network Analyzer with two North Hills 0301BB 50:100 Ω wideband transformers performs the insertion-loss measurements.

The transformers are for measuring the insertion loss of the modules under 100-Ω impedance conditions, which is equal to the line impedance over the VDSL frequency spectrum. An HP 4195 low-frequency impedance analyzer performs capacitance measurements at 1 MHz with no bias.

IMPLEMENT A LOW-CAPACITANCE VDSL SOLUTION

In Figure 2, the circuit diagram shows a VDSL solution that effectively reduces capacitance and energy let-through while optimizing the circuit-protection scheme. Visible in the circuit diagram, GDT1 provides primary protection from 350 V to 1 kV, and the GDT2 and GDT3 components work in series with the thyristors. In this scenario, the thyristor helps lower the breakdown voltage of the GDT and reduces the let-through energy in the case of a surge. Additionally, the PPTC devices help coordinate the primary and secondary protection.

Test results for this protection method, found in Figures 3, 4, and 5, demonstrate that the GDT and thyristor combination does not break down under ringing voltage and does not clip the ringing voltage. In the oscilloscope screen shot seen in Figure 3, the input voltage rate is at 100 V/s and the dc-breakdown voltage is at 287 V, which is higher than the ringing voltage of 200 V.

The data shown in Figure 4 comes from a test performed with an ac voltage input at 150 VRMS. Results show no clipping, indicating that the GDT and thyristor combo does not break down under the ringing voltage and clip the ringing voltage. Here, the thyristor determines the static breakdown.

In Figure 5, we see the same test performed as per the ITU K.20 10/700-µs specification at a 4-kV level. Oscilloscope observations show the breakdown voltage of the GDT and thyristor at 392 V. Voltages are the GDT breakdown voltage of 330 V and the thyristor breakdown voltage of 250 V. Here, the GDT determines the dynamic breakdown voltage.

SPLITTER PROTECTION SOLUTIONS

When ADSL or VDSL services operate over the same copper pair, service providers rely on splitters to connect plain-old-telephone-service (POTS) devices (Fig. 6). The POTS splitter employs a low-pass filter to separate the low-end frequencies of the telephone audio spectrum from the higher frequencies of the xDSL signal, allowing traditional voice service to coexist on the line.

A splitter is necessary at both the customer premises and at the central office (CO) except for xDSL, which does not use a POTS splitter on customer premises. Although commonly called splitter-less xDSL, splitter-less xDSL does not actually exist, in that there is a splitter at the provider end, generally the CO. Whether a POTS splitter is necessary or not depends on the type of xDSL service. Figures 7 and 8 illustrate two common topologies for splitter applications.

In Figure 7, the thyristor surge protection devices provide lower capacitance and faster trigger voltage in a grounded system, whereas in Figure 8, two PolySwitch overcurrent devices stand before the GDT. The PPTC devices help limit the energy through the GDTs, preventing them from going into glow mode. If the current flow is limited to below what’s necessary for a glow-to-arc transition, typically 200 mA to 1.5 A depending on design, the GDTs can experience significant power loss.

Employing fuses in a design, with a typical rating at 1.25 A in splitter applications, may not provide sufficient protection in the area of sneaker currents, which are in the 100-mA to 1-A range. Resettable PPTC devices can help provide protection against damage from such high-voltage, low-current conditions as well as help limit sneak currents that can degrade GDTs.

GDT GLOW VOLTAGE

Because of their switching action and rugged construction, GDTs offer greater current-carrying capability than other surge protection components. Many telecommunications GDTs can easily carry surge currents as high as 10 kA, 8/20. Depending on design and size values, currents in excess of 100 kA are possible.

Regardless of compliance issues, designers must consider how the GDT’s glow voltage region can influence two operational areas: dc holdover and low-ac power loss. When a GDT interfaces with conductors sourcing dc power, a current-limited dc source voltage can maintain the GDT in the glow region after a surge event. If the glow voltage is higher than the dc source voltage, latch-up in the glow region cannot occur.

In ac-power fault conditions, when current flow is limited to below what’s necessary for a glow-to-arc transition, the GDT can experience a significant power loss. When selecting an overcurrent protection device to help protect the GDT, it must be able to function in this region and should also help provide coordinated circuit protection.

DEVICE SELECTION FOR AGENCY APPROVALS

Circuit protection for telecommunications network equipment typically meets the requirements of Telcordia GR-1089 for North America installations and ITU-T K.20 for installations in the rest of the world. On the other end of the wire, protection for customer premise equipment typically complies with the requirements of UL60950 and TIA-968-A for North American use and IEC60950 and ITU-T K.21 for use in the rest of the world.

Designers should choose PPTC devices with voltage ratings based on the regulatory standards that the equipment needs to meet. Thyristors with upper-voltage (VDM) ratings of 200 V are applicable for most ringing systems with 48-V loops. For higher or lower loop voltage requirements, designers should adjust for and select the VDM ratings that will meet their requirements.

SUMMARY

GDTs are commonly used to help protect sensitive telecom equipment from damage caused by transient surge voltages that may result from lightning strikes and equipment switching operations. Designers place GDTs in front of, and in parallel with, the sensitive equipment acting as a high-impedance component without influencing the signal in normal operation. Due to their low capacitance, GDTs exhibit lower insertion losses than many other overvoltage protection technologies.

Due to their fast and accurate break-over voltage, GDTs suit applications such as main distribution frame modules, high data-rate telecom applications (VDSL and xDSL), and surge protection on power lines. When used in a coordinated protection scheme with PPTC devices and thyristors, they can help equipment manufacturers meet the most stringent regulatory standards.

As with any type of protection scheme, the effectiveness of a solution will depend on the individual layout, board type, specific components, and unique design considerations. Most circuit protection device manufacturers will work with OEM customers to help identify and implement the best approach. 

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