WHITE PAPER
Gas Discharge Tubes
Help Protect VDSL Equipment and xDSL Splitters
VDSL (very-high-speed digital subscriber line) technology is
similar to the well-known ADSL, and facilitates the delivery of
information at speeds of up to 52 Mb/s. Standard VDSL
deployment uses a frequency spectrum up to 12 MHz, whereas
VDSL2 allows for up to 30 MHz as an option.
The capabilities of VDSL are dependent 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 is able to
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
may be exposed to overcurrent and overvoltage hazards from AC
power cross, power induction, and lightning surges.
PolySwitch PPTC (Polymeric Positive Temperature Coefficient) devices can provide coordinated protection with overvoltage devices, such as
GTC Series gas discharge tubes and SiBar thyristors from Tyco Electronics, against these types of faults and can help reduce failures and
warranty costs.
Cost of Survival
Telecommunications equipment must be able to survive surges
and power faults as defined in the relevant standards.
Survivability can be achieved by providing protection either
remotely or at the terminals of the equipment, or both. In
addition, or alternatively, protection can be achieved by making
the equipment more robust.
the protection scheme may be diminished, but other components
must then be made more robust to compensate. In such a case,
the cost of enhancing reliability of the downstream components
may exceed the cost savings of a less robust protector. A good
design will optimize the trade-offs.
When designing a circuit protection strategy it is important to
consider the complete system. To reduce cost, the capabilities of
Reducing Insertion and Return Loss In VDSL Designs
Because signal spectrum is increasing from 10 MHz to 30 MHz,
VDSL system designers are faced with 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 circuit protection design and PCB layout are
also critical elements in optimizing system performance.
A variety of circuit protection methods are used in telecom
applications around the world, as illustrated in the following
figures:
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Figure 1. Typical circuit with GDT as the primary protection device, and
with fuses and SiBar thyristors as secondary protection devices.
Figure 2. Typical circuit with GDT as the primary protection device, and
with dual PolySwitch devices and SiBar thyristors as secondary
protection devices.
Figure 1 shows how a 3-pole Gas Discharge Tube (GDT) helps
provide primary protection, and how fuses and SiBar thyristors
are employed as secondary protection devices.
Figure 2 shows how a 3-pole GDT helps provide primary
protection, and how PolySwitch devices and SiBar thyristors are
employed as secondary protection devices.
Figure 3 illustrates the capacitance effects on insertion loss of
several overvoltage protection configurations. It shows that low
capacitance GDTs (1pF) have the lowest insertion loss, with the
standard 50A thyristors (15pF at 50V DC bias) and 100A microcapacitance TVB270SC (20pF at 50V DC bias) devices having
slightly greater insertion loss.
In both of these protection schemes the GDT, available in both
through-hole and small surface mount form factors, serves as the
primary protection device with the highest surge rating. The
SiBar thyristor offers fast secondary overvoltage protection. The
PolySwitch devices are purely resistive and help provide
resettable overcurrent protection.
The inset modules shown in this test diagram consist of either a
230V 3-Pole GDT or two 270V in-series SiBar thyristors, attached
to two 0.3m pieces of Cat 5e twisted pair. An Agilent 8753ES
Vector Network Analyzer with two North Hills’ 0301BB 50:100
Ohm wide band transformers were used to make the insertion
loss measurements.
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 cause increased
system insertion loss. Tyco Electronics’ low-capacitance SiBar
devices and GDTs, with inherently low capacitance, are suitable
for high data rate circuits, including VDSL.
The transformers were used to measure the insertion loss of the
modules under 100 Ohm impedance conditions, which is equal to
the line impedance over the VDSL frequency spectrum.
Capacitance at 1MHz with no bias was measured using an HP 4195
Low Frequency Impedance Analyzer.
Figure 3. Capacitance effects of overvoltage protection devices.
-2-
Implementing a Low-Capacitance Solution
for VDSL
The circuit diagram in Figure 4 shows a VDSL
solution that effectively reduces capacitance and
energy let-through, and optimizes the circuit
protection scheme.
As shown in this circuit diagram, GDT1 provides
primary protection (at 350V to 1000V). The
GDT2 and GDT3 devices are connected in series
with the SiBar 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. The PolySwitch devices help
coordinate
protection.
the
primary
and
Figure 4. Coordinated circuit protection helps reduce energy let-through
secondary
Figures 5, 6 and 7 show test results for this
protection method and demonstrate that the
GDT and SiBar combination does not break down
under ringing voltage and does not clip the
ringing voltage.
In the oscilloscope screen shot in Figure 5, the
input voltage rate is at 100V/s. The DC breakdown
voltage at 287V is achieved, which is higher than
the ringing voltage of 200V.
Figure 6 shows the data from a test performed
with an AC voltage input at 150Vrms. Results
show no clipping, indicating that the GDT and
SiBar combination does not break down under
the ringing voltage and clip the ringing voltage.
Here, the SiBar device determines the static
breakdown.
Figure 5: Test results of GDT and SiBar thyristor in series at 100V/s.
In Figure 7, the same test was performed per the
ITU K.20 10/700µS at 4kV level. Oscilloscope
observations show the breakdown voltage of the
GDT and SiBar combination at 392V. Voltages
also noted are the GDT breakdown voltage of
330V, and the SiBar breakdown voltage of 250V.
Here, the dynamic breakdown voltage is
determined by the GDT.
Figure 6: Test results of GDT and SiBar thyristor at 150Vrms.
Figure 8: Capacitance and breakdown
comparison of three protection schemes.
voltage
Figure 8 compares the capacitance and
breakdown voltage characteristics of GDTs and
SiBar thyristors, and shows the benefit of using
them in a coordinated protection scheme.
Figure 7: Test results of GDT and SiBar thyristor at 4kV level
-3-
xDSL Splitter Protection Solutions
Figure 9: DSL architecture featuring DSLAM and POTS/DSL splitters.
Splitters are used to help typical POTS devices when ADSL or
In Figure 11, two PolySwitch overcurrent devices are placed
VDSL services are deployed on the same copper pair. The POTS
before the GDT. They help limit the energy through the GDTs,
splitter uses a low-pass filter to separate the low-end frequencies
preventing them from going into glow mode. If the current flow
of the telephone audio spectrum from the higher frequencies of
is limited to below that need for a glow-to-arc transition, typically
the xDSL signal, allowing traditional voice service.
200 mA to 1.5A depending on design, the GDTs can experience
significant power loss in this high-voltage, low-current condition.
A splitter is required at both the customer premises and at the
central office (CO). xDSL that does not use a POTS splitter on
Employing fuses in one’s design –– typically rated at 1.25A in
customer premises is commonly referred to as "splitter-less
splitter applications –– may not provide sufficient protection in
xDSL." However, splitter-less xDSL does not actually exist, in that
the area of sneaker currents, which are in the 100mA ~ 1A range.
the splitter function in these cases is performed at the provider,
The PolySwitch devices help provide protection against such
generally the CO. Whether a POTS splitter is required or not
high-voltage, low-current conditions. When the PPTC device is
depends on the type of xDSL service being provided.
installed in the circuit it helps limit sneak currents that can
degrade GDTs.
Figures 10 and 11 illustrate two common topologies for splitter
applications. In Figure 10, SiBar surge protection devices provide
lower capacitance and faster trigger voltage in a grounded
system.
Figure 10. SiBar thyristors help provide low-capacitance surge
protection.
Figure 11. GDTs help provide robust surge protection.
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Device Selection for Agency Approval Requirements
Circuit protection for telecommunications network equipment is
TRF600 series devices are applicable for North American GR-
typically designed to meet the requirements of Telcordia GR-1089
1089 standards and for UL60950 standards, while surface-mount
for North America installations and ITU-T K.20 for installations in
TS250 and TSV250 and radial-leaded TRF250 devices are
the rest of the world. Protection for customer premise equipment
applicable for ITU-T K.20/21 and IEC60950 standards, as well as
is typically designed to meet the requirements of UL60950 and
for Telcordia GR-1089 Intrabuilding level protection.
TIA-968-A for North American use, and IEC60950 and ITU-T K.21
TVB thyristor surge protection devices with VDM ratings of 200V
for rest-of-world use.
are applicable for most ringing systems with 48V loops. For
PolySwitch devices should be selected with voltage ratings based
higher or lower loop voltage requirements, designers should
on the regulatory standards for which the equipment is being
adjust for and select the VDM ratings that will meet their
designed. Surface-mount TSM600 series and radial-leaded
requirements.
GDT Glow Voltage
Because of their switching action and rugged construction, GDTs
GDT in the glow region after a surge event. If the glow voltage is
exceed other surge protection components in current-carrying
higher than the DC source voltage, latch-up in the glow region
capability. Many telecommunications GDTs can easily carry surge
cannot occur.
currents as high as 10 kA, 8/20. Depending on design and size
In AC power fault conditions, if the current flow is limited to
values, currents of >100kA can be achieved.
below that which is needed for a glow-to-arc transition, typically
Regardless of compliance issues, the designer must consider how
100mA to 1.5A, the GDT can experience a significant power loss.
the GDT’s glow voltage region can influence two operational
When selecting an overcurrent protection device to help protect
areas – DC holdover and low AC power loss.
the GDT, it must be able to function in this region, and also should
help provide coordination.
When a GDT is connected to conductors sourcing DC power it is
possible for a current-limited DC source voltage to maintain the
Telecom Specs: Quick Review
North America:
Worldwide:
• CO and Remote Access Equipment – GR-1089-Core
• K.20 – Central Office
• GR-1089-Core Intrabuilding applies to CO ports that do not
• K.45 for Remote Access Equipment
leave the building
• K.21 for CPE
• CPE – TIA-968-A for lightning and UL60950 for AC Power
• ICASA TE001 and TE010 for South Africa
Cross that interface to external communication lines
• CPE equipment ports that do not leave the building do not
currently require any impulse or AC protection, but protection
is desirable to improve reliability and minimize field returns
• Some Customers are now meeting K.21, but prefer GR-1089
Importance of Bonding and Grounding
The objective of bonding is to equalize the potential between
Circuit boards should be laid out with all the protection grounds
grounds. Bonding conductors need to be of sufficient cross-
bonded together with the shortest and heaviest traces possible.
sectional area to safely conduct anticipated currents, and should
The grounds so bonded should be connected to earth or frame
be as short and straight as practicable.
ground with the largest and shortest connection possible. Poor
bonding and grounding practices can defeat even the best
The objective of grounding (earthing) is to provide a means for
protection scheme.
maintaining grounding conductors at or near earth potential. In
this respect it is important that the ground be a true earth or
frame ground, and not a signal ground.
-5-
Summary
Tyco Electronics’ GDTs are commonly used to help protect
Distribution Frame) modules, high data-rate telecom applications
sensitive telecom equipment from damage caused by transient
(e.g., VDSL and xDSL), and surge protection on power lines.
surge voltages that typically result from lightning strikes and
When used in a coordinated protection scheme with PolySwitch
equipment switching operations. GDTs are placed in front of, and
devices and SiBar thyristors, they can help equipment
in parallel with, the sensitive equipment acting as a high
manufacturers meet the most stringent regulatory standards.
impedance component while not influencing the signal in normal
operation. Due to their low capacitance, the GDTs exhibit lower
insertion
loss
than
many
other
overvoltage
protection
technologies.
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. Tyco Electronics
works with OEM customers to help identify and implement the
Due to their fast and accurate break-over voltage, Tyco
best approach.
Electronics’ GDTs are suitable for applications such as MDF (Main
Raychem Circuit Protection Products
308 Constitution Drive, Building H
Menlo Park, CA USA 94025-1164
Tel : (800) 227-7040, (650) 361-6900
Fax : (650) 361-4600
PolySwitch, Raychem, SiBar, TE logo and Tyco Electronics are trademarks. All information, including
illustrations, is believed to be reliable. Users, however, should independently evaluate the suitability of each
product for their application. Tyco Electronics Corporation makes no warranties as to the accuracy or
completeness of the information, and disclaims any liability regarding its use. Tyco Electronics’ only obligations
are those in the Tyco Electronics Standard Terms and Conditions of Sale for this product, and in no case will
Tyco Electronics be liable for any incidental, indirect, or consequential damages arising from the sale, resale,
use, or misuse of the product. Specifications are subject to change without notice. In addition, Tyco Electronics
reserves the right to make changes without notification to Buyer—to materials or processing that do not
affect compliance with any applicable specification.
© 2008 Tyco Electronics Corporation. All rights reserved. RCP0045E.1008
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