AN2053
Application note
SLIC protection for both classical and new networks
Introduction
Even with booming digital technologies, telecom analog lines remain the most used link in
the world. The market opening to new operators, reserved so far to national telecom
administration, makes an increase of new applications using this simple and cheap way to
supply speech information. POTS (plain old telephone set) is still alive.
Figure 1.
Proposed solutions for the subscriber
Central office
Pots
Suscriber house
Pots
Long line
Short line
High speed digital link
High speed to
Pots coupling
Figure 1
shows possibilities subscribers already have got and which will be in a growing
phase in the near future. This will split the
SLIC
(Subscriber Line Interface Circuit) in two
different types according to the application:
■
■
The long lines using the classical copper twisted pairs up to several kilometers long
The short lines (only a few tens of meter long)
In the second case shown at the bottom of
Figure 1,
the long distance carrying of the signal
is assumed by modern digital supports like optical fibers, coax, RF link etc.
For both of these applications the protection needs remain one of the major issues of the
system design, so STMicroelectronics, which is one of the major players in the world of
telecom protection, already proposes optimized solutions for these two topologies.
June 2011
Doc ID 10917 Rev 2
1/15
www.st.com
Protect against what?
AN2053
1
Protect against what?
Telecommunication lines are submitted mainly to two kinds of disturbances. The first one is
linked to atmospheric effects while the second one is produced by the 50/60 Hz mains
network (see
Figure 2).
These disturbances are well defined in individual country standards
and
Table 1
shows the main standards in use.
Figure 2.
Main telecom line disturbance causes
Atmospheric effects
Central office
ESD
50/60 Hz mains effects
Figure 3
shows an example of lightning surge definition. This is given by the ITU-T K20
standard (International Telecommunication Union). This simulation is based on the
discharge of a 20 µF capacitance through resistances. The 20 µF capacitance and the 50
Ω
resistance define the surge wave duration while the 15
Ω
resistance and the 0.2 µF
capacitance manage the rise time. In this case the surge is defined as a 10/700 µs wave.
The tests shall be managed in both transversal and longitudinal modes.
Figure 3.
ITU-T K20 lightning surge test definition
R3 = 25
Ω
Surge
Surge
generator
generator
Figure 1
Coupling
Coupling
network
network
A Equipment
Equipment
under test
under test
B E
R2 = 15
Ω
Uc
20 µF
R1 = 50
Ω
0.2 µF
K20 transversal test
R4 = 25
Ω
Surge
generator
R5 = 25
Ω
Coupling
network
A
Equipment
under test
K20 surge generator
B E
K20 longitudinal test
2/15
Doc ID 10917 Rev 2
AN2053
Protect against what?
Figure 4
shows the ITU-T requirement for both the mains induction and contact test circuits.
This simulation is based on the application of 50/60 Hz through resistance during a
programmed duration (i.e. 0.2 s for induction and 15 min. for contact).
Figure 4.
ITU-T K20 power induction and power contact surge test definition
10
Ω
160
Ω
Surge
generator
Uac
R = 600
Ω
R = 600
Ω
Timing circuit
600Ω
A Equipment
under test
B E
Uac
10
Ω
Timing circuit
160
Ω
600
Ω
T2
S
Equipment
under test
T1
A
B E
K20 power induction surge generator
K20 power contact surge generator
Table 1.
Country
Worldwide
Worldwide
Worldwide
USA
USA
Main line card lightning surge standards
Standard
ITU-T K20
IEC 61000-4-5
IEC 61000-4-5
GR-1089 Core (Telcordia)
GR-1089 Core (Telcordia)
Surge
voltage (V)
1500
1000/4000
1000/4000
2500
1000
Waveform
10/700 µs
10/700 µs
1.2/50 µs
2/10 µs
10/1000 µs
Current (A)
37.5
25/100
25/100
500
100
Table 2.
Test
1
2
GR-1089 Core Intra-building lightning surge standard
Surge Voltage (V)
±800
±1500
Waveform
2/10 µs
2/10 µs
Surge Current per
conductor (A)
100
100
Repetitions
Each
polarity
1
1
Table 1
and
2
show the main worldwide lightning surge standards.
Table 1
is dedicated to
classical wired telecom line cards while the
Table 2
is dedicated to intra-building
applications. The main worldwide standards for the 50/60 Hz disturbances can be defined
by 2 parameters: the applied voltage, between 60 to 1000 V and the test duration, between
0.2 s to 15 min. This type of disturbances obliges the designer to put series elements, like
PTC or a fuse between line and protection devices.
Section 2
presents the protection concept used to protect both short and long lines.
Doc ID 10917 Rev 2
3/15
LCP concept
AN2053
2
LCP concept
Figure 5.
LCP15xx concept behavior
Rs1
L1
TIP
Ig
GND
-Vbat
C
Rs2
L2
V Ring
Gate
ID1
T1
Th1
D1
GND
V Tip
RING
Figure 5
shows the classical protection circuit using the LCP15xx crowbar concept. This
topology has been developed to protect the new high voltage SLICs. It allows the system to
be programmed for the negative firing threshold while the positive clamping value is fixed at
GND.
When a negative surge occurs on one wire (L1 for example), a current I
g
flows through the
base of the transistor T1 and then injects a current in the gate of the thyristor Th1. Th1 turns
on and all the surge current is short circuited to ground. After the surge, when the current
flowing through Th1 becomes lower than the holding current I
h
, then Th1 switches off.
When a positive surge occurs on one wire (L1 for example) the diode D1 conducts and the
surge current is short circuited to ground.
In order to minimize the remaining voltage across the SLIC inputs during the surge, a 4 point
structure has been implemented (Pins 1 and 8 for TIP / Pins 4 and 5 for RING). This fact
allows the board designer to connect the track as designed in
Figure 6.
With such a PCB
design, extra voltages caused by track stray inductance and current slope (L
di/dt
) remain
located on the line side of the LCP and do not affect its SLIC side.
The capacitor C is used to speed up the crowbar structure firing during the fast negative
surge edges. This allows the dynamic breakover voltage at the SLIC Tip and Ring inputs to
be minimized during fast strikes. Please note that this capacitor is generally present around
the SLIC -V
bat
pin. So to be efficient it has to be moved as close as possible to the LCP15xx
Gate pin and to the reference ground track (or plan) (see
Figure 6).
Optimized value for C is
220 nF.
4/15
Doc ID 10917 Rev 2
AN2053
Figure 6.
Example of PCB layout based on LCP15xx protection
LCP concept
The series resistors Rs1 an Rs2 designed in
Figure 5
represent the fuse resistors or the
PTC which are mandatory to withstand the power contact or the power induction tests
imposed by the different country standards. Taking into account this fact, the actual lightning
surge current flowing through the LCP is equal to:
I
surge
= V
surge
/ (R
g
+ R
s
)
With:
●
●
●
V
surge
= peak surge voltage imposed by the standard
R
g
= series resistor of the surge generator
R
s
= series resistor of the line card (e.g. PTC)
For a line card with 30
Ω
of series resistors which has to be qualified under GR-1089 Core
1000 V, 10/1000 µs surge, the actual current through the LCP1521S is:
I
surge
= 1000 / (10 + 30) = 25 A
Doc ID 10917 Rev 2
220 nF
To
line side
GND
To
SLIC side
5/15
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