I INTRODUCTION
The goal of the telecommunication network
(fig.1) is to permit the data exchange (speech or
digital) between two (or more) subscribers.
The network is made up of different parts which
are subject to various disturbances.
The most susceptible elements are the lines,
due to their lenght and their geographical
location.
Disturbances strike the lines and are then
propagated to the extremities of the lines at
which lie telephone set and the subscriber line
interface card (SLIC).
So the lines receive two kinds of overvoltages:
Surges of short duration with high peak voltage
value (a few hundred micro-seconds for a few
thousand volts). These are generated by
atmospheric phenomena.
Surges of long duration with medium voltage
value (greater than one second for a few
hundred volts RMS) which are due to the mains
AC power networks.
The purpose of this application note is to
analyse these 2 kinds of overvoltages and to
propose different protection solutions dedicated
to the SLIC.
II OVERVOLTAGES ACROSS TELECOM-
MUNICATION LINES :
II.1 Atmosphericeffects :
Lightning phenomena are the most common
surge causes. They are mainly due to a voltage
difference between the ground and the clouds (a
few 100 kV). Two kinds of strikes may occur.
Negative discharge with a peak current of 50 kA,
rise time of 10µsto15µs and 100µs duration.
Positive discharge with a peak value of 150 kA,
rise time between 20µs and 50µs and a
duration between 100ms and 200ms.
The lightning effect appears on the lines in two
ways.
–Direct shock.
–Induced shock.
APPLICATION NOTE
AN386/0898
SLIC PROTECTION
A. BREMOND
Fig.1 Classical telecommunication network
topology.
MODEM
PABX
MODEM
EXCHANGE EXCHANGE
PABX
Fig.2 Lightning phenomenum
+++++++
-------
GROUND
IONOSPHERE
CLOUD
1/16
The Fig.3 shows the first case which is produced
mainly on overhead lines.
Induced shock is more frequent than a direct
shock. Lightning strike the ground and a current
flows in the cable shield. This current produces a
voltage gradient which in some places is above
the insulation capability of the cable material
(Fig.4).
II.2 Proximity and crossing with AC mains lines :
For these kinds of surges two cases may be
seen :
The first one is due to the falling of an AC mains
cable on a telephoneline.
The second case is produced by the proximity of
a subcriber line with an AC mains line or
equipment (mainly capacitive coupling).
It is interesting to note for these types of
disturbances a RMS value of a few Amps for a
duration of between 1 s and 15 mn.
III PRIMARY AND SECONDARY PROTEC-
TION :
The figures in chapter II give us an idea of the
energy which may appear on the lines. (so in the
field these surge values are lower due to the
losses of ground resistance, the capacitive
coupling and so on, but are signifiant
nevertheless).
We have to divide these disturbances into two
families :
High peak value and short duration (lightning)
Short peak value and long duration (crossing
with AC power).
For both cases the present state of the art of
silicon protection devices does not permit the
suppression of these levels of energy.
A second parameter to keep in mind is the very
low clamping factor (1) needed by the IC’s used
to realize the line interface. This fact
necessitates the designer to use a protection
solution with silicon (fast response time/low
clamping factor).
High energy values and low clamping factor
impose two protection levels.
The first level called primary protection (fig.5)
located on the connecting terminal of the
exchange, suppresses the major part of the
disturbance. The second level called secondary
protection reduces the remaining overvoltage.
(1) the clamping factor is the ratio of the normal
operating voltage over the maximum clamping
voltage.
Fig.4 induced strike.
SUBSCRIBER
CENTRAL
OFFICE
Fig.5 Primary/secondary protection topology.
LINE
PRIMARY
PROTECTION
SECONDARY
PROTECTION
SLIC
Fig.3 Direct lightning strike.
123
56
4
789
*0
CENTRAL
OFFICE
APPLICATION NOTE
2/16
The figure 6 shows the goal of both protection
levels.
In this example the surge across the line without
protection will be a few 10 kV peak value for a
few 10 ms length (Fig.6A)
After the primary protection the major part of the
energy is cancelled (Fig.6.B). The remaining
overvoltage may be a few kV (depend on the
dv/dt of the surge and the surge arrestor
technologyused).
Across the second level protection the voltage
does not exceed a few 10 Volts.
III.1 Primary protection :
Actually two kinds of primary protection are
used :
–carbon gaps.
–gas tubes.
III.1.A Carbon gaps :
These components are made by two carbon
electrodes. In fact the carbon gap is the low cost
primary protection but it has two major
disadvantages:
–its short life duration
–its variable spark threshold.
III.1.B Gas tubes :
These components are made by two metallic
electrodes in a sealed case. Generaly the sealed
tub contains a low pressure gas.
The major disadvantage of this kind of device is
its response time, in fact the maximum voltage
across the gas tube depends on the dv/dt of the
surge.
Fig.7 Carbon gap based primary protection.
Zl
Zl
LINE EXCHANGE
Fig.8 Gas tube based primary protection.
Zl
Zl
LINE EXCHANGE
Fig.9 Gas tube characteristics.
V
I
3
1
2
1/ Sparkover voltage.
2/ Glow discharge voltage.
3/ Remaining voltagein switch on mode.
Fig.6 Primary/secondaryprotection levels effects
t<1ms
1to4
kV
B/
t > 50ms
V peak
>
20kV
A/
Few 10V
C/
A/ Overvoltage acrossthe line withoutprotection.
B/ Remaining voltageafter the primarylevel action.
C/ Remaining voltage after the secondary level action.
APPLICATION NOTE
3/16
III.2 Secondaryprotection :
III.2.A Series and parallel protection :
The secondary protection level is generaly
achieved with two types of devices :
The series protection ensures the protection
against the proximity or the crossing with AC
power lines.
The parallel protection operates to suppress the
overvoltages due to the lightning effects.
* Series devices :
Series devices operate by opening of the circuit
or by an increment of the resistance.
The fuse is classical case of protection by
opening of the circuit. Figure 11 shows an
exchange protected by fuses and figure 12
represents an example of the limit curve of the
fusing action.
These components provide an absolute security
after action, but their major disadvantage is the
need for maintenance.
The PTC thermistor is a device which operates
by very rapid resistance increase as a function
of the temperature.
When the surge occurs across the line, the
parallel protection PP is activated.
The surge current Is, generated by PP action,
flows through the PTC and increases its internal
temperature. As shown on the figure 14 the
resistance value of the PTC rises quikly with the
temperature.
Fig.10 Series and parallel protection.
MODULE
TO BE
PROTECTED
1
2
1/ Series protection.
2/ Parallel protection.
Fig.11 Fuse protection.
LINE EXCHANGE
FUSE
FUSE
Fig.12 Fuse fusion function.
.01
.1
1
10
100
t (s)
I (A)10 100
2.5 A
1
Fig.13 PTC based protection.
LINE EXCHANGE
PTC
PTC
Is PP
PP
APPLICATION NOTE
4/16
The major disadvantage of the fuse does not
exist with the PTC. Unfortunatly this kind of
component presents a big tolerance, a long time
to return to its stand off point and a drift of its
value.
An other series device is the resistance which
permits to limit the current through the parallel
protector.
* Parallel devices :
The parallel protection function may be assumed
by different devices based on different
technologies.
In fact it is clear that the future in term of SLIC
topology is based on the use of IC.So the
consequent requirement for good response
times and high clamping factor necessitates the
use of silicon protection.
Parallel silicon protection functions in two
different modes.
* Clamping mode :
This device named TRANSIL is based on the
breakdown effect. During the stand off time the
working point is located between 0 and VRM (see
curve of fig.15) and the device draws a very low
leakage current ( < 5 µAat25°C). When a surge
occurs across the line the working point is
located between VBR and VCL the increase of the
voltage is low (good clamping factor) and the
current drawn very high.
The TRANSIL may be uni or bidirectional.
During the clamping action the major part of the
energy is dissipated in the TRANSIL.
* Crowbar mode :
The CROWBAR device named TRISIL is based
on the breakover effect. In fact in normal
operating the device operates in the area
between 0 and VRM (see curve fig.17) and the
bias current through the protection is very low
( < a few µAat25°C).
Fig.14 Resistance versus temperature.
10
100
1k
10k
100k
R(ohms)
25 150 t(C)
Fig.15 Clamping characteristics.
Ipp
Irm
0 Vrm Vbr Vcl
I
V
Fig.16 TRANSIL symbols.
UNIDIRECTIONAL BIDIRECTIONAL
Fig.17 CROWBAR characteristics.
Ipp
Irm
0Vrm Vbr
I
VVbo
Ih
Vt
APPLICATION NOTE
5/16
When the surge occurs the TRISIL begins to
work in the clamping mode between VBR and
VBO. After that the device fires and operates like
a short circuit.
After the surge, the current decreases and falls
below the holding current IH. This condition
causes the TRISIL to switch off and the return to
the stand off area.
There are two kinds of TRISIL, the fixed
breakover voltage type and the device with
adjustable breakover voltage.
During the surge suppression action of the
TRISIL the major part of the energy is dissipated
in the series elements of the line.
This last fact makes the TRISIL better adapted
to protect againt the high energy of the lightning
overvoltages.
IV STANDARDS :
These standards define the two kinds of
overvoltage (lightning and crossing).
IV.1 Lightning simulation :
The lightning overvoltage is simulated by a bi-
exponentional wave, which are defined by the
rise time t1 and the duration t2 between the start
and the passage of the deacrising edge at half
peak value (fig.19).
Each country published its standard, which can
be summarized by the times t1 and t2, the peak
voltage of the wave and the surge generator
diagram.
The table here after gives a unexhaustive list of
the standard:
The following figures give us the diagram of the
surge generatorsmainly used :
Fig.18 TRISIL symbols.
FIXED VBO AJUSTABLEVBO
G
Fig. 19 Standardwave.
V
Vp
Vp/2
0t1
t2
t
COUNTRY AUTORITY WAVEFORM
(µs)
ENGLAND
FRANCE
GERMANY
ITALY
SPAIN
SWEDEN
SWITZERLAND
USA
BRITISH
TELECOM
PTT
BUNDESPOST
SIP
COMPANY
TELEPHONICA
DE ESPANA
TELEVERKET
PTT - BETRIEBE
BELL
FCC
10/700
0.5/700
10/700
0.5/700
1/100
1/1000
10/700
10/700
1.2/50
10/1000
10/360
2/10
10/560
10/160
2/10
APPLICATION NOTE
6/16
Fig. 20 10/700µs wave generator.
20uF 50
15 25
0.2uF
Vp
MODULE
TO BE
TESTED
all resistances are given in ohms
Fig. 21 0.5/700µs wave generator.
20uF 50
15 25
10nF
Vp
MODULE
TO BE
TESTED
all resistances are given in ohms
Fig. 22 1/1000µs wave generator.
20uF
Vp
MODULE
TO BE
TESTED
all resistancesare given in ohms
60
1.5uH 15
2
22nF
Fig. 23 1.2/50µs wave generator.
Fig. 24 10/560µs wave generator.
Vp
MODULE
TO BE
TESTED
all resistances are given in ohms
50uF
36uH
27.1
5.2
1080
Fig. 25 10/160µs wave generator.
Vp
MODULE
TO BE
TESTED
all resistances are given in ohms
50uF
10uH
6.3
5.2
1080
Vp
MODULE
TO BE
TESTED
all resistances are given in ohms
4uF
4 33.5
0.1uF
33
APPLICATION NOTE
7/16
IV.2 Crossing or proximity with main ac lines :
Crossing or proximity are simulated by a sinus
generator (50 or 60 Hz) which injects through a
series resistor during a defined time (fig.27).
Here after are given some examples of crossing
simulation
.V SLIC FUNCTION :
V.1 SLIC generalities:
The SLIC function is defined by the
nemotechnic word BORCHT :
–Battery feeding
–Overvoltageprotection
–Ringing
–Signalling
–Cofidec
–Hybrid
–Test
The important parameters to define the
OVERVOLTAGE PROTECTION are the battery
feeding and the ringing signal.
Fig. 26 2/10µs wave generator.
Vp
MODULE
TO BE
TESTED
all resistancesare given in ohms
3M
110Vac
0.002uF 0.004uF
100uH
2uF
3uH
10
Fig. 27 Crossing simulation generator.
MODULE
TO BE
TESTED
RS
COUNTRY VOLTAGE
Volts RMS SERIES
RESISTOR
(Ohms)
DURATION
ENGLAND
FRANCE
GERMANY
ITALY
USA
0 TO 250
0 TO 650
0 TO 430
(50 Hz)
O TO 1000
> 1000
(50 Hz)
300
(50 Hz or
16.6 Hz)
300
650
220
0-50
50 - 100
100 - 600
40 TO 400
150
150
20
3000
600
600
200
10 or 600
150
600
600
15 mn
1s
2s
trains of 1 s
”ON” 2 s
”OFF” 10
times
with 3 mn
between
each trains
200 ms
500 ms
500 ms
15 mn
15 mn
15 mn
60 x 1 s
application
APPLICATION NOTE
8/16
V.1.A Battery feeding :
This sub-function of the SLIC is characterized
by :–the battery voltage typical value (generaly
between 45 and 65 V)
–the toleranceof the voltage value
–the possibility to switch from one value to
another one (case of line cards designed to
operate equally on normal and long lines)
V.1.B Ringing signal :
For the ringing two parameters are to be taken
into account :
–the voltage value (generaly between 70
and 100 V RMS)
–the ringing configuration (fig.28)
V.2 Different kinds of SLIC :
There are two SLIC families :
–the SLIC without integrated ring generator.
–the SLIC with integrated ring generator.
V.2A SLIC without integrated ring generator :
For this case the SLIC IC is supplied between
the ground and the battery voltage (- Vbat).
The relay operates the selection of functions,
ringing mode in position 1 and the other modes
in position 2.
V.2 B SLIC with integrated ring generator :
This kind of SLIC, only composed by the L3000
family of SGS THOMSON, is supplied between
the ground, the battery (-Vbat) and a positive
voltage (+VB) up to +72 V.
V.3 Goal of the SLIC protection :
The purpose of the protection is to suppress all
overvoltages out of the normal operating range
voltage of the SLIC.
We have to take into account the two kinds of
SLIC seen in paragraph V.2.
Fig. 28 Different ringing configurations.
Vbat Vbat Vbat
LINE
LINE
LINE
LINE
LINE
LINE
Ground-backed Battery-backed balanced
ringing ringing ringing
Fig. 29 SLIC without integrated ring generator.
-Vbat
1
1
2
2
Rp
Rp
T
R
SLIC
RING GENERATOR
Fig. 30 SLIC with integrated ring generator.
-Vbat
Rp
Rp
T
R
SLIC
+Vb
APPLICATION NOTE
9/16
V.3.A SLIC without integrated ring generator :
As shown in the Fig.31 the protection areas are
located differently before and after the ring
relay.
Before the relay the protection must operate
over the peak value of the ring signal (generally
+90V and -190V). As the relay protection does
not require a very precise clamping threshold,
we usually use a symetrical overvoltage
suppressor (generaly + or - 200V).
After the relay the protection acts to suppress
all spikes over the ground and under the battery
voltage (-Vbat).
It is important to note that the integrated
circuit needs a protection threshold closest than
the supply voltage.
In certain cases (the TDB7711/7722 of SGS
THOMSON) an internal network of diodes
permits to oversupply the output stages of the
SLIC (see Fig.32).
V.3.B SLIC with integrated ring generator :
The integrated circuit L3000 of SGS THOMSON
is presently the only SLIC of this kind. It
operates between ground and battery voltage for
all the modes except for the ringing where the
normal area is included between +VB (up to
72V) and battery voltage (-VBat) (see Fig.33).
The protection will take into account this fact and
operates over +VB and under -VBat.
Fig. 31 Goal of the protection for the SLIC without integrated ring generator.
-Vbat
Rp
Rp
T
R
SLIC
RING GENERATOR
PROTECTION
AREA
PROTECTION
AREA
PROTECTION
AREA
PROTECTION
AREA
NORMAL OPERATING
NORMAL OPERATING
AREA AREA
-Vbat
Vring peak
- Vbat
-Vbat
-Vbat
-Vring peak
Fig. 33 External diodes network used with the
L3000.
LINE
-Vbat
L3000
+Vb
NMT
POWER
SWITCH
LINE
D1
D2
Fig. 32 Internal diodes network of the TDB7722.
D1
D2
D3
LINE
-Vbat
APPLICATION NOTE
10/16
The diodes D1 and D2 (fig 33) act when the
L3000 operates out of the ringing mode and
when a positive overvoltage is clamped at +VB.
The output stages are then temporarily
oversupplied at +VB.
VI APPLICATION DIAGRAMS :
VI.1 SLIC without integrated ring generator
This basic diagram (fig 34) uses TRISIL TPA or
TPB solution. Before the ring relay both lines are
protected to the ground by 200 V bidirectional
devices.
After the relay the positive surges are clamped
to the ground by two diodes (one per line). For
the negative one two diodes switch the
overvoltage to a 62 V TRISIL.
Remark : The diodes choice is very important in
order to minimize their peak forward voltage
(VFP).
Fig. 34 Axial leaded solution
RING GENERATOR
INTEGRATED
SLIC
RING
RELAY
TEST
RELAY
LINE
LINE
-Vbat
R or PTC
R or PTC TPA or TPB62
2xTPA
or TPB
Fig. 35 TO 220 or SIP 3 version
RING GENERATOR
INTEGRATED
SLIC
RING
RELAY
TEST
RELAY
LINE
LINE
-Vbat
R or PTC
R or PTC
THDT58D
THBT200D
or
THBT200S
or
THDT58S
APPLICATION NOTE
11/16
This topology (fig. 35) assumes the same
protection function as precedent one with the
following advantages :
–Only two packages.
–Reduction of printed board area used by
the protection.
–Cost effective solution.
The figure 36 is exactly the same silicon solution
with :
–Surface mount packages (SOD 15).
–Better cost solution in SMD version.
–Same cost as the TO 220 version.
The protection behaviour is improved by a
breakover value very close to the battery voltage
(fig. 37).
This kind of solution is well adapted to the
variable battery voltage application, for example
short / long line switching.
Fig. 36 Surface mount solution
RING GENERATOR
INTEGRATED
SLIC
RING
RELAY
TEST
RELAY
LINE
LINE
-Vbat
R orPTC
R or PTC
2 x SMTHDT 58D
2 x SMTHBT 200D
Fig. 37 Programmable breakover voltage solution (2 x L3100B1)
RING GENERATOR
INTEGRATED
SLIC
RING
RELAY
TEST
RELAY
LINE
LINE
-Vbat
R or PTC
R or PTC
THBT200D
or
THBT200S
2 x L3100B1
C
A
Gn
C
AGn
APPLICATION NOTE
12/16
The figure 38 has the same electrical behaviour
as the precedentone with.
–Lower cost
Remark : The maximum voltage across the line
during the device firing is increased by the VFP
of the diode.
This solution (fig. 39) performs the new
generation of SLIC protection. It is the full
integration of protection devices and peripherical
diodes.
Fig. 38 Programmable breakover voltage solution (1 x L3100B1)
RING GENERATOR
INTEGRATED
SLIC
RING
RELAY
TEST
RELAY
LINE
LINE
-Vbat
R orPTC
R or PTC
THBT200D
or
THBT200S
CA
Gn
L3100B1
Fig. 39 Monochip programmable breakover voltage solution
RING GENERATOR
INTEGRATED
SLIC
RING
RELAY
TEST
RELAY
LINE
LINE
-Vbat
R orPTC
R or PTC
THBT200D
or
THBT200S
LCP150S
APPLICATION NOTE
13/16
.VI.2 SLIC with integrated ring generator
(L3000).
This topology (fig. 40) is the most efficient one
for this kind of SLIC.
The figure 41 has the same electrical behaviour
as previous one but with low cost.
VI.3 Common protections for several SLIC
These types (fig. 42 and 43) of application allow
to decrease the cost of the protection per line.
The major problems of these solutions are.
–The short circuit of all the lines when the
protectionfire
–The remaining current through the protec-
tion device after the surge is too high to
permit the automatic switch off of the pro-
tection. In fact only a software action (low
power state) permits to assume the switch
off.
Fig. 40 L3000 protection with 2 L3121B
R1
R2
R3
R4
-Vbat +VB
L3121B SLIC
L3000
L3121B
1
1
2
2
3
+VB-Vbat
22nF
22nF
TIP
RING
MNT
3
LINE
LINE
Fig. 41 L3000 protection with 2 L3100B
R1
R2
R3
R4
-Vbat +VB
SLIC
L3000
TIP
RING
MNT
LINE
LINE
-Vbat
+VB GnGp
A
A
C
C
L3100B L3100B
APPLICATION NOTE
14/16
In conclusion SGS-THOMSON have got a large
range of protection solutions in order to optimize
your application diagram. These solutions take
into account.
The type of SLIC.
The battery voltage.
The pc board surface.
The cost.
And will permit you to solve your SLIC protection
problem.
Fig. 42 Common protection for SLIC without integrated ring generator
Fig. 43 Common protection for SLIC with integrated ring generator
R1
R2
R3
R4
-Vbat +VB
SLIC
TIP
RING
MNT
LINE
LINE
R1
R2
R3
R4
-Vbat +VB
SLIC
TIP
RING
MNT
LINE
LINE
-Vbat
Gn
A
CL3100B
1
N
R1
R2
R3
R4
-Vbat +VB
SLIC
TIP
RING
MNT
LINE
LINE
R1
R2
R3
R4
-Vbat +VB
SLIC
TIP
RING
MNT
LINE
LINE
-Vbat
Gn
A
C
L3100B
1
N
Gp
A
C
+VB
L3100B
APPLICATION NOTE
15/16
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APPLICATION NOTE
16/16
