ST AN2053 APPLICATION NOTE

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

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.

High speed to Pots coupling

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 Ta bl e 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

Surge

generator

Figure 1

R2 = 15 Ω

20 µF

R1 = 50 Ω

K20 surge generator

0.2 µF

2/15 Doc ID 10917 Rev 2

Figure 1

Surge

generator

R3 = 25 Ω

K20 transversal test

R4 = 25 Ω

R5 = 25 Ω

K20 longitudinal test

Coupling

network

Coupling

network

Equipment

under test

Equipment

under test

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

Surge

generator

Uac

Timing circuit

R = 600 Ω

Equipment

under test

Uac

Timing circuit

160 Ω

600Ω

10 Ω

160 Ω

600 Ω

Equipment under test

K20 power induction surge generator

Table 1. Main line card lightning surge standards

Country Standard

Surge

voltage (V)

K20 power contact surge generator

Waveform Current (A)

Worldwide ITU-T K20 1500 10/700 µs 37.5

Worldwide IEC 61000-4-5 1000/4000 10/700 µs 25/100

Worldwide IEC 61000-4-5 1000/4000 1.2/50 µs 25/100

USA GR-1089 Core (Telcordia) 2500 2/10 µs 500

USA GR-1089 Core (Telcordia) 1000 10/1000 µs 100

Table 2. GR-1089 Core Intra-building lightning surge standard

Repetitions

Each

polarity

Test Surge Voltage (V) Waveform

Surge Current per

conductor (A)

1 ±800 2/10 µs 100 1

2 ±1500 2/10 µs 100 1

Ta bl e 1 and 2 show the main worldwide lightning surge standards. Tab l e 1 is dedicated to

classical wired telecom line cards while the Ta bl e 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

L 1

GND

-Vbat

Gate

Th1

TIP

ID1

V Tip

GND

Rs2

L 2

RING

V 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

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

, 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

) remain

di/dt

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

pin. So to be efficient it has to be moved as close as possible to the LCP15xx

bat

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 LCP concept

Figure 6. Example of PCB layout based on LCP15xx protection

line side

GND

nF 0

SLIC side

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:

surge

= V

/ (Rg + Rs)

surge

With:

● V

● R

= peak surge voltage imposed by the standard

surge

= series resistor of the surge generator

= 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:

= 1000 / (10 + 30) = 25 A

surge

Doc ID 10917 Rev 2 5/15

+ 10 hidden pages