ST AN821 Application note

AN821

APPLICATION NOTE

NEW LAYOUT INSULATION REQUIREMENT

INTRODUCTION

In order to assume better level of security for telecommunication system users, some new rules have been
published. This is the case of the IEC950 standard which dedicates one chapter to this subject. This kind
of requirement is beginning to be active in Germany for example.

REQUIREMENT

Figure 1 shows the principle of the test circuit. The test consists of the insulation measurement of the tele­communication network and the earth. During this test it is permitted to remove components. In fact in the
field the procedure is done as follow:

– The protection device is removed from the board.

– The telecommunication lines are connected together. Tip and Ring lines from all the subscribers for an

exchange equipment, or A and B lines for a telephone set.

– A voltage of 2 kV is applied between the lines and ground.

– The current flowing in the test circuit shall not exceed the requested limit, for example 10 mA in the

IEC950 case.

Figure 1. Test Circuit

TEST

GENERATOR

mA

TELECOM. EQUIPMENT

REV. 2

1/4May 2004

AN821 APPLICATION NOTE

OUR PROPOSAL

To withstand these new requirements STMicroelectronics proposes an adapted input/output arrangement
for the THBT series.

Figure 2. STMicroelectronics proposal

TIP TIP

TO THE
TELECOM.
NETWORK

TO THE
TELECOM.
NETWORK

RING

RING

WITH COMPONENT ON BOARD

TIP TIP

RING

RING

LAYOUT WITHOUT COMPONENT ON BOARD

TO THE

TELECOM.

EQUIPMENT

TO THE

TELECOM.

EQUIPMENT

Figure 2 shows our proposal that consists of following points:

– Insulation between inputs and outputs. For example TIP coming from the telecommunication network

is isolated from the TIP connected to the sensitive part of the circuit. That means, when the surge
suppressor is removed, no current can flow through the telecommunication equipment.

– For the component which has got four ground pins (like the THBT200xx), only two pins are enough to

withstand the maximum surge current capability of the component. That means only the two ground
pins located on the telecommunication equipment side are connected.

SUMMARY

New standards concerning the protection of the telecommunication network service personnel, and other
users of this network, from hazards in the equipment seems to be generalized. STMicroelectronics has
taken into account this fact and proposes to meet these requirements by layout like shown in Figure 2.

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